Identification of multiple magnetizations of the Ediacaran strata in South China

Identification of multiple magnetizations of the Ediacaran strata in South China Abstract A suspected Silurian remagnetization of the Ediacaran strata of South China was proposed decades ago by many researchers, but, there has been no systematic study of its causes and mechanisms. In this study, we investigate the multiphase remagnetization processes that affected the Ediacaran strata and the possible mechanisms of these remagnetization events. We conducted detailed palaeomagnetic, rock magnetic and scanning electron microscope (SEM) studies of samples from the Ediacaran strata in the Jiulongwan (JLWE, JLWS), Qinglinkou (QLK) and Sanxiarenjia (SXRJ) sections in the Three Gorges Area, South China. After removal of a recent viscous remanent magnetization below 150 °C, an intermediate temperature component (ITC; Dg = 27.6°, Ig = 45.3°, N = 12 sites, kg = 184.3, α95 = 3.2° for JLWE; Dg = 22°, Ig = 45.3°, N = 11 sites, kg = 789.2, α95 = 1.6° for JLWS; and Dg = 25.5°, Ig = 52.5°, N = 6 sites, kg = 533.4, α95 = 2.9° for SXRJ) was removed below 300 °C which coincides with the Jurassic results from South China, suggesting a pervasive Jurassic remagnetization. In addition, a high temperature component (HTC; Ds = 84.8°, Is = 19.2°, N = 9 sites, ks = 35.5, α95 = 8.8° for JLWE; Ds = 74.1°, Is = 49.4°, N = 7 sites, ks = 218.9, α95 = 4.1° for JLWS; and Ds = 89.5°, Is = 30.7°, N = 8 sites, ks = 129.2, α95 = 4.9° for SXRJ) was isolated between 300 and 480–540 °C. Rock magnetic and SEM studies suggest that the ITC and HTC are carried by pyrrhotite and magnetite, respectively. SEM observations also demonstrate the occurrence of massive authigenic magnetite in cavities or cracks, mineralogical changes from pyrite to Fe oxides, and the reaction between gypsum and Fe oxides. Based on similarities to the Silurian poles of South China, together with the SEM observations, we suggest that the HTC from the JLWE and SXRJ sections is a Silurian age remagnetization. The oxidation of iron sulphides and thermochemical sulphate reduction induced by the multiple generations of oil and gas in the Ediacaran and Cambrian strata are suggested as the main mechanism for remagnetization. Despite the pervasive Silurian remagnetization of the Ediacaran strata, most of the HTC from the thick-bedded dolostone of Doushantuo Formation Member 3 at the JLWS section appears to carry a primary remanence, because its pole differs from other poles of South China and the results pass both the fold and reversal tests. The relatively low-geothermic conditions and the absence of both hydrocarbon and smectite/illite explain why most results from the Doushantuo Member 3 of JLWS section were not affected by the Silurian remagnetization. This new Ediacaran pole supersedes the previous suspected remagnetized poles, which can be used to constrain the palaeoposition of South China both in Rodinia and Gondwana. Asia, Palaeomagnetism, Remagnetization, Rock and mineral magnetism 1 INTRODUCTION Worldwide, the palaeomagnetic data for the Ediacaran interval are complex (McCausland et al.2007, 2011; Abrajevitch & Van der Voo 2010; Schmidt & Williams 2010; Meert 2014; Schmidt 2014; Halls et al.2015; Jing et al.2015; Klein et al.2015). For instance, the contradictory results with almost the same age, obtained from Laurentia and Baltica, place these two cratons either near the equator or at high latitudes during 615–550 Ma (McCausland et al.2007, 2011; Abrajevitch & Van der Voo 2010; Meert 2014; Halls et al.2015; Klein et al.2015). In addition, palaeomagnetic poles from Australia during 625–550 Ma demonstrate a small circle distribution and a strong oscillation (Schmidt & Williams 2010; Jing et al.2015). Different hypotheses have been proposed for these contradictory results, including: rapid plate motions (Meert et al.1993), true polar wander (Evans 1998, 2003), local rotation (for Australia, Schmidt & Williams 2010; Jing et al.2015) and anomalous field behaviour (Abrajevitch & Van der Voo 2010). In contrast to the Ediacaran palaeomagnetic results from other continents, which are usually of dual polarity, passing the fold or baked-contact tests, with a precise age, and with rapid apparent motion, the data from the South China block (Zhang 1994; Macouin et al.2004) are unipolar, and close to the Silurian results of South China. Recently, Zhang et al. (2015) reported a new result from the uppermost 14 m of the Doushantuo Formation Member 3, which differs from the result obtained by Macouin et al. (2004) and is of dual polarity. However, as was noted, there are systematic oscillating trends in the inclinations through their upper polarity zone strata (Zhang et al.2015), implying a possible complex magnetization acquisition process. In fact, several researchers have suspected that a Silurian remagnetization event may have affected the Ediacaran-Cambrian strata across the South China Block (Wu et al.1988; Macouin et al.2004; Yang et al.2004; Zhang et al.2013, 2015; Jing et al.2015). Although many palaeomagnetic studies have been conducted on the Ediacaran strata over the last few decades (Zhang et al.1983; Zhang 1994; Macouin et al.2004, 2012; Zhang et al.2015), there has been no study of the remagnetization processes that affected the Ediacaran strata of South China. Here, we present the results of detailed palaeomagnetic, rock magnetic and scanning electron microscope (SEM) analyses of the Ediacaran strata in the Three Gorges Area, Hubei Province, China. The objective was to detect whether the occurrence of multiple magnetizations on the Ediacaran strata of South China took place and to understand the mechanisms responsible. Eventually, the primary palaeomagnetic direction isolated from these carbonate strata may provide further constraint on the palaeogeographic position of South China both in Rodinia and Gondwana. 2 GEOLOGICAL SETTING AND PALAEOMAGNETIC SAMPLING The study areas are in Yichang, Hubei Province, in the northern part of the Yangtze Block, where typical Precambrian sedimentary sequences have been identified by geological survey (Fig. 1, Regional geological survey team of Hubei Province 1970). Neoproterozoic strata in the Three Gorges Area are well preserved and consist of pre-Cryogenian siliciclastic units (Liantuo Formation) in the lower part, with ages ranging from ∼784 to ∼714 Ma (SIMS U-Pb zircon dating; Lan et al.2015). Cryogenian glacial and interglacial deposits occur in the middle part, including two glacial diamictite intervals (the Chang’an Formation and the Nantuo Formation), separated by interglacial manganese-bearing shale of the Datangpo Formation, dated to between ∼663 and ∼635 Ma (SHRIMP U-Pb zircon dating; Zhou et al.2004; Zhang et al.2008a,b), and Ediacaran mixed carbonate-siliciclastic units (Doushantuo and Dengying Formations) form the top of the sequence. Figure 1. View largeDownload slide (A) Simplified map of South China showing the location of the sampling area. (B) Geological sketch map of the sampling area in Yichang, Hubei Provence. Three sections were sampled: Jiulongwan (including JLWE and JLWS), Qinglinkou (QLK) and Sanxiarenjia (SXRJ). Strike and dip are also shown in the inset. Figure 1. View largeDownload slide (A) Simplified map of South China showing the location of the sampling area. (B) Geological sketch map of the sampling area in Yichang, Hubei Provence. Three sections were sampled: Jiulongwan (including JLWE and JLWS), Qinglinkou (QLK) and Sanxiarenjia (SXRJ). Strike and dip are also shown in the inset. Most of the Doushantuo Formation in the Yangtze platform was deposited on a rimmed carbonate shelf with a shelf margin shoal complex that restricted the shelf lagoon from the open ocean during the Early Ediacaran (Jiang et al.2011). In the Three Gorges Area, the Doushantuo Formation consists of 160–250 m of carbonate and black shale, subdivided into four members (Fig. 2). In the study area, basal Member 1 of the Doushantuo Formation is a 6–10 m thick cap carbonate overlying the glacial diamictite of the Nantuo Formation. Member 2 consists of about 70–80 m of interbedded black shale and shaly limestone with abundant pea-sized chert and less-common phosphatic nodules overlying the cap carbonate. Member 3 consists of ∼60 m of medium- to thick-bedded dolostone intercalated with concretions and layers of chert (in the lower part) and thin-bedded marlstone (in the upper part). Member 4 consists of ∼10 m of thick black shale with scattered thin layers or lenses of dolomite (McFadden et al.2008; Fig. 2). Figure 2. View largeDownload slide Lithostratigraphy of the three sampled sections and sampling layer positions for each section. Figure 2. View largeDownload slide Lithostratigraphy of the three sampled sections and sampling layer positions for each section. The Dengying Formation in the Sanxiarenjia section can be subdivided into three lithostratigraphic members, namely Hamajing, Shibantan, and Baimatuo, in ascending order (Fig. 2; Zhao et al.1985, 1988; Zhu et al.2007). The Hamajing Member is a ∼130 m thick white or pink-white dolostone, characterized by massive intraclastic and oolitic dolomitic grainstone with oncoids. The Shibantan Member is a ca. 80 m thick dark grey thin- to intermediate-bedded, laminated micritic limestone with trace fossils and fragments rich in vendotaenids (algal or bacterial colonies). Cherty laminae occur in the upper part of this member. The Baimatuo Member consists of massive micritic and sparitic white or pink-white dolostones. Cross-stratification begins to occur in the middle part of this member, leading upwards into miarolitic bird-eye structures or boxworks, indicating that the water depth had become shallower (Zhao et al.1988). In the Three Gorges Area, the Doushantuo Formation records the onset of a long interval of carbonate-dominated sedimentation on the Yangtze craton, which was interrupted by uplift and erosion in the late Silurian and then continued after a rifting phase until the Early Triassic (Peters et al.1996; Vernhet & Reijmer 2010). The study area experienced three major tectonic episodes, the Caledonian, the Indosinian and the Yanshanian orogenies, and there were three main periods of oil generation and expulsion in the central Yangtze area (Zhu et al.2015). The Caledonian orogeny only resulted in the absence of the lower Devonian strata, and subsequently the area continued to accumulate marine sediments. However, the transgression was terminated by the Indosinian orogeny, which led to exposure of the strata. During the Yanshanian orogeny, the Ediacaran strata overlying the Huangling Granite suites were folded and formed the nearly north-south-trending Huangling anticline (Xiao et al.1965). Exposures of the Doushantuo Formation in the eastern Three Gorges Area are either not well formed or inaccessible, being either covered by vegetation or in the form of cliffs. Therefore, in 2012 we collected a total of 99 block samples from Member 2 of the Doushantuo Formation in the Jiulongwan section (JLWE) (Figs 1 and 2; 30.81°N, 111.07°E; dip direction: ∼130°) and 92 (9 sites) drill samples from the Dengying Formation Shibantan Member in the Sanxiarenjia section (SXRJ; Figs 1 and 2; 30.82°N, 111.16°E; dip direction: ∼127°). In the laboratory, 2.54-cm-diameter core samples were drilled from the block samples. Subsequently, in 2015, we revisited the Three Gorges Area and obtained 138 drill samples (11 sites) from Member 3 of the Doushantuo Formation in the Jiulongwan section (JLWS; Figs 1 and 2; 30.81°N, 111.07°E; dip direction: ∼90°). To conduct a fold test, we also collected 53 drill samples (4 sites) from Member 2 and Member 3 of the Doushantuo Formation in the Qinglinkou section (QLK; 30.8°N, 110.92°E; Figs 1 and 2; dip direction: ∼240°; An et al.2015a,b). 3 METHODS At the Paleomagnetism Laboratory of Nanjing University, cores were cut into standard cylindrical specimens of 2.3 cm height and 2.54 cm diameter. Selected samples were used for acquisition of isothermal remanent magnetization (IRM) experiments. Fields of up 2.4 T were first imparted to the Z-axis using an ASC IM-10–30 impulse magnetizer and measured using a JR-6A spinner magnetometer housed in a magnetically shielded room. Subsequently, these samples were magnetized sequentially along the Z-, Y-, and X-axes in fields of 2.4, 0.4, and 0.12 T, respectively, and subjected to stepwise thermal demagnetization (Lowrie 1990). Also at the Paleomagnetism Laboratory of Nanjing University, progressive thermal demagnetization of untreated samples from 44 sites was carried out in 14–18 steps with a 50 °C increment for low temperatures (<300 °C) and a 20 or 30 °C increment for high temperatures (>300 °C), up to 530–580 °C. All samples were thermally demagnetized step-by-step using an ASC-TD48 oven. Remanences were measured using a 2 G Enterprises Inc. cryogenic magnetometer (2G-755) housed in a magnetically shielded room. The remanence directions were analysed using the principal component analysis method (Kirschvink 1980) and the mean directions were calculated using Fisher spherical statistics (Fisher 1953). The palaeomagnetic data were analysed using the PMGSC software package of R. Enkin, and graphs were plotted using the PaleoMac program of Cogné (2003). At the Key Paleomagnetic Laboratory of the Chinese Academy of Geological Sciences, Beijing, China, susceptibility–temperature (κ–T) curves were obtained using a KLY4 Kappa bridge (Czech Agico Company). Powder samples were heated from room temperature to 405 °C and then cooled to room temperature in air. One of the samples was heated to 710 °C in an argon atmosphere. At the State Key Laboratory of Mineral Deposits Research of Nanjing University, SEM observations and energy-dispersive X-ray spectra (EDS) analysis were performed on gold-coated rock fragments cut from selected specimens using a Jeol JSM-6490 scanning electron microscope and Oxford INCA energy 350 EDS Analyzer. 4 RESULTS 4.1 Rock magnetic results To identify the potential magnetic carriers in the samples we studied, we selected representative samples from the Doushantuo and the Dengying Formations for rock magnetic studies. Cumulative log Gaussian (CLG) functions analysis was performed on the IRM data for 19 representative specimens from all four sections using the software developed by Kruiver et al. (2001; Fig. 3, Supporting Information Table S1). Five coercivity components are revealed. A very low-coercivity component (1.6–22 mT), constituting ∼15 per cent of the total IRM (Supporting Information Table S1), is present in 14 specimens. Another low coercivity component (30–62 mT; Supporting Information Table S1) is present in all specimens. A medium coercivity component (126–160 mT), constituting 6–34 per cent of the total IRM, is present in 11 specimens (Supporting Information Table S1). A medium to high-coercivity component (251–398 mT), constituting 10–37 per cent of the total IRM, is present in five specimens. In addition, five specimens contained a high coercivity (1548–2089 mT) component. Based on their coercivity ranges (Peters & Dekkers 2003; Manning & Elmore 2015), we interpreted these five coercivity components as maghemite or multidomain magnetite, single domain magnetite, pyrrhotite, hematite and goethite, respectively, from low to high coercivity. Figure 3. View largeDownload slide Examples of coercivity component analysis of IRM in linear acquisition plots (LAP), gradient acquisition plots (GAP) and standardized acquisition plots (SAP), respectively, processed using the cumulative log-Gaussian model (Kruiver et al.2001). The thick red line shows the sum of all the components. Figure 3. View largeDownload slide Examples of coercivity component analysis of IRM in linear acquisition plots (LAP), gradient acquisition plots (GAP) and standardized acquisition plots (SAP), respectively, processed using the cumulative log-Gaussian model (Kruiver et al.2001). The thick red line shows the sum of all the components. In most of the specimens, the results of thermal demagnetization of the tri-axial IRMs show dominance by the low-coercivity component, which has a laboratory unblocking temperature ranging from 530–580 °C (Fig. 4), suggesting the presence of magnetite (Lowrie 1990). A major inflection in the thermal demagnetization decay curves of the low- and medium-coercivity components around 400 °C (Figs 4a and c) suggests the presence of greigite or mineral alteration (Roberts et al.2011). For the specimens with a minor high coercivity component, complete demagnetization did not occur until 680 °C (Figs 4b and c), suggesting the presence of hematite (Lowrie 1990). Figure 4. View largeDownload slide Results of stepwise thermal demagnetization of a three-component IRM imparted to representative samples. Orthogonal IRMs were imparted in fields of 0.12 T (circles), 0.4 T (triangles) and 2.4 T (squares). Figure 4. View largeDownload slide Results of stepwise thermal demagnetization of a three-component IRM imparted to representative samples. Orthogonal IRMs were imparted in fields of 0.12 T (circles), 0.4 T (triangles) and 2.4 T (squares). Thermomagnetic curves (κ–T) of representative specimens (Figs 5a and b) show a marked decrease between 260 and 305 °C, with a minimum susceptibility value reached at ∼305 °C. This minimum is followed by a slow increase until ∼400 °C, followed by a dramatic susceptibility increase. In addition, a specimen was heated to 710 °C (Figs 5c and d), and in this case, a minimum susceptibility value reached at ∼310 °C, followed by a small peak at ∼350 °C (Fig. 5d). Subsequently, a slow increase in susceptibility occurred at ∼400 °C, a dramatic increase until ∼450 °C, and then the heating curve reached a minimum at ∼580 °C. These results indicate the presence of pyrrhotite and magnetite in the samples. Figure 5. View largeDownload slide Thermomagnetic curves (κ–T) of the representative specimens. Figure 5. View largeDownload slide Thermomagnetic curves (κ–T) of the representative specimens. 4.2 Palaeomagnetic results In most of the samples, three natural remanent magnetization (NRM) components were isolated. First, a low-temperature component (LTC) was removed below 150 °C (Figs 6–9). The mean directions of this component in all sections are similar to the local present field direction (Tables 1–3). Figure 6. View largeDownload slide Orthogonal vector projection of the results of thermal demagnetization of NRM for representative samples from the JLWE section in geographic coordinates. Figure 6. View largeDownload slide Orthogonal vector projection of the results of thermal demagnetization of NRM for representative samples from the JLWE section in geographic coordinates. Table 1. HTC palaeomagnetic data for the Doushantuo Fm Member 2 of the Jiulongwan section (JLWE), Yichang (111.069°E, 30.81°N). Site  Component  N/n  Dg  Ig  Ds  Is  k  α95  JLWE10  HTC  9  74.2  26.1  77.3  17.1  33.5  9  JLWE11  HTC  17  58.2  37.1  66.2  33.1  88.8  3.8  JLWE12  HTC  7  94.3  39.4  93.4  38.9  59.9  7.9  JLWE13  HTC  10  81.2  23.7  82.5  20.3  42.5  7.5  JLWE14  HTC  6  93  26.1  97.1  16.2  62.3  8.6  JLWE15  HTC  10  88.6  13.8  89.8  10.4  58.6  6.4  JLWE16  HTC  12  90.2  16.6  91.2  11.7  67.5  5.3  JLWE17  HTC  7  86  14.7  87  9.2  79.3  6.8  JLWE6–7  HTC  6  75.5  19.5  77.1  13.4  117.5  6.2  Mean of HTC components                  9/84  82.6  24.5      35.3  8.8            84.8  19.2  35.2  8.8  Palaeopole        12.8°N, 193.6°E, dp/dm = 5.1°/9.4°(in situ)          9.5°N, 195.2°E, dp/dm = 4.8°/9.2°(tilt corrected)  Mean of LTC components                  13/133  358.5  55.9  8.4  59.4  ks/kg = 356/282  2.5  Palaeopole        84.2°N, 99°E, dp/dm = 2.6°/3.6°(in situ)  Site  Component  N/n  Dg  Ig  Ds  Is  k  α95  JLWE10  HTC  9  74.2  26.1  77.3  17.1  33.5  9  JLWE11  HTC  17  58.2  37.1  66.2  33.1  88.8  3.8  JLWE12  HTC  7  94.3  39.4  93.4  38.9  59.9  7.9  JLWE13  HTC  10  81.2  23.7  82.5  20.3  42.5  7.5  JLWE14  HTC  6  93  26.1  97.1  16.2  62.3  8.6  JLWE15  HTC  10  88.6  13.8  89.8  10.4  58.6  6.4  JLWE16  HTC  12  90.2  16.6  91.2  11.7  67.5  5.3  JLWE17  HTC  7  86  14.7  87  9.2  79.3  6.8  JLWE6–7  HTC  6  75.5  19.5  77.1  13.4  117.5  6.2  Mean of HTC components                  9/84  82.6  24.5      35.3  8.8            84.8  19.2  35.2  8.8  Palaeopole        12.8°N, 193.6°E, dp/dm = 5.1°/9.4°(in situ)          9.5°N, 195.2°E, dp/dm = 4.8°/9.2°(tilt corrected)  Mean of LTC components                  13/133  358.5  55.9  8.4  59.4  ks/kg = 356/282  2.5  Palaeopole        84.2°N, 99°E, dp/dm = 2.6°/3.6°(in situ)  N: number of sites for statistical analysis; n: number of samples for statistical analysis; Dg, Ig, Ds, Is: declination and inclination in geographic and stratigraphic coordinates; k: Fisher precision parameter of the mean; α95: confidence of the mean direction; dp/dm: semi-axes of elliptical error around the pole at a probability of 95 per cent. View Large Second, an intermediate-temperature component (ITC), with a northeast declination and moderate positive inclination (Figs 6–9, Tables 4–6), was demagnetized mainly between 200 and 300 °C (Figs 6–9). The mean ITC directions for the JLWE, JLWS and SXRJ sections, in geographic coordinates, are Dg = 27.6°, Ig = 45.3°, N = 12 sites, kg = 184.3, α95 = 3.2° (Fig. 10a, Table 4); Dg = 22°, Ig = 45.3°, N = 11 sites, kg = 789.2, α95 = 1.6° (Fig. 10c, Table 5); and Dg = 25.5°, Ig = 52.5°, N = 6 sites, kg = 533.4, α95 = 2.9° (Fig. 10e, Table 6), respectively. Figure 7. View largeDownload slide Orthogonal vector projection of the results of thermal demagnetization of NRM for representative samples from the JLWS section in geographic coordinates. Figure 7. View largeDownload slide Orthogonal vector projection of the results of thermal demagnetization of NRM for representative samples from the JLWS section in geographic coordinates. Figure 8. View largeDownload slide Orthogonal vector projection of the results of thermal demagnetization of NRM for representative samples from the QLK section in geographic coordinates. Figure 8. View largeDownload slide Orthogonal vector projection of the results of thermal demagnetization of NRM for representative samples from the QLK section in geographic coordinates. Figure 9. View largeDownload slide Orthogonal vector projection of the results of thermal demagnetization of NRM for representative samples from the SXRJ section in geographic coordinates. Figure 9. View largeDownload slide Orthogonal vector projection of the results of thermal demagnetization of NRM for representative samples from the SXRJ section in geographic coordinates. Figure 10. View largeDownload slide Equal-area stereographic projections of site-mean directions of the ITC components, in geographic and stratigraphic coordinates. Red stars indicate the mean directions. Figure 10. View largeDownload slide Equal-area stereographic projections of site-mean directions of the ITC components, in geographic and stratigraphic coordinates. Red stars indicate the mean directions. Finally, a high-temperature component (HTC) with a more east-directed declination and shallow to moderate positive inclination (Figs 6–9 and 11) was removed mainly between 330 °C and 480–530 °C (Figs 6–9). Only site JLWS6 exhibits a southwest declination and moderate negative inclination (Figs 7 and 11c and d). The overall mean HTC directions for the JLWE and SXRJ sections, in stratigraphic coordinates, are Ds = 84.8°, Is = 19.2°, N = 9 sites, ks = 35.5, α95 = 8.8° (Table 1); and Ds = 89.5°, Is = 30.7°, N = 8 sites, ks = 129.2, α95 = 4.9° (Table 3), respectively. Figure 11. View largeDownload slide Equal-area stereographic projections of site-mean directions of the HTC components, in geographic and stratigraphic coordinates. HTC components from the QLK section (blue diamonds) are also plotted in the JLWS section for a fold test. Figure 11. View largeDownload slide Equal-area stereographic projections of site-mean directions of the HTC components, in geographic and stratigraphic coordinates. HTC components from the QLK section (blue diamonds) are also plotted in the JLWS section for a fold test. Although the direction of sites JLWS6 has a negative (upward) inclination, the reversal test is negative at the 95 per cent probability level in the JLWS section (McFadden & McElhinny 1990; γcritical = 11.6, angle difference is 18.1). In addition, the directions of sites JLWS1, 6, 9 and QLK2, 3 are distributed along great circles which are fitted by using the LTC and ITC of JLWS, the Silurian results recalculated based on Opdyke et al. (1987) and Huang et al. (2000) and the HTC of JLWS and QLK (Figs 12a and b). In addition, the HTC of JLWS7 is rather different from the other results for this section (Figs 12a and b), so we exclude it from the following analysis. Figure 12. View largeDownload slide (A,B) Equal-area stereographic projections of the HTC components from the JLWS section (black dots), QLK section (yellow triangles), the results from Zhang et al. (2015) (blue crosses) and the Silurian result in our study area recalculated based on the results of Huang et al. (2000) and Opdyke et al. (1987) (green diamonds). Two red great circles are fitted using the LTC, ITC, Silurian results and the HTC components of the JLWS and QLK; (C,D) the HTC components used to calculate the primary remanent magnetization from the JLWS section (black dots), the results from Zhang et al. (2015) (blue crosses) and the QLK result (yellow triangles), before and after tilt correction. Red stars indicate the mean directions. Figure 12. View largeDownload slide (A,B) Equal-area stereographic projections of the HTC components from the JLWS section (black dots), QLK section (yellow triangles), the results from Zhang et al. (2015) (blue crosses) and the Silurian result in our study area recalculated based on the results of Huang et al. (2000) and Opdyke et al. (1987) (green diamonds). Two red great circles are fitted using the LTC, ITC, Silurian results and the HTC components of the JLWS and QLK; (C,D) the HTC components used to calculate the primary remanent magnetization from the JLWS section (black dots), the results from Zhang et al. (2015) (blue crosses) and the QLK result (yellow triangles), before and after tilt correction. Red stars indicate the mean directions. The rest of the HTC magnetizations from the JLWS and QLK sections are situated at the end of Fit 2 (Fig. 12b), which suggests that they were probably not affected by the Silurian magnetization. The overall mean directions for all the JLWS sites are Dg = 75.1, Ig = 51.5, N = 7 sites, kg = 97.3, α95 = 6.1 in geographic coordinates; and Ds = 74.1°, Is = 49.4°, N = 7 sites, ks = 218.9, α95 = 4.1° in stratigraphic coordinates (Fig. 11d). 4.3 SEM observations SEM observations of the fresh surface of representative specimens show the presence of numerous pyrites grains in the form of detritus, ultrafine crystals, framboidal aggregates and fine crystal aggregates (Fig. 13). Most of them cave into the matrix, which suggests that they formed during early diagenesis (Wang et al.2012). In addition, the presence of several ultrafine framboidal aggregates formed on the surface of detrital magnetite suggests that they may have formed during late diagenesis (Fig. 14, QLK4–7). Figure 13. View largeDownload slide Scanning electron microscope (SEM) secondary electron image and energy-dispersive X-ray spectra (EDS) analysis of Fe sulphides from the JLWS and QLK sections. Figure 13. View largeDownload slide Scanning electron microscope (SEM) secondary electron image and energy-dispersive X-ray spectra (EDS) analysis of Fe sulphides from the JLWS and QLK sections. Figure 14. View largeDownload slide Scanning electron microscope (SEM) secondary electron image and energy-dispersive X-ray spectra (EDS) analysis of specimens from the QLK section. Detrital magnetite is indicated by the EDS in QLK4–7. Mineral changes both from pyrite to magnetite and the reverse are indicated (QLK3–8b, 4–7) by the EDS. Reaction between gypsum and Fe oxides is indicated by the EDS (QLK3–8a). Figure 14. View largeDownload slide Scanning electron microscope (SEM) secondary electron image and energy-dispersive X-ray spectra (EDS) analysis of specimens from the QLK section. Detrital magnetite is indicated by the EDS in QLK4–7. Mineral changes both from pyrite to magnetite and the reverse are indicated (QLK3–8b, 4–7) by the EDS. Reaction between gypsum and Fe oxides is indicated by the EDS (QLK3–8a). No greigite was recognized, but some pyrrhotite grains were indicated by the EDS analysis and by their shape (Fig. 15, JLWE5–5). The magnetite has three different forms: detrital (Fig. 14, QLK4–7), framboidal or botryoidal aggregates (Fig. 15, JLWE18–1, SXRJ15–5), and a thin coating on some framboidal pyrites (Fig. 14, QLK3–8b). The individual magnetite crystals that comprise the framboids and aggregates are usually <1 μm in diameter and the framboids themselves are mainly <10 μm. Except for the detrital magnetite, all the other forms of magnetite occur in cavities or fractures (Figs 14 and 15), which suggests that they formed during late diagenesis. A clear replacement process of framboidal pyrite by magnetite is revealed by the EDS analysis (Fig. 14, QLK3–8b), as reported by Suk et al. (1990a). Much previous research (McCabe et al.1983; Elmore et al.1987, 2006; Suk et al.1990a,b, 1991, 1993; Sun & Jackson 1994; Weil & Van der Voo 2002; Font et al.2006; Zechmeister et al.2012) has documented the occurrence of framboidal magnetites in remagnetized carbonate rocks. Figure 15. View largeDownload slide Scanning electron microscope (SEM) secondary electron image and energy-dispersive X-ray spectra (EDS) analysis of specimens from the JLWE and SXRJ sections. (A) pyrrhotite; (B) framboidal magnetite in a cavity; (C,D) magnetite aggregates distributed along a crack, from the SXRJ section. Figure 15. View largeDownload slide Scanning electron microscope (SEM) secondary electron image and energy-dispersive X-ray spectra (EDS) analysis of specimens from the JLWE and SXRJ sections. (A) pyrrhotite; (B) framboidal magnetite in a cavity; (C,D) magnetite aggregates distributed along a crack, from the SXRJ section. There is also abundant gypsum in the samples (Fig. 14, QLK3–8a). Several spherical or framboidal grains contain Fe and Ti close to the gypsum indicating a reaction between the Fe oxides and the gypsum by thermochemical sulphate reduction (TSR), which may have produced the pyrrhotite and pyrite (Zhu et al.2015). 5 DISCUSSION 5.1 Acquisition time of different components Due to the similarity between the local present field and the LTC direction (Tables 1–3), we interpret it as a recent viscous remanent magnetization. As to the ITC, palaeopoles calculated from these sections are 65.6°N, 203.5°E, dp/dm = 2.6°/4.1°; 70.7°N, 207.6°E, dp/dm = 1.3°/2.0°; and 68.3°N, 188.4°E, dp/dm = 2.7°/4°, for the JLWE, JLWS and SXRJ sections, respectively (Fig. 16, Tables 4–6). These palaeopoles overlap with the Early-Middle Jurassic poles of South China (Fig. 16), which suggests that the ITC is a Jurassic remagnetization remanence. In addition, a fold test (McElhinny 1964; McFadden 1990) on the JLWE, JLWS, QLK and SXRJ sections is negative both at the 95 per cent and 99 per cent probability levels, which confirms that the ITC is a post Yanshanian magnetization. Figure 16. View largeDownload slide Palaeomagnetic poles calculated from ITC and HTC components from all three sampled sections. The existing Neoproterozoic and Phanerozoic palaeopoles (grey shading) from the South China block are shown for comparison. Details of the poles before the late Devonian are listed in Table 7; for other poles see table 1 in Huang et al. (2008). Figure 16. View largeDownload slide Palaeomagnetic poles calculated from ITC and HTC components from all three sampled sections. The existing Neoproterozoic and Phanerozoic palaeopoles (grey shading) from the South China block are shown for comparison. Details of the poles before the late Devonian are listed in Table 7; for other poles see table 1 in Huang et al. (2008). The HTC from the JLWE and SXRJ sections are similar to each other (Figs 6 and 9, Tables 1 and 3) although they have an age gap of 80–50 Ma (Condon et al.2005). A fold test cannot be conducted on both of sections, since the sites are all distributed on the east limb of the fold (Fig. 1). However, the folding time in the study area was during the Yanshanian orogeny (Xiao et al.1965), and the directions of the HTC of JLWE and SXRJ sections, both in geographic and stratigraphic coordinates, are different from the post-late Triassic results of South China (Figs 11a, b, e and f). This suggests that the HTC of the JLWE and SXRJ sections should be of pre-Jurassic age. In addition, palaeopoles for the JLWE and SXRJ sections plot close to the Silurian poles of South China block (Fig. 16), which suggests that they are Silurian remagnetization remanence. The HTC from the QLK and JLWS sections are rather complicated. Sites JLWS1, 6, 9 and QLK2, 3 are distributed along great circles (Figs 12a and b), which may reflect the effects of a remagnetization process at these sites during or after the Silurian. The remaining sites of the QLK and JLWS sections are distributed on both limbs of the fold (Fig. 1). The fold test (McElhinny 1964) conducted on these results is positive at both the 95 per cent and 99 per cent confidence levels. In addition, the palaeopole derived from the HTC of the JLWS section in stratigraphic coordinates differs from any previously reported poles of the South China block (Fig. 16), although the strata relationship of JLWS, JLWE and SXRJ resembles a ‘sandwich’, with JLWS in middle (Fig. 2). Based on these observations and the end point distribution on the great circle (Fig. 12), we suggest that the results from JLWS and QLK4 are a primary remanence which was acquired during or shortly after the deposition of Doushantuo Formation Member 3. 5.2 Silurian remagnetization A pulse of Silurian remagnetization, which affected the Ediacaran-Early Palaeozoic strata across the South China block, has long been suspected (Wu et al.1988; Macouin et al.2004; Yang et al.2004; Zhang et al.2013, 2015; Jing et al.2015). Lin et al. (1985a) first reported an east-west shallow inclination component for lower Cambrian rocks from Hubei, Zhejiang and Yunnan provinces (Yang et al.2004, in their fig. 7). Subsequently, many studies of late Precambrian rocks from Hubei, Hunan and Jiangxi provinces (Zhang 1994; Macouin et al.2004) have yielded results that are almost identical to the Silurian results reported by Opdyke et al. (1987) and Huang et al. (2000). Recently, Zhang et al. (2015) reported a palaeopole from the Doushantuo Formation Member 3, which is different from the known reliable Phanerozoic poles of the South China block (Fig. 16, Table 7) and passes the reversal test (B class). Rock magnetic and thermal demagnetization results suggest it is carried by SD magnetite (Zhang et al.2015). Therefore, this result may be the primary remanence of the Doushantuo Formation. However, as the authors observed, there is a large and rapid change in inclination values through the sampling section (Zhang et al.2015, in their fig. 2). In their study, they proposed four possible explanations, including differing degrees of unresolved secondary contamination of the HTC remanence. However, they dismissed this possibility because their results passed the reversal test. The results of Zhang et al. (2015), together with our HTC results from the JLWS and QLK sections, are plotted in Figs 12(a) and (b). Clearly, many of the results of Zhang et al. (2015) are distributed along either Fit 1 or Fit 2 great circles. This distribution characteristic suggests that some of their results may not only be affected by the Silurian remagnetization but also by a younger remagnetization (Jing et al.2015). The HTC components from the JLWE and SXRJ sections that resemble the Silurian results (Fig. 16) are of normal polarity with a maximum unblocking temperature of 540 °C, indicative of a magnetite carrier. The presence of magnetite is confirmed by the rock magnetic and SEM results (Figs 3–5, 14 and 15). The Doushantuo Formation experienced moderate burial temperatures (180–200 °C), as indicated by bitumen reflectance data and the calibration curve for illitization from the lower-middle Member 2 and Member 4 (Derkowski et al.2013). The minimum burial temperature for the maximum laboratory unblocking temperature range of 530–540 °C should have exceeded 420 °C according to magnetite relaxation-time–unblocking-temperature curves (Dunlop et al.2000). This indicates that this Silurian-similar magnetization carried by magnetite is a chemical remanent magnetization (CRM), rather than a thermoviscous magnetization. The SEM observations demonstrate that the authigenic magnetite grains occur in cavities and cracks in the JLWE and SXRJ sections (Fig. 15), which also indicate that their magnetization is a CRM and may be related to the activity of hydrothermal fluids (Suk et al.1990a). This explanation is also supported by previous studies (Jackson et al.1988; McCabe & Elmore 1989; Jackson 1990; Lu et al.1990), which did not find thermoviscous magnetization characteristics in carbonate rocks that had undergone burial temperatures below 250 °C. Interestingly, from the carbonate clumped isotope thermometry and clay mineralogical evidence, Bristow et al. (2011) suggested that the highly 13C-depleted carbonate cements in the cap dolostone (Member 1 of the Doushantuo Formation) were formed from hydrothermal fluids. Furthermore, based on clay mineral transformations (illitization of dioctahedral smectite/conversion of saponite to chlorite via corrensite) and the degree of organic maturation, Derkowski et al. (2013) postulated the occurrence of hydrothermal fluid activity in the underlying, relatively permeable Nantuo Formation, in the lower Doushantuo Formation (Member 2) and in the uppermost part of the Doushantuo Formation (Member 4). K-Ar dating of different size fractions of fine-grained illite of the Doushantuo Formation (Member 4) shale in the Jiulongwan section, provides an age estimate of ∼430 Ma for the hydrothermal fluid activity (Derkowski et al.2013), which is coincident with the age of the Silurian-similar magnetization. Based on these observations, we suggest that pervasive Silurian remagnetization has affected the Ediacaran age strata in the Three Gorges area, and that this remagnetization event was induced by hydrothermal fluid activity. Because illite is present in the Doushantuo Formation (Bristow et al.2011; Derkowski et al.2013), the illitization of the smectite may be an alternative mechanism for this Silurian remagnetization (McCabe & Elmore 1989; Katz et al.1998, 2000; Woods et al.2002; Tohver et al.2008). However, clay mineral analysis of the Doushantuo Formation (Bristow et al.2011; Derkowski et al.2013) suggested that illitization/chloritization only occurs in the lower 30 m of Member 2 and Member 4 of the Doushantuo Formation. This illitization/chloritization was induced by high-temperature hydrothermal fluids (Derkowski et al.2013, in their fig. 3). In addition, if illitization of the smectite was the main mechanism for the Silurian remagnetization event in this case, then the upper part (ca. 50 m) of Member 2 and Member 3 of the Doushantuo Formation should not record this Silurian remagnetization. However, all the results from Member 2 of the Doushantuo Formation resemble the Silurian results. This evidence indicates that the illitization cannot be the dominant mechanism of the Silurian remagnetization in the study area. It is noteworthy that during the Silurian–Devonian, the Ediacaran–lower Cambrian source rocks reached their peak in oil generation in the study area and in the central Yangtze area (Peters et al.1996; Zhu et al.2015). The accumulated oil was preserved in the source beds (Zhu et al.2015). This type of oil distribution in the Yangtze Block (e.g. in Sichuan and Hubei) may have induced a pervasive Silurian-Devonian remagnetization event that only affected the Ediacaran-lower Cambrian strata, but did not affect the Ordovician strata. The results of geochemical and isotopic analyses of the Ediacaran–lower Cambrian strata (Sawaki et al.2010) also suggest that the fluid was derived from the maturation of organic matter (Elmore et al.2001). The results of previous palaeomagnetic studies of the Ediacaran-lower Cambrian strata in the Yangtze block are consistent with this remagnetization pattern (Lin et al.1985a; Zhang 1994; Macouin et al.2004). Palaeomagnetic studies of the Palaeozoic sedimentary rocks in Yichang (Lin 1983; Lin et al.1985b; Kent et al.1987) confirmed that the Silurian-Devonian remagnetization did not affect the Ordovician age strata in this region. Thus, it seems clear that the HTC component, identified at the JLWE and SXRJ sections, is a CRM produced by hydrocarbon accumulation during the middle to late Silurian period. 5.3 Jurassic remagnetization The post-folding ITC component is likely a Middle Jurassic remagnetization residing in pyrrhotite. The possible remagnetization mechanism for the pyrrhotite is probably related to TSR, and this hypothesis is confirmed by the palaeomagnetic, rock magnetic and SEM observation results. By modelling the burial process, Zhu et al. (2015) proposed that the TSR may initially have occurred in the Jurassic and ceased in the Cretaceous. Palaeopoles of the ITC overlap with the Early-Middle Jurassic poles of South China (Fig. 16). The presence of the gypsum and its reaction with the Fe oxides (Fig. 14, QLK3–8a), in addition to the results of geochemical and isotope analyses of the H2S (Zhu et al.2005; Hao et al.2008; Ma et al.2008; Tian et al.2008; Liu et al.2013) and the temperature conditions, all favour a TSR (Zhu et al.2010; Zhu et al.2015). 5.4 Primary remanence of Doushantuo Formation Member 3 The distribution of the hydrothermal fluids, now revealed as hydrocarbons, indicates that the Doushantuo Formation Member 3 was relatively cool in the JLWS section (Derkowski et al.2013; Loyd et al.2015) and may only have been affected by minor hydrocarbon migration. The rock magnetic results suggest that magnetite is the main remanence carrier (Figs 3 and 4), and no framboidal iron oxide spherules are present. In addition, clay minerals, which can induce remagnetization in carbonate rocks (McCabe & Elmore 1989; Katz et al.1998, 2000; Woods et al.2002; Tohver et al.2008), are rare in Member 3 (<5 per cent, Bristow et al.2009). All this evidence, together with the palaeopoles comparison (Fig. 16), suggests that the HTC component from the JLWS section is a primary remanence. Thus, the directions of magnetization at the end of Fit 2, which plot away from those of the Silurian age (in Figs 12c and d and Table 2), including JLWS2, 3, 4, 5, 10, 11, QLK4, and ZG1, 2, 3, 4, JLW3, 5, 7, 12 reported by Zhang et al. (2015), were used to calculate a mean direction (Dg = 72.9°, Ig = 52.8°, kg = 43.9, α95 = 5.8° in situ; and Ds = 73.3°, Is = 59.5°, ks = 156.6, α95 = 3.1° after tilt correction; Table 2). The fold test of these directions (McElhinny 1964; McFadden 1990) is positive at both the 95 per cent and 99 per cent levels, and a positive reversal test (McFadden & McElhinny 1990) further suggests that it is a primary remanent magnetization. The palaeopole calculated from this primary remanent magnetization (DSTM3: 31.3°N, 169.8°E, dp/dm = 3.5°/4.7° after tilt correction, Fig. 16, Tables 2 and 7) is different from all known poles from rocks of the South China block. Table 2. HTC palaeomagnetic data for the Doushantuo Fm Member 3 of Jiulongwan section (JLWS), Yichang (111.069°E, 30.81°N). Results from the Qinglinkou section (QLK) and Zhang et al. (2015) are also listed. Site  Component  N/n  Dg  Ig  Ds  Is  k  α95    HTC  12  74.5  45.4  78.5  39.9  84.4  4.8  JLWS02  HTC  10  60.2  52  66.7  47.6  127.9  4.3  JLWS03  HTC  12  68.6  56  71.3  52.5  54.8  5.9  JLWS04  HTC  9  72.7  54.6  74.8  51  51.2  7.3  JLWS05  HTC  8  70.7  57.6  73.2  54  18.6  13.2  JLWS06a  HTC  7  226.9  −43.4  233  −39.3  154.1  4.9  JLWS07a  HTC  3  329.6  51.8  338  57.6  158.1  9.8  JLWS09a  HTC  6  53.3  43.8  42.7  40.8  57  8.9  JLWS10  HTC  8  85.7  48  74.6  51.5  17.4  13.7  JLWS11  HTC  13  88.8  44.4  79.1  48.5  23.2  8.8  QLK02a  HTC  4  51.2  35.8  27.6  66.4  67  11.3  QLK03a  HTC  11  80.5  6.7  84.8  34.4  24.6  9.4  QLK04  HTC  7  70.9  19.3  76.5  46  60.5  7.8  JLW03b  HTC  5  251.9  −51.8  255.7  −44.4  50.6  7.9  JLW05b  HTC  18  63.3  47.2  69.2  41.8  65.1  4.3  JLW07b  HTC  10  248.7  −50.9  252.3  −44.2  27.8  7.4  JLW12b  HTC  15  71.2  51.3  75.3  44.2  127.9  3.4  ZG01b  HTC  6  78.6  57.6  75.9  51.9  40.2  9.8  ZG02b  HTC  6  75.6  64.7  72.6  58.9  124.6  5.5  ZG03b  HTC  10  64.2  67.0  63.0  61.1  129.8  4.0  ZG04b  HTC  14  82.2  64.1  78.1  58.5  125.7  3.4  Mean of HTC components      15/151  72.9  52.8      43.9  5.8            73.3  59.5  156.6  3.1  Palaeopole (DSTM3)      29.5°N, 177.6°E, dp/dm = 5.5°/8.0°(in situ)          31.3°N, 169.8°E, dp/dm = 3.5°/4.7°(tilt corrected)  Mean of LTC components                  11/111  4.1  41.5  6.9  40  ks/kg  = 110.8/602.8  1.9  Palaeopole        82.2°N, 262.4°E, dp/dm = 1.4°/2.3°(in situ)  Site  Component  N/n  Dg  Ig  Ds  Is  k  α95    HTC  12  74.5  45.4  78.5  39.9  84.4  4.8  JLWS02  HTC  10  60.2  52  66.7  47.6  127.9  4.3  JLWS03  HTC  12  68.6  56  71.3  52.5  54.8  5.9  JLWS04  HTC  9  72.7  54.6  74.8  51  51.2  7.3  JLWS05  HTC  8  70.7  57.6  73.2  54  18.6  13.2  JLWS06a  HTC  7  226.9  −43.4  233  −39.3  154.1  4.9  JLWS07a  HTC  3  329.6  51.8  338  57.6  158.1  9.8  JLWS09a  HTC  6  53.3  43.8  42.7  40.8  57  8.9  JLWS10  HTC  8  85.7  48  74.6  51.5  17.4  13.7  JLWS11  HTC  13  88.8  44.4  79.1  48.5  23.2  8.8  QLK02a  HTC  4  51.2  35.8  27.6  66.4  67  11.3  QLK03a  HTC  11  80.5  6.7  84.8  34.4  24.6  9.4  QLK04  HTC  7  70.9  19.3  76.5  46  60.5  7.8  JLW03b  HTC  5  251.9  −51.8  255.7  −44.4  50.6  7.9  JLW05b  HTC  18  63.3  47.2  69.2  41.8  65.1  4.3  JLW07b  HTC  10  248.7  −50.9  252.3  −44.2  27.8  7.4  JLW12b  HTC  15  71.2  51.3  75.3  44.2  127.9  3.4  ZG01b  HTC  6  78.6  57.6  75.9  51.9  40.2  9.8  ZG02b  HTC  6  75.6  64.7  72.6  58.9  124.6  5.5  ZG03b  HTC  10  64.2  67.0  63.0  61.1  129.8  4.0  ZG04b  HTC  14  82.2  64.1  78.1  58.5  125.7  3.4  Mean of HTC components      15/151  72.9  52.8      43.9  5.8            73.3  59.5  156.6  3.1  Palaeopole (DSTM3)      29.5°N, 177.6°E, dp/dm = 5.5°/8.0°(in situ)          31.3°N, 169.8°E, dp/dm = 3.5°/4.7°(tilt corrected)  Mean of LTC components                  11/111  4.1  41.5  6.9  40  ks/kg  = 110.8/602.8  1.9  Palaeopole        82.2°N, 262.4°E, dp/dm = 1.4°/2.3°(in situ)  aResults excluded in calculating the mean of HTC. bHT components from Zhang et al. (2015) used to calculate the primary remanent magnetization direction. Other abbreviations are the same as Table 1. View Large Table 3. HTC palaeomagnetic data for the Dengying formation of Sanxiarenjia section (SXRJ), Yichang (111.165°E, 30.817°N). Site  Component  N/n  Dg  Ig  Ds  Is  k  α95  SXRJ10  HTC  8  92.5  29  96.1  21.8  60.3  7.2  SXRJ11  HTC  10  81.5  46.6  89  43.7  26.2  9.6  SXRJ12  HTC  8  85.2  36.1  90.6  30.7  98.3  5.6  SXRJ13,14  HTC  7  76.7  39.3  85.3  34.8  50.6  8.6  SXRJ15  HTC  10  79.1  37.1  86.3  30.1  109.4  4.6  SXRJ16  HTC  8  79.1  36.9  87.4  30.4  199.7  3.9  SXRJ17  HTC  11  87.1  34.2  91.7  27.7  111  4.4  SXRJ18  HTC  8  83.3  32.6  88.5  26.4  42.4  8.6  Mean of HTC components                  8/70  83.2  36.6      148.4  4.6            89.5  30.7  129.2  4.9  Palaeopole        15.9°N, 186.6°E, dp/dm = 3.1°/5.4° (in situ)          8.8°N, 187.1°E, dp/dm = 3.0°/5.5° (tilt corrected)  Mean of LTC components                  7/52  357.5  52.6  7.3  61.7  kg/ks = 206.7/148.2  3.9  Palaeopole        86.8°N, 70.0°E, dp/dm = 3.7°/5.4° (in situ)  Site  Component  N/n  Dg  Ig  Ds  Is  k  α95  SXRJ10  HTC  8  92.5  29  96.1  21.8  60.3  7.2  SXRJ11  HTC  10  81.5  46.6  89  43.7  26.2  9.6  SXRJ12  HTC  8  85.2  36.1  90.6  30.7  98.3  5.6  SXRJ13,14  HTC  7  76.7  39.3  85.3  34.8  50.6  8.6  SXRJ15  HTC  10  79.1  37.1  86.3  30.1  109.4  4.6  SXRJ16  HTC  8  79.1  36.9  87.4  30.4  199.7  3.9  SXRJ17  HTC  11  87.1  34.2  91.7  27.7  111  4.4  SXRJ18  HTC  8  83.3  32.6  88.5  26.4  42.4  8.6  Mean of HTC components                  8/70  83.2  36.6      148.4  4.6            89.5  30.7  129.2  4.9  Palaeopole        15.9°N, 186.6°E, dp/dm = 3.1°/5.4° (in situ)          8.8°N, 187.1°E, dp/dm = 3.0°/5.5° (tilt corrected)  Mean of LTC components                  7/52  357.5  52.6  7.3  61.7  kg/ks = 206.7/148.2  3.9  Palaeopole        86.8°N, 70.0°E, dp/dm = 3.7°/5.4° (in situ)  Abbreviations are the same as Table 1. View Large Table 4. ITC palaeomagnetic data for the Doushantuo Fm Member 2 of Jiulongwan section (JLWE), Yichang (111.069°E, 30.81°N). Site  Component  N/n  Dg  Ig  Ds  Is  k  α95  JLWE01  ITC  5  22.2  23.4  25.2  25.8  132.7  6.7  JLWE10  ITC  9  23.9  44.3  34.7  44.2  419.1  2.5  JLWE11  ITC  14  26.2  43.4  37.6  46.0  117.6  3.7  JLWE12  ITC  9  34.8  53.9  34.8  53.2  100.0  5.2  JLWE13  ITC  9  31.2  46.3  35.4  45.5  252.5  3.2  JLWE14  ITC  4  21.6  42.3  35.1  47.9  999.9  2.6  JLWE15  ITC  9  26.0  45.8  31.9  49.6  133.9  4.5  JLWE16  ITC  9  24.8  49.4  32.5  50.5  285.2  3.1  JLWE17  ITC  5  23.4  46.7  32.4  48.7  375.3  4.0  JLWE2,3  ITC  7  28.6  45.6  37.0  45.4  52.0  8.4  JLWE4,5  ITC  7  38.3  35.0  43.9  33.1  44.8  9.1  JLWE6,7  ITC  10  28.0  40.5  35.3  41.0  34.5  8.3  JLWE8,9  ITC  6  23.5  49.0  32.4  48.7  150.1  5.5  Mean of ITC components      12/98  27.6  45.3  35.5  46.2  ks/kg = 200.3/184.3  3.2  Palaeopole        65.6°N, 203.5°E, dp/dm = 2.6°/4.1°(in situ)  Site  Component  N/n  Dg  Ig  Ds  Is  k  α95  JLWE01  ITC  5  22.2  23.4  25.2  25.8  132.7  6.7  JLWE10  ITC  9  23.9  44.3  34.7  44.2  419.1  2.5  JLWE11  ITC  14  26.2  43.4  37.6  46.0  117.6  3.7  JLWE12  ITC  9  34.8  53.9  34.8  53.2  100.0  5.2  JLWE13  ITC  9  31.2  46.3  35.4  45.5  252.5  3.2  JLWE14  ITC  4  21.6  42.3  35.1  47.9  999.9  2.6  JLWE15  ITC  9  26.0  45.8  31.9  49.6  133.9  4.5  JLWE16  ITC  9  24.8  49.4  32.5  50.5  285.2  3.1  JLWE17  ITC  5  23.4  46.7  32.4  48.7  375.3  4.0  JLWE2,3  ITC  7  28.6  45.6  37.0  45.4  52.0  8.4  JLWE4,5  ITC  7  38.3  35.0  43.9  33.1  44.8  9.1  JLWE6,7  ITC  10  28.0  40.5  35.3  41.0  34.5  8.3  JLWE8,9  ITC  6  23.5  49.0  32.4  48.7  150.1  5.5  Mean of ITC components      12/98  27.6  45.3  35.5  46.2  ks/kg = 200.3/184.3  3.2  Palaeopole        65.6°N, 203.5°E, dp/dm = 2.6°/4.1°(in situ)  Abbreviations are the same as Table 1. View Large Table 5. ITC palaeomagnetic data for the Doushantuo Fm Member 3 of Jiulongwan section (JLWS), Yichang (111.069°E, 30.81°N). Site  Component  N/n  Dg  Ig  Ds  Is  k  α95  JLWS01  ITC  13  22.2  43.1  28.7  43.1  317.8  2.3  JLWS02  ITC  11  21.3  43.2  27.9  43.3  864.3  1.6  JLWS03  ITC  13  18.9  43.5  22.5  42.6  146.6  3.4  JLWS04  ITC  14  22.1  44.5  24.4  45.3  78.5  4.5  JLWS05  ITC  8  21.6  46.7  25.6  45.7  55.8  7.5  JLWS06  ITC  8  16.6  44.6  24.5  44.3  999.9  1.8  JLWS07  ITC  10  18.8  43.5  26.3  43.0  528.7  2.1  JLWS08  ITC  8  24.0  47.3  35.8  46.4  243.3  3.6  JLWS09  ITC  10  23.4  44.1  15.9  36.0  212.4  3.3  JLWS10  ITC  11  23.6  49.1  15.7  42.6  127.1  4.1  JLWS11  ITC  16  25.6  48.5  17.6  42.3  298.4  2.1  Mean of ITC components                  11/122  21.6  45.3  23.9  43.3  ks/kg  = 245/789.2  1.6  Palaeopole        70.7°N, 207.6°E, dp/dm = 1.3°/2.0°(in situ)  QLK02  ITC  7  17.9  44.7  335.7  55.2  151.1  4.9  QLK03  ITC  13  26.6  46.8  350.0  66.1  43.4  6.4  QLK04  ITC  9  16.4  49.3  337.9  64.9  105.6  5.0  Site  Component  N/n  Dg  Ig  Ds  Is  k  α95  JLWS01  ITC  13  22.2  43.1  28.7  43.1  317.8  2.3  JLWS02  ITC  11  21.3  43.2  27.9  43.3  864.3  1.6  JLWS03  ITC  13  18.9  43.5  22.5  42.6  146.6  3.4  JLWS04  ITC  14  22.1  44.5  24.4  45.3  78.5  4.5  JLWS05  ITC  8  21.6  46.7  25.6  45.7  55.8  7.5  JLWS06  ITC  8  16.6  44.6  24.5  44.3  999.9  1.8  JLWS07  ITC  10  18.8  43.5  26.3  43.0  528.7  2.1  JLWS08  ITC  8  24.0  47.3  35.8  46.4  243.3  3.6  JLWS09  ITC  10  23.4  44.1  15.9  36.0  212.4  3.3  JLWS10  ITC  11  23.6  49.1  15.7  42.6  127.1  4.1  JLWS11  ITC  16  25.6  48.5  17.6  42.3  298.4  2.1  Mean of ITC components                  11/122  21.6  45.3  23.9  43.3  ks/kg  = 245/789.2  1.6  Palaeopole        70.7°N, 207.6°E, dp/dm = 1.3°/2.0°(in situ)  QLK02  ITC  7  17.9  44.7  335.7  55.2  151.1  4.9  QLK03  ITC  13  26.6  46.8  350.0  66.1  43.4  6.4  QLK04  ITC  9  16.4  49.3  337.9  64.9  105.6  5.0  Abbreviations are the same as Table 1. View Large Table 6. ITC palaeomagnetic data for the Dengying formation of Sanxiarenjia section (SXRJ), Yichang (111.165°E, 30.817°N). Site  Component  N/n  Dg  Ig  Ds  Is  k  α95  SXRJ10  ITC  5  32.6  54.8  48.3  56.3  278.8  4.6  SXRJ11  ITC  3  18.7  53.6  27.8  58.9  239.5  8.0  SXRJ12  ITC  7  28.6  50.7  40.3  53.9  119.9  5.5  SXRJ16  ITC  7  25.3  52.9  44.5  58.0  156.2  4.8  SXRJ17  ITC  4  24.3  53.2  37.1  56.2  272.8  5.6  SXRJ18  ITC  6  23.9  49.1  36.8  53.3  158.4  5.3  Mean of ITC components                  6/32  25.5  52.5  39.2  56.3  kg/ks = 533.4/332  2.9  Palaeopole        68.3°N, 188.4°E, dp/dm = 2.7°/4° (in situ)  Site  Component  N/n  Dg  Ig  Ds  Is  k  α95  SXRJ10  ITC  5  32.6  54.8  48.3  56.3  278.8  4.6  SXRJ11  ITC  3  18.7  53.6  27.8  58.9  239.5  8.0  SXRJ12  ITC  7  28.6  50.7  40.3  53.9  119.9  5.5  SXRJ16  ITC  7  25.3  52.9  44.5  58.0  156.2  4.8  SXRJ17  ITC  4  24.3  53.2  37.1  56.2  272.8  5.6  SXRJ18  ITC  6  23.9  49.1  36.8  53.3  158.4  5.3  Mean of ITC components                  6/32  25.5  52.5  39.2  56.3  kg/ks = 533.4/332  2.9  Palaeopole        68.3°N, 188.4°E, dp/dm = 2.7°/4° (in situ)  Abbreviations are the same as Table 1. View Large Table 7. Known palaeopoles of the South China block from the Neoproterozoic to the Devonian. Pole name  Formation  Age (dating method)  1  2  3  4  5  6  7  Q  Plat (N)  Plong (E)  A95  References  Xiaofeng Dyke  Xiaofeng Dyke  802 ± 10 Ma (SHRIMP U-Pb zircon)  y  y  y  n  y  y  y  6  13.5  91  11  Li et al. (2004)  Liantuo  Liantuo Fm  720 ± 10 Ma (SIMS-U-Pb zircon)  y  y  y  n  y  y  y  6  13.2  155.2  5.3  Jing et al. (2015)                              Lan et al. (2015)  Nantuo  Nantuo Fm  636.3 ± 4.9 Ma (SHRIMP U-Pb zircon)  y  y  y  y  y  y  y  7  9.3  165.9  4.3  Zhang et al. (2008b, 2013)  DST  Doushantuo Fm  635–550 Ma (TIMS U-Pb zircon)  n  y  y  y  y  n  n  4  0.6  196.9  7.1  Macouin et al. (2004)                              Condon et al. (2005)  DS3  Doushantuo Fm  595 ± 22 Ma (Re–Os isochron age)  y  y  y  n  y  y  y  6  25.9  185.5  6.7  Zhang et al. (2015)                              Zhu et al. (2013)  DSTM3  Doushantuo Fm  595 ± 22 Ma (Re–Os isochron age)  n  y  y  y  y  y  y  6  31.3  169.8  4.1  This study                              Zhu et al. (2013)  Є2  Douposi Fm  510 ± 11 Ma (stratigraphy and fossils)  n  y  y  y  y  y  y  6  −51.3  166  6.8  Yang et al. (2004)  O3  Pagoda Fm  458 ± 6 Ma (stratigraphy and fossils)  n  y  y  y  y  y  y  6  −45.8  191.3  3.2  Han et al. (2015)  Sx  Xiushan Fm  430 ± 2 Ma (stratigraphy and fossils)  n  y  y  y  y  n  y  5  4.9  194.7  5.6  Opdyke et al. (1987)  Sc1  Rongxi, Huixingshao, Niugundang Fm  430 ± 7 Ma (stratigraphy and fossils)  n  y  y  y  y  n  y  5  14.9  196.1  5.1  Huang et al. (2000)  D1–2  Longdongshui Fm  419–382 Ma (International Chronostratigraphic Chart, 2016/04)  n  y  y  n  y  y  y  5  36.1  231.4  12.1  Zhang et al. (2001)    Jipao Fm et al.                            Pole name  Formation  Age (dating method)  1  2  3  4  5  6  7  Q  Plat (N)  Plong (E)  A95  References  Xiaofeng Dyke  Xiaofeng Dyke  802 ± 10 Ma (SHRIMP U-Pb zircon)  y  y  y  n  y  y  y  6  13.5  91  11  Li et al. (2004)  Liantuo  Liantuo Fm  720 ± 10 Ma (SIMS-U-Pb zircon)  y  y  y  n  y  y  y  6  13.2  155.2  5.3  Jing et al. (2015)                              Lan et al. (2015)  Nantuo  Nantuo Fm  636.3 ± 4.9 Ma (SHRIMP U-Pb zircon)  y  y  y  y  y  y  y  7  9.3  165.9  4.3  Zhang et al. (2008b, 2013)  DST  Doushantuo Fm  635–550 Ma (TIMS U-Pb zircon)  n  y  y  y  y  n  n  4  0.6  196.9  7.1  Macouin et al. (2004)                              Condon et al. (2005)  DS3  Doushantuo Fm  595 ± 22 Ma (Re–Os isochron age)  y  y  y  n  y  y  y  6  25.9  185.5  6.7  Zhang et al. (2015)                              Zhu et al. (2013)  DSTM3  Doushantuo Fm  595 ± 22 Ma (Re–Os isochron age)  n  y  y  y  y  y  y  6  31.3  169.8  4.1  This study                              Zhu et al. (2013)  Є2  Douposi Fm  510 ± 11 Ma (stratigraphy and fossils)  n  y  y  y  y  y  y  6  −51.3  166  6.8  Yang et al. (2004)  O3  Pagoda Fm  458 ± 6 Ma (stratigraphy and fossils)  n  y  y  y  y  y  y  6  −45.8  191.3  3.2  Han et al. (2015)  Sx  Xiushan Fm  430 ± 2 Ma (stratigraphy and fossils)  n  y  y  y  y  n  y  5  4.9  194.7  5.6  Opdyke et al. (1987)  Sc1  Rongxi, Huixingshao, Niugundang Fm  430 ± 7 Ma (stratigraphy and fossils)  n  y  y  y  y  n  y  5  14.9  196.1  5.1  Huang et al. (2000)  D1–2  Longdongshui Fm  419–382 Ma (International Chronostratigraphic Chart, 2016/04)  n  y  y  n  y  y  y  5  36.1  231.4  12.1  Zhang et al. (2001)    Jipao Fm et al.                            1–7 and Q: reliability criteria of Van der Voo (1990). 1–Well-dated rock age; 2–sufficient number of samples; 3–adequate demagnetization; 4–field tests; 5–structural control and tectonic coherence with craton or block involved; 6–presence of reversals; 7–no resemblance to palaeopoles of younger age. Plat: latitude of the palaeopoles; Plong: longitude of the palaeopoles; A95: radius of circle with 95 per cent confidence about the palaeopole. View Large Our study demonstrates that a combination of rock magnetic, palaeomagnetic and SEM analyses can be used to determine the different magnetization characteristics of different carbonate lithologies. The application of this approach provides a new perspective on the resolution of the issue of multi-phase magnetization in South China. The proposed mechanism of multiple magnetizations may explain the pervasive Silurian and Jurassic-Cretaceous remagnetization events in the stable Yangtze block. In addition, the new DSTM3 pole can be used as a reliable Ediacaran pole of South China to constrain its position in the reconstruction of supercontinents (e.g. Rodinia or Gondwana). 6 CONCLUSIONS Detailed palaeomagnetic and rock magnetic studies reveal multiple magnetizations in the Ediacaran strata in South China. These secondary remagnetizations seem to be characteristic of the Ediacaran and lower Palaeozoic strata across a large region of the Central Yangtze. Several lines of evidence, including magnetic studies, SEM observations, clay mineral analysis and geochemical and isotope analysis, suggest that the remagnetization events were induced by multiple generations of oil and gas formation in the Ediacaran and Lower Cambrian strata. In addition, our study demonstrates that reliable palaeomagnetic results can be obtained from the relatively massive Ediacaran limestone of the South China block at localities where only minor hydrocarbon circulation has occurred and that future work should focus on strata of low permeability and with minimal organic matter content. This new Ediacaran palaeopole was proved by the fold and reversal tests, and petrographic observation, indicating a reliable pole for the South China block. The pole can be used for the reconstruction of South China in Rodinian and Gondwanan continents. Acknowledgements We gratefully acknowledge the careful and constructive comments of Andrew Biggin, Eduard Petrovsk and an anonymous reviewer who considerably improved the manuscript. Meanwhile, we are particularly grateful to Dr Pengju Liu for guidance in the field. This work was funded by the Chinese National Natural Science Foundation (Grant No. 41230208). REFERENCES Abrajevitch A., Van der Voo R., 2010. Incompatible Ediacaran paleomagnetic directions suggest an equatorial geomagnetic dipole hypothesis, Earth planet. Sci. Lett. , 293, 164– 170. https://doi.org/10.1016/j.epsl.2010.02.038 Google Scholar CrossRef Search ADS   An Z., Jiang G., Tong J., Tian L., Ye Q., Song H., Song H., 2015a. Stratigraphic position of the Ediacaran Miaohe biota and its constrains on the age of the upper Doushantuo δ 13 C anomaly in the Yangtze Gorges area, South China, Precambrian Res. , 271, 243– 253. https://doi.org/10.1016/j.precamres.2015.10.007 Google Scholar CrossRef Search ADS   An Z., Tong J., Ye Q., Tian L., Song H., Zhao X., 2015b. Neoproterozoic stratigraphic sequence and sedimentary evolution at Qinglinkou section, east Yangtze Gorges area, J. China Univ. Geosci.-Earth Sci. , 39( 7), 795– 806. Bristow T.F., Kennedy M.J., Derkowski A., Droser M.L., Jiang G., Creaser R.A., 2009. Mineralogical constraints on the paleoenvironments of the Ediacaran Doushantuo Formation, Proc. Natl. Acad. Sci. USA , 106, 13 190–13 195. https://doi.org/10.1073/pnas.0901080106 Google Scholar CrossRef Search ADS   Bristow T.F., Bonifacie M., Derkowski A., Eiler J.M., Grotzinger J.P., 2011. A hydrothermal origin for isotopically anomalous cap dolostone cements from south China, Nature , 474, 68– 71. https://doi.org/10.1038/nature10096 Google Scholar CrossRef Search ADS PubMed  Cogné J.P., 2003. PaleoMac: A Macintosh (TM) application for treating paleomagnetic data and making plate reconstructions, Geochem. Geophys. Geosyst. , 4( 1), 1– 8. Google Scholar CrossRef Search ADS   Condon D., Zhu M., Bowring S., Wang W., Yang A., Jin Y., 2005. U-Pb ages from the Neoproterozoic Doushantuo Formation, China, Science , 308, 95– 98. https://doi.org/10.1126/science.1107765 Google Scholar CrossRef Search ADS PubMed  Derkowski A., Bristow T.F., Wampler J., Środoń J., Marynowski L., Elliott W.C., Chamberlain C.P., 2013. Hydrothermal alteration of the Ediacaran Doushantuo Formation in the Yangtze Gorges area (South China), Geochim. Cosmochim. Acta , 107, 279– 298. https://doi.org/10.1016/j.gca.2013.01.015 Google Scholar CrossRef Search ADS   Dunlop D., Özdemir Ö., Clark D., Schmidt P., 2000. Time–temperature relations for the remagnetization of pyrrhotite (Fe 7 S 8) and their use in estimating paleotemperatures, Earth planet. Sci. Lett. , 176, 107– 116. https://doi.org/10.1016/S0012-821X(99)00309-X Google Scholar CrossRef Search ADS   Elmore R.D., Engel M.H., Crawford L., Nick K., Imbus S., Sofer Z., 1987. Evidence for a relationship between hydrocarbons and authigenic magnetite, Nature , 325, 428– 430. https://doi.org/10.1038/325428a0 Google Scholar CrossRef Search ADS   Elmore D.R., Kelley J., Evans M., Lewchuk M.T., 2001. Remagnetization and orogenic fluids: testing the hypothesis in the central Appalachians, Geophys. J. Int. , 144, 568– 576. https://doi.org/10.1111/j.1365-246X.2001.00349.x Google Scholar CrossRef Search ADS   Elmore R.D., Foucher J.L.-E., Evans M., Lewchuk M., Cox E., 2006. Remagnetization of the Tonoloway Formation and the Helderberg Group in the Central Appalachians: testing the origin of syntilting magnetizations, Geophys. J. Int. , 166, 1062– 1076. https://doi.org/10.1111/j.1365-246X.2006.02875.x Google Scholar CrossRef Search ADS   Evans D.A., 1998. True polar wander, a supercontinental legacy, Earth planet. Sci. Lett. , 157, 1– 8. https://doi.org/10.1016/S0012-821X(98)00031-4 Google Scholar CrossRef Search ADS   Evans D.A., 2003. True polar wander and supercontinents, Tectonophysics , 362, 303– 320. https://doi.org/10.1016/S0040-1951(02)000642-X Google Scholar CrossRef Search ADS   Fisher R., 1953. Dispersion on a sphere, Proc. R. Soc. A , 217, 295– 305. https://doi.org/10.1098/rspa.1953.0064 Google Scholar CrossRef Search ADS   Font E., Trindade R., Nédélec A., 2006. Remagnetization in bituminous limestones of the Neoproterozoic Araras Group (Amazon craton): Hydrocarbon maturation, burial diagenesis, or both?, J. geophys. Res. , 111( B06204), 1– 17. https://doi.org/10.1029/2005JB004106 Google Scholar PubMed  Halls H.C., Lovette A., Hamilton M., Söderlund U., 2015. A paleomagnetic and U–Pb geochronology study of the western end of the Grenville dyke swarm: Rapid changes in paleomagnetic field direction at ca. 585 Ma related to polarity reversals?, Precambrian Res. , 257, 137– 166. https://doi.org/10.1016/j.precamres.2014.11.029 Google Scholar CrossRef Search ADS   Han Z., Yang Z., Tong Y., Jing X., 2015. New paleomagnetic results from Late Ordovician rocks of the Yangtze Block, South China, and their paleogeographic implications, J. geophys. Res. , 120, 4759– 4772. https://doi.org/10.1002/2015JB012005 Google Scholar CrossRef Search ADS   Hao F., Guo T., Zhu Y., Cai X., Zou H., Li P., 2008. Evidence for multiple stages of oil cracking and thermochemical sulfate reduction in the Puguang gas field, Sichuan Basin, China, AAPG Bull. , 92, 611– 637. https://doi.org/10.1306/01210807090 Google Scholar CrossRef Search ADS   Huang B., Zhou Y., Zhu R., 2008. Discussions on Phanerozoic evolution and formation of continental China, based on paleomagnetic studies, Earth Sci. Frontiers , 15, 348– 359. https://doi.org/10.1016/S1872-5791(08)60039-1 Google Scholar CrossRef Search ADS   Huang K.N., Opdyke N.D., Zhu R.X., 2000. Further paleomagnetic results from the Silurian of the Yangtze Block and their implications, Earth planet. Sci. Lett. , 175, 191– 202. https://doi.org/10.1016/S0012-821X(99)00302-7 Google Scholar CrossRef Search ADS   Jackson M., 1990. Diagenetic sources of stable remanence in remagnetized Paleozoic cratonic carbonates: A rock magnetic study, J. geophys. Res. , 95, 2753– 2761. https://doi.org/10.1029/JB095iB03p02753 Google Scholar CrossRef Search ADS   Jackson M., McCabe C., Ballard M.M., Van der Voo R., 1988. Magnetite authigenesis and diagenetic paleotemperatures across the northern Appalachian basin, Geology , 16, 592– 595. https://doi.org/10.1130/0091-7613(1988)0160592:MAADPA2.3.CO;2 Google Scholar CrossRef Search ADS   Jiang G., Shi X., Zhang S., Wang Y., Xiao S., 2011. Stratigraphy and paleogeography of the Ediacaran Doushantuo Formation (ca. 635–551 Ma) in South China, Gondwana Res. , 19, 831– 849. https://doi.org/10.1016/j.gr.2011.01.006 Google Scholar CrossRef Search ADS   Jing X.-Q., Yang Z., Tong Y., Han Z., 2015. A revised paleomagnetic pole from the mid-Neoproterozoic Liantuo Formation in the Yangtze block and its paleogeographic implications, Precambrian Res. , 268, 194– 211. https://doi.org/10.1016/j.precamres.2015.07.007 Google Scholar CrossRef Search ADS   Katz B., Elmore R., Cogoini M., Ferry S., 1998. Widespread chemical remagnetization: Orogenic fluids or burial diagenesis of clays?, Geology , 26, 603– 606. https://doi.org/10.1130/0091-7613(1998)0260603:WCROFO2.3.CO;2 Google Scholar CrossRef Search ADS   Katz B., Elmore R.D., Cogoini M., Engel M.H., Ferry S., 2000. Associations between burial diagenesis of smectite, chemical remagnetization, and magnetite authigenesis in the Vocontian trough, SE France, J. geophys. Res. , 105, 851– 868. https://doi.org/10.1029/1999JB900309 Google Scholar CrossRef Search ADS   Kent D.V., Zeng X., Wen Y.Z., Opdyke N.D., 1987. Widespread late Mesozoic to Recent remagnetization of Paleozoic and lower Triassic sedimentary rocks from South China, Tectonophysics , 139, 133– 143. https://doi.org/10.1016/0040-1951(87)90202-2 Google Scholar CrossRef Search ADS   Kirschvink J.L., 1980. The least-squares line and plane and the analysis of paleomagnetic data, Geophys. J. R. astr. Soc. , 62, 699– 718. https://doi.org/10.1111/j.1365-246X.1980.tb02601.x Google Scholar CrossRef Search ADS   Klein R., Salminen J., Mertanen S., 2015. Baltica during the Ediacaran and Cambrian: A paleomagnetic study of Hailuoto sediments in Finland, Precambrian Res. , 267, 94– 105. https://doi.org/10.1016/j.precamres.2015.06.005 Google Scholar CrossRef Search ADS   Kruiver P.P., Dekkers M.J., Heslop D., 2001. Quantification of magnetic coercivity components by the analysis of acquisition curves of isothermal remanent magnetisation, Earth planet. Sci. Lett. , 189, 269– 276. https://doi.org/10.1016/S0012-821X(01)00367-3 Google Scholar CrossRef Search ADS   Lan Z., Li X.-H., Zhu M., Zhang Q., Li Q.-L., 2015. Revisiting the Liantuo Formation in Yangtze Block, South China: SIMS U–Pb zircon age constraints and regional and global significance, Precambrian Res. , 263, 123– 141. https://doi.org/10.1016/j.precamres.2015.03.012 Google Scholar CrossRef Search ADS   Li Z., Evans D., Zhang S., 2004. A 90 spin on Rodinia: possible causal links between the Neoproterozoic supercontinent, superplume, true polar wander and low-latitude glaciation, Earth planet. Sci. Lett. , 220, 409– 421. https://doi.org/10.1016/S0012-821X(04)00064-0 Google Scholar CrossRef Search ADS   Lin J.-L., 1983. The Apparent Polar Wander Paths for the North and South China Blocks , Universyty of California Santa Barbara. Lin J.-L., Fuller M., Zhang W.-Y., 1985a. Paleogeography of the North and South China blocks during the Cambrian, J. Geodyn. , 2, 91– 114. https://doi.org/10.1016/0264-3707(85)90003-1 Google Scholar CrossRef Search ADS   Lin J.L., Fuller M., Zhang W.Y., 1985b. Preliminary Phanerozoic polar wander paths for the North and South China blocks, Nature , 313, 444– 449. https://doi.org/10.1038/313444a0 Google Scholar CrossRef Search ADS   Liu Q.et al.  , 2013. TSR versus non-TSR processes and their impact on gas geochemistry and carbon stable isotopes in Carboniferous, Permian and Lower Triassic marine carbonate gas reservoirs in the Eastern Sichuan Basin, China, Geochim. Cosmochim. Acta , 100, 96– 115. https://doi.org/10.1016/j.gca.2012.09.039 Google Scholar CrossRef Search ADS   Lowrie W., 1990. Identification of ferromagnetic minerals in a rock by coercivity and unblocking temperature properties, Geophys. Res. Lett. , 17, 159– 162. https://doi.org/10.1029/GL017i002p00159 Google Scholar CrossRef Search ADS   Loyd S.J., Corsetti F.A., Eagle R.A., Hagadorn J.W., Shen Y., Zhang X., Bonifacie M., Tripati A.K., 2015. Evolution of Neoproterozoic Wonoka–Shuram Anomaly-aged carbonates: evidence from clumped isotope paleothermometry, Precambrian Res. , 264, 179– 191. https://doi.org/10.1016/j.precamres.2015.04.010 Google Scholar CrossRef Search ADS   Lu G., Marshak S., Kent D.V., 1990. Characteristics of magnetic carriers responsible for Late Paleozoic remagnetization in carbonate strata of the mid-continent, USA, Earth planet. Sci. Lett. , 99, 351– 361. https://doi.org/10.1016/0012-821X(90)90139-O Google Scholar CrossRef Search ADS   Ma Y., Zhang S., Guo T., Zhu G., Cai X., Li M., 2008. Petroleum geology of the Puguang sour gas field in the Sichuan Basin, SW China, Mar. Pet. Geol. , 25, 357– 370. https://doi.org/10.1016/j.marpetgeo.2008.01.010 Google Scholar CrossRef Search ADS   Macouin M., Besse J., Ader M., Gilder S., Yang Z., Sun Z., Agrinier P., 2004. Combined paleomagnetic and isotopic data from the Doushantuo carbonates, South China: implications for the ‘snowball Earth’ hypothesis, Earth planet. Sci. Lett. , 224, 387– 398. https://doi.org/10.1016/j.epsl.2004.05.015 Google Scholar CrossRef Search ADS   Macouin M., Ader M., Moreau M.-G., Poitou C., Yang Z., Sun Z., 2012. Deciphering the impact of diagenesis overprint on negative δ 13 C excursions using rock magnetism: case study of Ediacaran carbonates, Yangjiaping section, South China, Earth planet. Sci. Lett. , 351, 281– 294. https://doi.org/10.1016/j.epsl.2012.06.057 Google Scholar CrossRef Search ADS   Manning E.B., Elmore R., 2015. An integrated paleomagnetic, rock magnetic, and geochemical study of the Marcellus shale in the Valley and Ridge province in Pennsylvania and West Virginia, J. geophys. Res. , 120, 705– 724. https://doi.org/10.1002/2014JB011418 Google Scholar CrossRef Search ADS   McCabe C., Elmore R.D., 1989. The occurrence and origin of Late Paleozoic remagnetization in the sedimentary rocks of North America, Rev. Geophys. , 27, 471– 494. https://doi.org/10.1029/RG027i004p00471 Google Scholar CrossRef Search ADS   McCabe C., Van der Voo R., Peacor D.R., Scotese C.R., Freeman R., 1983. Diagenetic magnetite carries ancient yet secondary remanence in some Paleozoic sedimentary carbonates, Geology , 11, 221– 223. https://doi.org/10.1130/0091-7613(1983)11221:DMCAYS2.0.CO;2 Google Scholar CrossRef Search ADS   McCausland P.J., Van der Voo R., Hall C.M., 2007. Circum-Iapetus paleogeography of the Precambrian–Cambrian transition with a new paleomagnetic constraint from Laurentia, Precambrian Res. , 156, 125– 152. https://doi.org/10.1016/j.precamres.2007.03.004 Google Scholar CrossRef Search ADS   McCausland P.J., Hankard F., Van der Voo R., Hall C.M., 2011. Ediacaran paleogeography of Laurentia: Paleomagnetism and 40Ar–39Ar geochronology of the 583 Ma Baie des Moutons syenite, Quebec, Precambrian Res. , 187, 58– 78. https://doi.org/10.1016/j.precamres.2011.02.004 Google Scholar CrossRef Search ADS   McElhinny M.W., 1964. Statistical significance of the fold test in palaeomagnetism, Geophys. J. R. astr. Soc. , 8, 338– 340. https://doi.org/10.1111/j.1365-246X.1964.tb06300.x Google Scholar CrossRef Search ADS   McFadden K.A., Huang J., Chu X., Jiang G., Kaufman A.J., Zhou C., Yuan X., Xiao S., 2008. Pulsed oxidation and biological evolution in the Ediacaran Doushantuo Formation, Proc. Natl. Acad. Sci. USA , 105, 3197– 3202. https://doi.org/10.1073/pnas.0708336105 Google Scholar CrossRef Search ADS   McFadden P., 1990. A new fold test for palaeomagnetic studies, Geophys. J. Int. , 103, 163– 169. https://doi.org/10.1111/j.1365-246X.1990.tb01761.x Google Scholar CrossRef Search ADS   McFadden P., McElhinny M., 1990. Classification of the reversal test in palaeomagnetism, Geophys. J. Int. , 103, 725– 729. https://doi.org/10.1111/j.1365-246X.1990.tb05683.x Google Scholar CrossRef Search ADS   Meert J.G., 2014. Ediacaran–Early Ordovician paleomagnetism of Baltica: a review, Gondwana Res. , 25, 159– 169. https://doi.org/10.1016/j.gr.2013.02.003 Google Scholar CrossRef Search ADS   Meert J.G., Van der Voo R., Powell C.M., Li Z.-X., McElhinny M.W., Chen Z., Symons D., 1993. A plate-tectonic speed limit?, Nature , 363, 216– 217. https://doi.org/10.1038/363216a0 Google Scholar CrossRef Search ADS   Opdyke N.D., Huang K., Xu G., Zhang W., Kent D.V., 1987. Paleomagnetic results from the Silurian of the Yangtze paraplatform, Tectonophysics , 139, 123– 132. https://doi.org/10.1016/0040-1951(87)90201-0 Google Scholar CrossRef Search ADS   Peters C., Dekkers M., 2003. Selected room temperature magnetic parameters as a function of mineralogy, concentration and grain size, Phys. Chem. Earth, Parts A/B/C , 28, 659– 667. https://doi.org/10.1016/S1474-7065(03)00120-7 Google Scholar CrossRef Search ADS   Peters K.E., Cunningham A.E., Walters C.C., Jigang J., Zhaoan F., 1996. Petroleum systems in the Jiangling-Dangyang area, Jianghan basin, China, Organic Geochem. , 24, 1035– 1060. https://doi.org/10.1016/S0146-6380(96)00080-0 Google Scholar CrossRef Search ADS   Roberts A.P., Chang L., Rowan C.J., Horng C.S., Florindo F., 2011. Magnetic properties of sedimentary greigite (Fe3S4): an update, Rev. Geophys. , 49( 1), 1– 46. https://doi.org/10.1029/2010RG000336 Google Scholar CrossRef Search ADS   Sawaki Y.et al.  , 2010. The Ediacaran radiogenic Sr isotope excursion in the Doushantuo Formation in the three Gorges area, South China, Precambrian Res. , 176, 46– 64. https://doi.org/10.1016/j.precamres.2009.10.006 Google Scholar CrossRef Search ADS   Schmidt P.W., 2014. A review of Precambrian palaeomagnetism of Australia: palaeogeography, supercontinents, glaciations and true polar wander, Gondwana Res. , 25, 1164– 1185. https://doi.org/10.1016/j.gr.2013.12.007 Google Scholar CrossRef Search ADS   Schmidt P.W., Williams G.E., 2010. Ediacaran palaeomagnetism and apparent polar wander path for Australia: no large true polar wander, Geophys. J. Int. , 182, 711– 726. https://doi.org/10.1111/j.1365-246X.2010.04652.x Google Scholar CrossRef Search ADS   Suk D., Peacor D., Van der Voo R., 1990a. Replacement of pyrite framboids by magnetite in limestone and implications for palaeomagnetism, Nature , 345, 611– 613. https://doi.org/10.1038/345611a0 Google Scholar CrossRef Search ADS   Suk D., Van Der Voo R., Peacor D.R., 1990b. Scanning and transmission electron microscope observations of magnetite and other iron phases in Ordovician carbonates from east Tennessee, J. geophys. Res. , 95, 12327– 12336. https://doi.org/10.1029/JB095iB08p12327 Google Scholar CrossRef Search ADS   Suk D., Van der Voo R., Peacor D.R., 1991. SEM/STEM observations of magnetite in carbonates of eastern North America: evidence for chemical remagnettzation during the Alleghenian Orogeny, Geophys. Res. Lett. , 18, 939– 942. https://doi.org/10.1029/91GL00916 Google Scholar CrossRef Search ADS   Suk D., Van Der Voo R., Peacor D.R., 1993. Origin of magnetite responsible for remagnetization of early Paleozoic limestones of New York State, J. geophys. Res. , 98, 419– 434. https://doi.org/10.1029/92JB01323 Google Scholar CrossRef Search ADS   Sun W.-W., Jackson M., 1994. Scanning electron microscopy and rock magnetic studies of magnetic carriers in remagnetized early Paleozoic carbonates from Missouri, J. geophys. Res., 99, 2935–2942. Tian H., Xiao X., Wilkins R.W., Tang Y., 2008. New insights into the volume and pressure changes during the thermal cracking of oil to gas in reservoirs: Implications for the in-situ accumulation of gas cracked from oils, AAPG Bull. , 92, 181– 200. https://doi.org/10.1306/09210706140 Google Scholar CrossRef Search ADS   Tohver E., Weil A., Solum J., Hall C., 2008. Direct dating of carbonate remagnetization by 40 Ar/39 Ar analysis of the smectite–illite transformation, Earth planet. Sci. Lett. , 274, 524– 530. https://doi.org/10.1016/j.epsl.2008.08.002 Google Scholar CrossRef Search ADS   Van der Voo R., 1990. The reliability of paleomagnetic data, Tectonophysics , 184, 1– 9. https://doi.org/10.1016/0040-1951(90)90116-P Google Scholar CrossRef Search ADS   Vernhet E., Reijmer J.J., 2010. Sedimentary evolution of the Ediacaran Yangtze platform shelf (Hubei and Hunan provinces, Central China), Sedimentary Geol. , 225, 99– 115. https://doi.org/10.1016/j.sedgeo.2010.01.005 Google Scholar CrossRef Search ADS   Wang L., Shi X., Jiang G., 2012. Pyrite morphology and redox fluctuations recorded in the Ediacaran Doushantuo Formation, Palaeogeog. Palaeoclimat. Palaeoecol. , 333, 218– 227. https://doi.org/10.1016/j.palaeo.2012.03.033 Google Scholar CrossRef Search ADS   Weil A.B., Van der Voo R., 2002. Insights into the mechanism for orogen-related carbonate remagnetization from growth of authigenic Fe-oxide: a scanning electron microscopy and rock magnetic study of Devonian carbonates from northern Spain, J. geophys. Res. , 107( B4), 1– 14. https://doi.org/10.1029/2001JB000200 Google Scholar CrossRef Search ADS   Woods S.D., Elmore R., Engel M., 2002. Paleomagnetic dating of the smectite-to-illite conversion: Testing the hypothesis in Jurassic sedimentary rocks, Skye, Scotland, J. geophys. Res. , 107( B5), 1– 10. https://doi.org/10.1029/2000JB000053 Google Scholar CrossRef Search ADS   Wu F., Van der Voo R., Liang Q., 1988. Reconnaissance magnetostratigraphy of the Precambrian-Cambrian boundary section at Meishucun, southwest China, Cuadernos de geología ibérica, J. Iberian Geol. , 12, 205– 222. Xiao D., Luo G., Bo Z., 1965. Primary disscussion on the Neo-structure activities near Yichang, West of Hubei Province, J. Nanjing Univ. (Nat. Sci.) , 9( 1), 133– 150. Yang Z., Sun Z., Yang T., Pei J., 2004. A long connection (750–380 Ma) between South China and Australia: paleomagnetic constraints, Earth planet. Sci. Lett. , 220, 423– 434. https://doi.org/10.1016/S0012-821X(04)00053-6 Google Scholar CrossRef Search ADS   Zechmeister M., Pannalal S., Elmore R., 2012. A multidisciplinary investigation of multiple remagnetizations within the Southern Canadian Cordillera, SW Alberta and SE British Columbia, Geol. Soc., London, Spec. Publ. , 371, SP371. 311, 123– 144. https://doi.org/10.1144/SP371.11 Google Scholar CrossRef Search ADS   Zhang H., 1994. Paleomagnetism of Proterozoic rock in Hunan and Guangxi provinces, South China, in Proceedings of the Annual Meeting of the Chinese Geophysics Society , Seismology Press, Beijing, pp. 333– 334. (in Chinese). Zhang H., Zhang W., Li P., 1983. Palaeomagnetism of the Sinian System of Eastern Yangzi Gorges in China, Bull. Tianjin Inst. Miner. Res. , 6, 57– 68. Zhang Q.-R., Li X.-H., Feng L.-J., Huang J., Song B., 2008a. A new age constraint on the onset of the Neoproterozoic glaciations in the Yangtze Platform, South China, J. Geol. , 116, 423– 429. https://doi.org/10.1086/589312 Google Scholar CrossRef Search ADS   Zhang S., Zhu H., Meng X., 2001. New paleomagnetic results from the Devonian-Carboniferous successions in the Southern Yangtze Block and their paleogeographic implications, Acta Geol. Sin. , 75, 303– 313. Zhang S., Jiang G., Han Y., 2008b. The age of the Nantuo Formation and Nantuo glaciation in South China, Terra Nova , 20, 289– 294. https://doi.org/10.1111/j.1365-3121.2008.00819.x Google Scholar CrossRef Search ADS   Zhang S.et al.  , 2013. Paleomagnetism of the late Cryogenian Nantuo Formation and paleogeographic implications for the South China Block, J. Asian Earth Sci. , 72, 164– 177. https://doi.org/10.1016/j.jseaes.2012.11.022 Google Scholar CrossRef Search ADS   Zhang S.et al.  , 2015. New paleomagnetic results from the Ediacaran Doushantuo Formation in South China and their paleogeographic implications, Precambrian Res. , 259, 130– 142. https://doi.org/10.1016/j.precamres.2014.09.018 Google Scholar CrossRef Search ADS   Zhao Z., Xing Y., Ma G., Chen Y., 1985. Biostratigraphy of the Yangtze Gorge Area,(1) Sinian , Geological Publishing House, Beijing, 1, 143. Zhao Z.-Q.et al.  , 1988. The Sinian System of Hubei , China University of Geosciences Press, Wuhan, p. 205. Zhou C., Tucker R., Xiao S., Peng Z., Yuan X., Chen Z., 2004. New constraints on the ages of Neoproterozoic glaciations in South China, Geology , 32, 437– 440. https://doi.org/10.1130/G20286.1 Google Scholar CrossRef Search ADS   Zhu B., Becker H., Jiang S.-Y., Pi D.-H., Fischer-Gödde M., Yang J.-H., 2013. Re–Os geochronology of black shales from the Neoproterozoic Doushantuo Formation, Yangtze platform, South China, Precambrian Res. , 225, 67– 76. https://doi.org/10.1016/j.precamres.2012.02.002 Google Scholar CrossRef Search ADS   Zhu G., Zhang S., Liang Y., Dai J., Li J., 2005. Isotopic evidence of TSR origin for natural gas bearing high H2S contents within the Feixianguan Formation of the northeastern Sichuan Basin, southwestern China, Sci. China D , 48, 1960– 1971. https://doi.org/10.1360/082004-147 Google Scholar CrossRef Search ADS   Zhu G., Zhang S., Huang H., Liu Q., Yang Z., Zhang J., Wu T., Huang Y., 2010. Induced H 2 S formation during steam injection recovery process of heavy oil from the Liaohe Basin, NE China, J. Pet. Sci. Eng. , 71, 30– 36. https://doi.org/10.1016/j.petrol.2010.01.002 Google Scholar CrossRef Search ADS   Zhu G., Wang T., Xie Z., Xie B., Liu K., 2015. Giant gas discovery in the Precambrian deeply buried reservoirs in the Sichuan Basin, China: Implications for gas exploration in old cratonic basins, Precambrian Res. , 262, 45– 66. https://doi.org/10.1016/j.precamres.2015.02.023 Google Scholar CrossRef Search ADS   Zhu M., Zhang J., Yang A., 2007. Integrated Ediacaran (Sinian) chronostratigraphy of South China, Palaeogeog. Palaeoclimat. Palaeoecol. , 254, 7– 61. https://doi.org/10.1016/j.palaeo.2007.03.025 Google Scholar CrossRef Search ADS   SUPPORTING INFORMATION Supplementary data are available at GJI online. Table S1. Results of the coercivity based on IRM component analysis (Kruiver et al. 2001). In the columns, the three distinguished components and their contributions are shown: saturation IRM (SIRM), the field at which half of the SIRM is reached (B1/2) and the dispersion parameter (DP) represents one standard deviation. Please note: Oxford University Press is not responsible for the content or functionality of any supporting materials supplied by the authors. Any queries (other than missing material) should be directed to the corresponding author for the paper. © The Authors 2017. Published by Oxford University Press on behalf of The Royal Astronomical Society. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Geophysical Journal International Oxford University Press

Identification of multiple magnetizations of the Ediacaran strata in South China

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The Royal Astronomical Society
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© The Authors 2017. Published by Oxford University Press on behalf of The Royal Astronomical Society.
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0956-540X
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1365-246X
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10.1093/gji/ggx396
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Abstract

Abstract A suspected Silurian remagnetization of the Ediacaran strata of South China was proposed decades ago by many researchers, but, there has been no systematic study of its causes and mechanisms. In this study, we investigate the multiphase remagnetization processes that affected the Ediacaran strata and the possible mechanisms of these remagnetization events. We conducted detailed palaeomagnetic, rock magnetic and scanning electron microscope (SEM) studies of samples from the Ediacaran strata in the Jiulongwan (JLWE, JLWS), Qinglinkou (QLK) and Sanxiarenjia (SXRJ) sections in the Three Gorges Area, South China. After removal of a recent viscous remanent magnetization below 150 °C, an intermediate temperature component (ITC; Dg = 27.6°, Ig = 45.3°, N = 12 sites, kg = 184.3, α95 = 3.2° for JLWE; Dg = 22°, Ig = 45.3°, N = 11 sites, kg = 789.2, α95 = 1.6° for JLWS; and Dg = 25.5°, Ig = 52.5°, N = 6 sites, kg = 533.4, α95 = 2.9° for SXRJ) was removed below 300 °C which coincides with the Jurassic results from South China, suggesting a pervasive Jurassic remagnetization. In addition, a high temperature component (HTC; Ds = 84.8°, Is = 19.2°, N = 9 sites, ks = 35.5, α95 = 8.8° for JLWE; Ds = 74.1°, Is = 49.4°, N = 7 sites, ks = 218.9, α95 = 4.1° for JLWS; and Ds = 89.5°, Is = 30.7°, N = 8 sites, ks = 129.2, α95 = 4.9° for SXRJ) was isolated between 300 and 480–540 °C. Rock magnetic and SEM studies suggest that the ITC and HTC are carried by pyrrhotite and magnetite, respectively. SEM observations also demonstrate the occurrence of massive authigenic magnetite in cavities or cracks, mineralogical changes from pyrite to Fe oxides, and the reaction between gypsum and Fe oxides. Based on similarities to the Silurian poles of South China, together with the SEM observations, we suggest that the HTC from the JLWE and SXRJ sections is a Silurian age remagnetization. The oxidation of iron sulphides and thermochemical sulphate reduction induced by the multiple generations of oil and gas in the Ediacaran and Cambrian strata are suggested as the main mechanism for remagnetization. Despite the pervasive Silurian remagnetization of the Ediacaran strata, most of the HTC from the thick-bedded dolostone of Doushantuo Formation Member 3 at the JLWS section appears to carry a primary remanence, because its pole differs from other poles of South China and the results pass both the fold and reversal tests. The relatively low-geothermic conditions and the absence of both hydrocarbon and smectite/illite explain why most results from the Doushantuo Member 3 of JLWS section were not affected by the Silurian remagnetization. This new Ediacaran pole supersedes the previous suspected remagnetized poles, which can be used to constrain the palaeoposition of South China both in Rodinia and Gondwana. Asia, Palaeomagnetism, Remagnetization, Rock and mineral magnetism 1 INTRODUCTION Worldwide, the palaeomagnetic data for the Ediacaran interval are complex (McCausland et al.2007, 2011; Abrajevitch & Van der Voo 2010; Schmidt & Williams 2010; Meert 2014; Schmidt 2014; Halls et al.2015; Jing et al.2015; Klein et al.2015). For instance, the contradictory results with almost the same age, obtained from Laurentia and Baltica, place these two cratons either near the equator or at high latitudes during 615–550 Ma (McCausland et al.2007, 2011; Abrajevitch & Van der Voo 2010; Meert 2014; Halls et al.2015; Klein et al.2015). In addition, palaeomagnetic poles from Australia during 625–550 Ma demonstrate a small circle distribution and a strong oscillation (Schmidt & Williams 2010; Jing et al.2015). Different hypotheses have been proposed for these contradictory results, including: rapid plate motions (Meert et al.1993), true polar wander (Evans 1998, 2003), local rotation (for Australia, Schmidt & Williams 2010; Jing et al.2015) and anomalous field behaviour (Abrajevitch & Van der Voo 2010). In contrast to the Ediacaran palaeomagnetic results from other continents, which are usually of dual polarity, passing the fold or baked-contact tests, with a precise age, and with rapid apparent motion, the data from the South China block (Zhang 1994; Macouin et al.2004) are unipolar, and close to the Silurian results of South China. Recently, Zhang et al. (2015) reported a new result from the uppermost 14 m of the Doushantuo Formation Member 3, which differs from the result obtained by Macouin et al. (2004) and is of dual polarity. However, as was noted, there are systematic oscillating trends in the inclinations through their upper polarity zone strata (Zhang et al.2015), implying a possible complex magnetization acquisition process. In fact, several researchers have suspected that a Silurian remagnetization event may have affected the Ediacaran-Cambrian strata across the South China Block (Wu et al.1988; Macouin et al.2004; Yang et al.2004; Zhang et al.2013, 2015; Jing et al.2015). Although many palaeomagnetic studies have been conducted on the Ediacaran strata over the last few decades (Zhang et al.1983; Zhang 1994; Macouin et al.2004, 2012; Zhang et al.2015), there has been no study of the remagnetization processes that affected the Ediacaran strata of South China. Here, we present the results of detailed palaeomagnetic, rock magnetic and scanning electron microscope (SEM) analyses of the Ediacaran strata in the Three Gorges Area, Hubei Province, China. The objective was to detect whether the occurrence of multiple magnetizations on the Ediacaran strata of South China took place and to understand the mechanisms responsible. Eventually, the primary palaeomagnetic direction isolated from these carbonate strata may provide further constraint on the palaeogeographic position of South China both in Rodinia and Gondwana. 2 GEOLOGICAL SETTING AND PALAEOMAGNETIC SAMPLING The study areas are in Yichang, Hubei Province, in the northern part of the Yangtze Block, where typical Precambrian sedimentary sequences have been identified by geological survey (Fig. 1, Regional geological survey team of Hubei Province 1970). Neoproterozoic strata in the Three Gorges Area are well preserved and consist of pre-Cryogenian siliciclastic units (Liantuo Formation) in the lower part, with ages ranging from ∼784 to ∼714 Ma (SIMS U-Pb zircon dating; Lan et al.2015). Cryogenian glacial and interglacial deposits occur in the middle part, including two glacial diamictite intervals (the Chang’an Formation and the Nantuo Formation), separated by interglacial manganese-bearing shale of the Datangpo Formation, dated to between ∼663 and ∼635 Ma (SHRIMP U-Pb zircon dating; Zhou et al.2004; Zhang et al.2008a,b), and Ediacaran mixed carbonate-siliciclastic units (Doushantuo and Dengying Formations) form the top of the sequence. Figure 1. View largeDownload slide (A) Simplified map of South China showing the location of the sampling area. (B) Geological sketch map of the sampling area in Yichang, Hubei Provence. Three sections were sampled: Jiulongwan (including JLWE and JLWS), Qinglinkou (QLK) and Sanxiarenjia (SXRJ). Strike and dip are also shown in the inset. Figure 1. View largeDownload slide (A) Simplified map of South China showing the location of the sampling area. (B) Geological sketch map of the sampling area in Yichang, Hubei Provence. Three sections were sampled: Jiulongwan (including JLWE and JLWS), Qinglinkou (QLK) and Sanxiarenjia (SXRJ). Strike and dip are also shown in the inset. Most of the Doushantuo Formation in the Yangtze platform was deposited on a rimmed carbonate shelf with a shelf margin shoal complex that restricted the shelf lagoon from the open ocean during the Early Ediacaran (Jiang et al.2011). In the Three Gorges Area, the Doushantuo Formation consists of 160–250 m of carbonate and black shale, subdivided into four members (Fig. 2). In the study area, basal Member 1 of the Doushantuo Formation is a 6–10 m thick cap carbonate overlying the glacial diamictite of the Nantuo Formation. Member 2 consists of about 70–80 m of interbedded black shale and shaly limestone with abundant pea-sized chert and less-common phosphatic nodules overlying the cap carbonate. Member 3 consists of ∼60 m of medium- to thick-bedded dolostone intercalated with concretions and layers of chert (in the lower part) and thin-bedded marlstone (in the upper part). Member 4 consists of ∼10 m of thick black shale with scattered thin layers or lenses of dolomite (McFadden et al.2008; Fig. 2). Figure 2. View largeDownload slide Lithostratigraphy of the three sampled sections and sampling layer positions for each section. Figure 2. View largeDownload slide Lithostratigraphy of the three sampled sections and sampling layer positions for each section. The Dengying Formation in the Sanxiarenjia section can be subdivided into three lithostratigraphic members, namely Hamajing, Shibantan, and Baimatuo, in ascending order (Fig. 2; Zhao et al.1985, 1988; Zhu et al.2007). The Hamajing Member is a ∼130 m thick white or pink-white dolostone, characterized by massive intraclastic and oolitic dolomitic grainstone with oncoids. The Shibantan Member is a ca. 80 m thick dark grey thin- to intermediate-bedded, laminated micritic limestone with trace fossils and fragments rich in vendotaenids (algal or bacterial colonies). Cherty laminae occur in the upper part of this member. The Baimatuo Member consists of massive micritic and sparitic white or pink-white dolostones. Cross-stratification begins to occur in the middle part of this member, leading upwards into miarolitic bird-eye structures or boxworks, indicating that the water depth had become shallower (Zhao et al.1988). In the Three Gorges Area, the Doushantuo Formation records the onset of a long interval of carbonate-dominated sedimentation on the Yangtze craton, which was interrupted by uplift and erosion in the late Silurian and then continued after a rifting phase until the Early Triassic (Peters et al.1996; Vernhet & Reijmer 2010). The study area experienced three major tectonic episodes, the Caledonian, the Indosinian and the Yanshanian orogenies, and there were three main periods of oil generation and expulsion in the central Yangtze area (Zhu et al.2015). The Caledonian orogeny only resulted in the absence of the lower Devonian strata, and subsequently the area continued to accumulate marine sediments. However, the transgression was terminated by the Indosinian orogeny, which led to exposure of the strata. During the Yanshanian orogeny, the Ediacaran strata overlying the Huangling Granite suites were folded and formed the nearly north-south-trending Huangling anticline (Xiao et al.1965). Exposures of the Doushantuo Formation in the eastern Three Gorges Area are either not well formed or inaccessible, being either covered by vegetation or in the form of cliffs. Therefore, in 2012 we collected a total of 99 block samples from Member 2 of the Doushantuo Formation in the Jiulongwan section (JLWE) (Figs 1 and 2; 30.81°N, 111.07°E; dip direction: ∼130°) and 92 (9 sites) drill samples from the Dengying Formation Shibantan Member in the Sanxiarenjia section (SXRJ; Figs 1 and 2; 30.82°N, 111.16°E; dip direction: ∼127°). In the laboratory, 2.54-cm-diameter core samples were drilled from the block samples. Subsequently, in 2015, we revisited the Three Gorges Area and obtained 138 drill samples (11 sites) from Member 3 of the Doushantuo Formation in the Jiulongwan section (JLWS; Figs 1 and 2; 30.81°N, 111.07°E; dip direction: ∼90°). To conduct a fold test, we also collected 53 drill samples (4 sites) from Member 2 and Member 3 of the Doushantuo Formation in the Qinglinkou section (QLK; 30.8°N, 110.92°E; Figs 1 and 2; dip direction: ∼240°; An et al.2015a,b). 3 METHODS At the Paleomagnetism Laboratory of Nanjing University, cores were cut into standard cylindrical specimens of 2.3 cm height and 2.54 cm diameter. Selected samples were used for acquisition of isothermal remanent magnetization (IRM) experiments. Fields of up 2.4 T were first imparted to the Z-axis using an ASC IM-10–30 impulse magnetizer and measured using a JR-6A spinner magnetometer housed in a magnetically shielded room. Subsequently, these samples were magnetized sequentially along the Z-, Y-, and X-axes in fields of 2.4, 0.4, and 0.12 T, respectively, and subjected to stepwise thermal demagnetization (Lowrie 1990). Also at the Paleomagnetism Laboratory of Nanjing University, progressive thermal demagnetization of untreated samples from 44 sites was carried out in 14–18 steps with a 50 °C increment for low temperatures (<300 °C) and a 20 or 30 °C increment for high temperatures (>300 °C), up to 530–580 °C. All samples were thermally demagnetized step-by-step using an ASC-TD48 oven. Remanences were measured using a 2 G Enterprises Inc. cryogenic magnetometer (2G-755) housed in a magnetically shielded room. The remanence directions were analysed using the principal component analysis method (Kirschvink 1980) and the mean directions were calculated using Fisher spherical statistics (Fisher 1953). The palaeomagnetic data were analysed using the PMGSC software package of R. Enkin, and graphs were plotted using the PaleoMac program of Cogné (2003). At the Key Paleomagnetic Laboratory of the Chinese Academy of Geological Sciences, Beijing, China, susceptibility–temperature (κ–T) curves were obtained using a KLY4 Kappa bridge (Czech Agico Company). Powder samples were heated from room temperature to 405 °C and then cooled to room temperature in air. One of the samples was heated to 710 °C in an argon atmosphere. At the State Key Laboratory of Mineral Deposits Research of Nanjing University, SEM observations and energy-dispersive X-ray spectra (EDS) analysis were performed on gold-coated rock fragments cut from selected specimens using a Jeol JSM-6490 scanning electron microscope and Oxford INCA energy 350 EDS Analyzer. 4 RESULTS 4.1 Rock magnetic results To identify the potential magnetic carriers in the samples we studied, we selected representative samples from the Doushantuo and the Dengying Formations for rock magnetic studies. Cumulative log Gaussian (CLG) functions analysis was performed on the IRM data for 19 representative specimens from all four sections using the software developed by Kruiver et al. (2001; Fig. 3, Supporting Information Table S1). Five coercivity components are revealed. A very low-coercivity component (1.6–22 mT), constituting ∼15 per cent of the total IRM (Supporting Information Table S1), is present in 14 specimens. Another low coercivity component (30–62 mT; Supporting Information Table S1) is present in all specimens. A medium coercivity component (126–160 mT), constituting 6–34 per cent of the total IRM, is present in 11 specimens (Supporting Information Table S1). A medium to high-coercivity component (251–398 mT), constituting 10–37 per cent of the total IRM, is present in five specimens. In addition, five specimens contained a high coercivity (1548–2089 mT) component. Based on their coercivity ranges (Peters & Dekkers 2003; Manning & Elmore 2015), we interpreted these five coercivity components as maghemite or multidomain magnetite, single domain magnetite, pyrrhotite, hematite and goethite, respectively, from low to high coercivity. Figure 3. View largeDownload slide Examples of coercivity component analysis of IRM in linear acquisition plots (LAP), gradient acquisition plots (GAP) and standardized acquisition plots (SAP), respectively, processed using the cumulative log-Gaussian model (Kruiver et al.2001). The thick red line shows the sum of all the components. Figure 3. View largeDownload slide Examples of coercivity component analysis of IRM in linear acquisition plots (LAP), gradient acquisition plots (GAP) and standardized acquisition plots (SAP), respectively, processed using the cumulative log-Gaussian model (Kruiver et al.2001). The thick red line shows the sum of all the components. In most of the specimens, the results of thermal demagnetization of the tri-axial IRMs show dominance by the low-coercivity component, which has a laboratory unblocking temperature ranging from 530–580 °C (Fig. 4), suggesting the presence of magnetite (Lowrie 1990). A major inflection in the thermal demagnetization decay curves of the low- and medium-coercivity components around 400 °C (Figs 4a and c) suggests the presence of greigite or mineral alteration (Roberts et al.2011). For the specimens with a minor high coercivity component, complete demagnetization did not occur until 680 °C (Figs 4b and c), suggesting the presence of hematite (Lowrie 1990). Figure 4. View largeDownload slide Results of stepwise thermal demagnetization of a three-component IRM imparted to representative samples. Orthogonal IRMs were imparted in fields of 0.12 T (circles), 0.4 T (triangles) and 2.4 T (squares). Figure 4. View largeDownload slide Results of stepwise thermal demagnetization of a three-component IRM imparted to representative samples. Orthogonal IRMs were imparted in fields of 0.12 T (circles), 0.4 T (triangles) and 2.4 T (squares). Thermomagnetic curves (κ–T) of representative specimens (Figs 5a and b) show a marked decrease between 260 and 305 °C, with a minimum susceptibility value reached at ∼305 °C. This minimum is followed by a slow increase until ∼400 °C, followed by a dramatic susceptibility increase. In addition, a specimen was heated to 710 °C (Figs 5c and d), and in this case, a minimum susceptibility value reached at ∼310 °C, followed by a small peak at ∼350 °C (Fig. 5d). Subsequently, a slow increase in susceptibility occurred at ∼400 °C, a dramatic increase until ∼450 °C, and then the heating curve reached a minimum at ∼580 °C. These results indicate the presence of pyrrhotite and magnetite in the samples. Figure 5. View largeDownload slide Thermomagnetic curves (κ–T) of the representative specimens. Figure 5. View largeDownload slide Thermomagnetic curves (κ–T) of the representative specimens. 4.2 Palaeomagnetic results In most of the samples, three natural remanent magnetization (NRM) components were isolated. First, a low-temperature component (LTC) was removed below 150 °C (Figs 6–9). The mean directions of this component in all sections are similar to the local present field direction (Tables 1–3). Figure 6. View largeDownload slide Orthogonal vector projection of the results of thermal demagnetization of NRM for representative samples from the JLWE section in geographic coordinates. Figure 6. View largeDownload slide Orthogonal vector projection of the results of thermal demagnetization of NRM for representative samples from the JLWE section in geographic coordinates. Table 1. HTC palaeomagnetic data for the Doushantuo Fm Member 2 of the Jiulongwan section (JLWE), Yichang (111.069°E, 30.81°N). Site  Component  N/n  Dg  Ig  Ds  Is  k  α95  JLWE10  HTC  9  74.2  26.1  77.3  17.1  33.5  9  JLWE11  HTC  17  58.2  37.1  66.2  33.1  88.8  3.8  JLWE12  HTC  7  94.3  39.4  93.4  38.9  59.9  7.9  JLWE13  HTC  10  81.2  23.7  82.5  20.3  42.5  7.5  JLWE14  HTC  6  93  26.1  97.1  16.2  62.3  8.6  JLWE15  HTC  10  88.6  13.8  89.8  10.4  58.6  6.4  JLWE16  HTC  12  90.2  16.6  91.2  11.7  67.5  5.3  JLWE17  HTC  7  86  14.7  87  9.2  79.3  6.8  JLWE6–7  HTC  6  75.5  19.5  77.1  13.4  117.5  6.2  Mean of HTC components                  9/84  82.6  24.5      35.3  8.8            84.8  19.2  35.2  8.8  Palaeopole        12.8°N, 193.6°E, dp/dm = 5.1°/9.4°(in situ)          9.5°N, 195.2°E, dp/dm = 4.8°/9.2°(tilt corrected)  Mean of LTC components                  13/133  358.5  55.9  8.4  59.4  ks/kg = 356/282  2.5  Palaeopole        84.2°N, 99°E, dp/dm = 2.6°/3.6°(in situ)  Site  Component  N/n  Dg  Ig  Ds  Is  k  α95  JLWE10  HTC  9  74.2  26.1  77.3  17.1  33.5  9  JLWE11  HTC  17  58.2  37.1  66.2  33.1  88.8  3.8  JLWE12  HTC  7  94.3  39.4  93.4  38.9  59.9  7.9  JLWE13  HTC  10  81.2  23.7  82.5  20.3  42.5  7.5  JLWE14  HTC  6  93  26.1  97.1  16.2  62.3  8.6  JLWE15  HTC  10  88.6  13.8  89.8  10.4  58.6  6.4  JLWE16  HTC  12  90.2  16.6  91.2  11.7  67.5  5.3  JLWE17  HTC  7  86  14.7  87  9.2  79.3  6.8  JLWE6–7  HTC  6  75.5  19.5  77.1  13.4  117.5  6.2  Mean of HTC components                  9/84  82.6  24.5      35.3  8.8            84.8  19.2  35.2  8.8  Palaeopole        12.8°N, 193.6°E, dp/dm = 5.1°/9.4°(in situ)          9.5°N, 195.2°E, dp/dm = 4.8°/9.2°(tilt corrected)  Mean of LTC components                  13/133  358.5  55.9  8.4  59.4  ks/kg = 356/282  2.5  Palaeopole        84.2°N, 99°E, dp/dm = 2.6°/3.6°(in situ)  N: number of sites for statistical analysis; n: number of samples for statistical analysis; Dg, Ig, Ds, Is: declination and inclination in geographic and stratigraphic coordinates; k: Fisher precision parameter of the mean; α95: confidence of the mean direction; dp/dm: semi-axes of elliptical error around the pole at a probability of 95 per cent. View Large Second, an intermediate-temperature component (ITC), with a northeast declination and moderate positive inclination (Figs 6–9, Tables 4–6), was demagnetized mainly between 200 and 300 °C (Figs 6–9). The mean ITC directions for the JLWE, JLWS and SXRJ sections, in geographic coordinates, are Dg = 27.6°, Ig = 45.3°, N = 12 sites, kg = 184.3, α95 = 3.2° (Fig. 10a, Table 4); Dg = 22°, Ig = 45.3°, N = 11 sites, kg = 789.2, α95 = 1.6° (Fig. 10c, Table 5); and Dg = 25.5°, Ig = 52.5°, N = 6 sites, kg = 533.4, α95 = 2.9° (Fig. 10e, Table 6), respectively. Figure 7. View largeDownload slide Orthogonal vector projection of the results of thermal demagnetization of NRM for representative samples from the JLWS section in geographic coordinates. Figure 7. View largeDownload slide Orthogonal vector projection of the results of thermal demagnetization of NRM for representative samples from the JLWS section in geographic coordinates. Figure 8. View largeDownload slide Orthogonal vector projection of the results of thermal demagnetization of NRM for representative samples from the QLK section in geographic coordinates. Figure 8. View largeDownload slide Orthogonal vector projection of the results of thermal demagnetization of NRM for representative samples from the QLK section in geographic coordinates. Figure 9. View largeDownload slide Orthogonal vector projection of the results of thermal demagnetization of NRM for representative samples from the SXRJ section in geographic coordinates. Figure 9. View largeDownload slide Orthogonal vector projection of the results of thermal demagnetization of NRM for representative samples from the SXRJ section in geographic coordinates. Figure 10. View largeDownload slide Equal-area stereographic projections of site-mean directions of the ITC components, in geographic and stratigraphic coordinates. Red stars indicate the mean directions. Figure 10. View largeDownload slide Equal-area stereographic projections of site-mean directions of the ITC components, in geographic and stratigraphic coordinates. Red stars indicate the mean directions. Finally, a high-temperature component (HTC) with a more east-directed declination and shallow to moderate positive inclination (Figs 6–9 and 11) was removed mainly between 330 °C and 480–530 °C (Figs 6–9). Only site JLWS6 exhibits a southwest declination and moderate negative inclination (Figs 7 and 11c and d). The overall mean HTC directions for the JLWE and SXRJ sections, in stratigraphic coordinates, are Ds = 84.8°, Is = 19.2°, N = 9 sites, ks = 35.5, α95 = 8.8° (Table 1); and Ds = 89.5°, Is = 30.7°, N = 8 sites, ks = 129.2, α95 = 4.9° (Table 3), respectively. Figure 11. View largeDownload slide Equal-area stereographic projections of site-mean directions of the HTC components, in geographic and stratigraphic coordinates. HTC components from the QLK section (blue diamonds) are also plotted in the JLWS section for a fold test. Figure 11. View largeDownload slide Equal-area stereographic projections of site-mean directions of the HTC components, in geographic and stratigraphic coordinates. HTC components from the QLK section (blue diamonds) are also plotted in the JLWS section for a fold test. Although the direction of sites JLWS6 has a negative (upward) inclination, the reversal test is negative at the 95 per cent probability level in the JLWS section (McFadden & McElhinny 1990; γcritical = 11.6, angle difference is 18.1). In addition, the directions of sites JLWS1, 6, 9 and QLK2, 3 are distributed along great circles which are fitted by using the LTC and ITC of JLWS, the Silurian results recalculated based on Opdyke et al. (1987) and Huang et al. (2000) and the HTC of JLWS and QLK (Figs 12a and b). In addition, the HTC of JLWS7 is rather different from the other results for this section (Figs 12a and b), so we exclude it from the following analysis. Figure 12. View largeDownload slide (A,B) Equal-area stereographic projections of the HTC components from the JLWS section (black dots), QLK section (yellow triangles), the results from Zhang et al. (2015) (blue crosses) and the Silurian result in our study area recalculated based on the results of Huang et al. (2000) and Opdyke et al. (1987) (green diamonds). Two red great circles are fitted using the LTC, ITC, Silurian results and the HTC components of the JLWS and QLK; (C,D) the HTC components used to calculate the primary remanent magnetization from the JLWS section (black dots), the results from Zhang et al. (2015) (blue crosses) and the QLK result (yellow triangles), before and after tilt correction. Red stars indicate the mean directions. Figure 12. View largeDownload slide (A,B) Equal-area stereographic projections of the HTC components from the JLWS section (black dots), QLK section (yellow triangles), the results from Zhang et al. (2015) (blue crosses) and the Silurian result in our study area recalculated based on the results of Huang et al. (2000) and Opdyke et al. (1987) (green diamonds). Two red great circles are fitted using the LTC, ITC, Silurian results and the HTC components of the JLWS and QLK; (C,D) the HTC components used to calculate the primary remanent magnetization from the JLWS section (black dots), the results from Zhang et al. (2015) (blue crosses) and the QLK result (yellow triangles), before and after tilt correction. Red stars indicate the mean directions. The rest of the HTC magnetizations from the JLWS and QLK sections are situated at the end of Fit 2 (Fig. 12b), which suggests that they were probably not affected by the Silurian magnetization. The overall mean directions for all the JLWS sites are Dg = 75.1, Ig = 51.5, N = 7 sites, kg = 97.3, α95 = 6.1 in geographic coordinates; and Ds = 74.1°, Is = 49.4°, N = 7 sites, ks = 218.9, α95 = 4.1° in stratigraphic coordinates (Fig. 11d). 4.3 SEM observations SEM observations of the fresh surface of representative specimens show the presence of numerous pyrites grains in the form of detritus, ultrafine crystals, framboidal aggregates and fine crystal aggregates (Fig. 13). Most of them cave into the matrix, which suggests that they formed during early diagenesis (Wang et al.2012). In addition, the presence of several ultrafine framboidal aggregates formed on the surface of detrital magnetite suggests that they may have formed during late diagenesis (Fig. 14, QLK4–7). Figure 13. View largeDownload slide Scanning electron microscope (SEM) secondary electron image and energy-dispersive X-ray spectra (EDS) analysis of Fe sulphides from the JLWS and QLK sections. Figure 13. View largeDownload slide Scanning electron microscope (SEM) secondary electron image and energy-dispersive X-ray spectra (EDS) analysis of Fe sulphides from the JLWS and QLK sections. Figure 14. View largeDownload slide Scanning electron microscope (SEM) secondary electron image and energy-dispersive X-ray spectra (EDS) analysis of specimens from the QLK section. Detrital magnetite is indicated by the EDS in QLK4–7. Mineral changes both from pyrite to magnetite and the reverse are indicated (QLK3–8b, 4–7) by the EDS. Reaction between gypsum and Fe oxides is indicated by the EDS (QLK3–8a). Figure 14. View largeDownload slide Scanning electron microscope (SEM) secondary electron image and energy-dispersive X-ray spectra (EDS) analysis of specimens from the QLK section. Detrital magnetite is indicated by the EDS in QLK4–7. Mineral changes both from pyrite to magnetite and the reverse are indicated (QLK3–8b, 4–7) by the EDS. Reaction between gypsum and Fe oxides is indicated by the EDS (QLK3–8a). No greigite was recognized, but some pyrrhotite grains were indicated by the EDS analysis and by their shape (Fig. 15, JLWE5–5). The magnetite has three different forms: detrital (Fig. 14, QLK4–7), framboidal or botryoidal aggregates (Fig. 15, JLWE18–1, SXRJ15–5), and a thin coating on some framboidal pyrites (Fig. 14, QLK3–8b). The individual magnetite crystals that comprise the framboids and aggregates are usually <1 μm in diameter and the framboids themselves are mainly <10 μm. Except for the detrital magnetite, all the other forms of magnetite occur in cavities or fractures (Figs 14 and 15), which suggests that they formed during late diagenesis. A clear replacement process of framboidal pyrite by magnetite is revealed by the EDS analysis (Fig. 14, QLK3–8b), as reported by Suk et al. (1990a). Much previous research (McCabe et al.1983; Elmore et al.1987, 2006; Suk et al.1990a,b, 1991, 1993; Sun & Jackson 1994; Weil & Van der Voo 2002; Font et al.2006; Zechmeister et al.2012) has documented the occurrence of framboidal magnetites in remagnetized carbonate rocks. Figure 15. View largeDownload slide Scanning electron microscope (SEM) secondary electron image and energy-dispersive X-ray spectra (EDS) analysis of specimens from the JLWE and SXRJ sections. (A) pyrrhotite; (B) framboidal magnetite in a cavity; (C,D) magnetite aggregates distributed along a crack, from the SXRJ section. Figure 15. View largeDownload slide Scanning electron microscope (SEM) secondary electron image and energy-dispersive X-ray spectra (EDS) analysis of specimens from the JLWE and SXRJ sections. (A) pyrrhotite; (B) framboidal magnetite in a cavity; (C,D) magnetite aggregates distributed along a crack, from the SXRJ section. There is also abundant gypsum in the samples (Fig. 14, QLK3–8a). Several spherical or framboidal grains contain Fe and Ti close to the gypsum indicating a reaction between the Fe oxides and the gypsum by thermochemical sulphate reduction (TSR), which may have produced the pyrrhotite and pyrite (Zhu et al.2015). 5 DISCUSSION 5.1 Acquisition time of different components Due to the similarity between the local present field and the LTC direction (Tables 1–3), we interpret it as a recent viscous remanent magnetization. As to the ITC, palaeopoles calculated from these sections are 65.6°N, 203.5°E, dp/dm = 2.6°/4.1°; 70.7°N, 207.6°E, dp/dm = 1.3°/2.0°; and 68.3°N, 188.4°E, dp/dm = 2.7°/4°, for the JLWE, JLWS and SXRJ sections, respectively (Fig. 16, Tables 4–6). These palaeopoles overlap with the Early-Middle Jurassic poles of South China (Fig. 16), which suggests that the ITC is a Jurassic remagnetization remanence. In addition, a fold test (McElhinny 1964; McFadden 1990) on the JLWE, JLWS, QLK and SXRJ sections is negative both at the 95 per cent and 99 per cent probability levels, which confirms that the ITC is a post Yanshanian magnetization. Figure 16. View largeDownload slide Palaeomagnetic poles calculated from ITC and HTC components from all three sampled sections. The existing Neoproterozoic and Phanerozoic palaeopoles (grey shading) from the South China block are shown for comparison. Details of the poles before the late Devonian are listed in Table 7; for other poles see table 1 in Huang et al. (2008). Figure 16. View largeDownload slide Palaeomagnetic poles calculated from ITC and HTC components from all three sampled sections. The existing Neoproterozoic and Phanerozoic palaeopoles (grey shading) from the South China block are shown for comparison. Details of the poles before the late Devonian are listed in Table 7; for other poles see table 1 in Huang et al. (2008). The HTC from the JLWE and SXRJ sections are similar to each other (Figs 6 and 9, Tables 1 and 3) although they have an age gap of 80–50 Ma (Condon et al.2005). A fold test cannot be conducted on both of sections, since the sites are all distributed on the east limb of the fold (Fig. 1). However, the folding time in the study area was during the Yanshanian orogeny (Xiao et al.1965), and the directions of the HTC of JLWE and SXRJ sections, both in geographic and stratigraphic coordinates, are different from the post-late Triassic results of South China (Figs 11a, b, e and f). This suggests that the HTC of the JLWE and SXRJ sections should be of pre-Jurassic age. In addition, palaeopoles for the JLWE and SXRJ sections plot close to the Silurian poles of South China block (Fig. 16), which suggests that they are Silurian remagnetization remanence. The HTC from the QLK and JLWS sections are rather complicated. Sites JLWS1, 6, 9 and QLK2, 3 are distributed along great circles (Figs 12a and b), which may reflect the effects of a remagnetization process at these sites during or after the Silurian. The remaining sites of the QLK and JLWS sections are distributed on both limbs of the fold (Fig. 1). The fold test (McElhinny 1964) conducted on these results is positive at both the 95 per cent and 99 per cent confidence levels. In addition, the palaeopole derived from the HTC of the JLWS section in stratigraphic coordinates differs from any previously reported poles of the South China block (Fig. 16), although the strata relationship of JLWS, JLWE and SXRJ resembles a ‘sandwich’, with JLWS in middle (Fig. 2). Based on these observations and the end point distribution on the great circle (Fig. 12), we suggest that the results from JLWS and QLK4 are a primary remanence which was acquired during or shortly after the deposition of Doushantuo Formation Member 3. 5.2 Silurian remagnetization A pulse of Silurian remagnetization, which affected the Ediacaran-Early Palaeozoic strata across the South China block, has long been suspected (Wu et al.1988; Macouin et al.2004; Yang et al.2004; Zhang et al.2013, 2015; Jing et al.2015). Lin et al. (1985a) first reported an east-west shallow inclination component for lower Cambrian rocks from Hubei, Zhejiang and Yunnan provinces (Yang et al.2004, in their fig. 7). Subsequently, many studies of late Precambrian rocks from Hubei, Hunan and Jiangxi provinces (Zhang 1994; Macouin et al.2004) have yielded results that are almost identical to the Silurian results reported by Opdyke et al. (1987) and Huang et al. (2000). Recently, Zhang et al. (2015) reported a palaeopole from the Doushantuo Formation Member 3, which is different from the known reliable Phanerozoic poles of the South China block (Fig. 16, Table 7) and passes the reversal test (B class). Rock magnetic and thermal demagnetization results suggest it is carried by SD magnetite (Zhang et al.2015). Therefore, this result may be the primary remanence of the Doushantuo Formation. However, as the authors observed, there is a large and rapid change in inclination values through the sampling section (Zhang et al.2015, in their fig. 2). In their study, they proposed four possible explanations, including differing degrees of unresolved secondary contamination of the HTC remanence. However, they dismissed this possibility because their results passed the reversal test. The results of Zhang et al. (2015), together with our HTC results from the JLWS and QLK sections, are plotted in Figs 12(a) and (b). Clearly, many of the results of Zhang et al. (2015) are distributed along either Fit 1 or Fit 2 great circles. This distribution characteristic suggests that some of their results may not only be affected by the Silurian remagnetization but also by a younger remagnetization (Jing et al.2015). The HTC components from the JLWE and SXRJ sections that resemble the Silurian results (Fig. 16) are of normal polarity with a maximum unblocking temperature of 540 °C, indicative of a magnetite carrier. The presence of magnetite is confirmed by the rock magnetic and SEM results (Figs 3–5, 14 and 15). The Doushantuo Formation experienced moderate burial temperatures (180–200 °C), as indicated by bitumen reflectance data and the calibration curve for illitization from the lower-middle Member 2 and Member 4 (Derkowski et al.2013). The minimum burial temperature for the maximum laboratory unblocking temperature range of 530–540 °C should have exceeded 420 °C according to magnetite relaxation-time–unblocking-temperature curves (Dunlop et al.2000). This indicates that this Silurian-similar magnetization carried by magnetite is a chemical remanent magnetization (CRM), rather than a thermoviscous magnetization. The SEM observations demonstrate that the authigenic magnetite grains occur in cavities and cracks in the JLWE and SXRJ sections (Fig. 15), which also indicate that their magnetization is a CRM and may be related to the activity of hydrothermal fluids (Suk et al.1990a). This explanation is also supported by previous studies (Jackson et al.1988; McCabe & Elmore 1989; Jackson 1990; Lu et al.1990), which did not find thermoviscous magnetization characteristics in carbonate rocks that had undergone burial temperatures below 250 °C. Interestingly, from the carbonate clumped isotope thermometry and clay mineralogical evidence, Bristow et al. (2011) suggested that the highly 13C-depleted carbonate cements in the cap dolostone (Member 1 of the Doushantuo Formation) were formed from hydrothermal fluids. Furthermore, based on clay mineral transformations (illitization of dioctahedral smectite/conversion of saponite to chlorite via corrensite) and the degree of organic maturation, Derkowski et al. (2013) postulated the occurrence of hydrothermal fluid activity in the underlying, relatively permeable Nantuo Formation, in the lower Doushantuo Formation (Member 2) and in the uppermost part of the Doushantuo Formation (Member 4). K-Ar dating of different size fractions of fine-grained illite of the Doushantuo Formation (Member 4) shale in the Jiulongwan section, provides an age estimate of ∼430 Ma for the hydrothermal fluid activity (Derkowski et al.2013), which is coincident with the age of the Silurian-similar magnetization. Based on these observations, we suggest that pervasive Silurian remagnetization has affected the Ediacaran age strata in the Three Gorges area, and that this remagnetization event was induced by hydrothermal fluid activity. Because illite is present in the Doushantuo Formation (Bristow et al.2011; Derkowski et al.2013), the illitization of the smectite may be an alternative mechanism for this Silurian remagnetization (McCabe & Elmore 1989; Katz et al.1998, 2000; Woods et al.2002; Tohver et al.2008). However, clay mineral analysis of the Doushantuo Formation (Bristow et al.2011; Derkowski et al.2013) suggested that illitization/chloritization only occurs in the lower 30 m of Member 2 and Member 4 of the Doushantuo Formation. This illitization/chloritization was induced by high-temperature hydrothermal fluids (Derkowski et al.2013, in their fig. 3). In addition, if illitization of the smectite was the main mechanism for the Silurian remagnetization event in this case, then the upper part (ca. 50 m) of Member 2 and Member 3 of the Doushantuo Formation should not record this Silurian remagnetization. However, all the results from Member 2 of the Doushantuo Formation resemble the Silurian results. This evidence indicates that the illitization cannot be the dominant mechanism of the Silurian remagnetization in the study area. It is noteworthy that during the Silurian–Devonian, the Ediacaran–lower Cambrian source rocks reached their peak in oil generation in the study area and in the central Yangtze area (Peters et al.1996; Zhu et al.2015). The accumulated oil was preserved in the source beds (Zhu et al.2015). This type of oil distribution in the Yangtze Block (e.g. in Sichuan and Hubei) may have induced a pervasive Silurian-Devonian remagnetization event that only affected the Ediacaran-lower Cambrian strata, but did not affect the Ordovician strata. The results of geochemical and isotopic analyses of the Ediacaran–lower Cambrian strata (Sawaki et al.2010) also suggest that the fluid was derived from the maturation of organic matter (Elmore et al.2001). The results of previous palaeomagnetic studies of the Ediacaran-lower Cambrian strata in the Yangtze block are consistent with this remagnetization pattern (Lin et al.1985a; Zhang 1994; Macouin et al.2004). Palaeomagnetic studies of the Palaeozoic sedimentary rocks in Yichang (Lin 1983; Lin et al.1985b; Kent et al.1987) confirmed that the Silurian-Devonian remagnetization did not affect the Ordovician age strata in this region. Thus, it seems clear that the HTC component, identified at the JLWE and SXRJ sections, is a CRM produced by hydrocarbon accumulation during the middle to late Silurian period. 5.3 Jurassic remagnetization The post-folding ITC component is likely a Middle Jurassic remagnetization residing in pyrrhotite. The possible remagnetization mechanism for the pyrrhotite is probably related to TSR, and this hypothesis is confirmed by the palaeomagnetic, rock magnetic and SEM observation results. By modelling the burial process, Zhu et al. (2015) proposed that the TSR may initially have occurred in the Jurassic and ceased in the Cretaceous. Palaeopoles of the ITC overlap with the Early-Middle Jurassic poles of South China (Fig. 16). The presence of the gypsum and its reaction with the Fe oxides (Fig. 14, QLK3–8a), in addition to the results of geochemical and isotope analyses of the H2S (Zhu et al.2005; Hao et al.2008; Ma et al.2008; Tian et al.2008; Liu et al.2013) and the temperature conditions, all favour a TSR (Zhu et al.2010; Zhu et al.2015). 5.4 Primary remanence of Doushantuo Formation Member 3 The distribution of the hydrothermal fluids, now revealed as hydrocarbons, indicates that the Doushantuo Formation Member 3 was relatively cool in the JLWS section (Derkowski et al.2013; Loyd et al.2015) and may only have been affected by minor hydrocarbon migration. The rock magnetic results suggest that magnetite is the main remanence carrier (Figs 3 and 4), and no framboidal iron oxide spherules are present. In addition, clay minerals, which can induce remagnetization in carbonate rocks (McCabe & Elmore 1989; Katz et al.1998, 2000; Woods et al.2002; Tohver et al.2008), are rare in Member 3 (<5 per cent, Bristow et al.2009). All this evidence, together with the palaeopoles comparison (Fig. 16), suggests that the HTC component from the JLWS section is a primary remanence. Thus, the directions of magnetization at the end of Fit 2, which plot away from those of the Silurian age (in Figs 12c and d and Table 2), including JLWS2, 3, 4, 5, 10, 11, QLK4, and ZG1, 2, 3, 4, JLW3, 5, 7, 12 reported by Zhang et al. (2015), were used to calculate a mean direction (Dg = 72.9°, Ig = 52.8°, kg = 43.9, α95 = 5.8° in situ; and Ds = 73.3°, Is = 59.5°, ks = 156.6, α95 = 3.1° after tilt correction; Table 2). The fold test of these directions (McElhinny 1964; McFadden 1990) is positive at both the 95 per cent and 99 per cent levels, and a positive reversal test (McFadden & McElhinny 1990) further suggests that it is a primary remanent magnetization. The palaeopole calculated from this primary remanent magnetization (DSTM3: 31.3°N, 169.8°E, dp/dm = 3.5°/4.7° after tilt correction, Fig. 16, Tables 2 and 7) is different from all known poles from rocks of the South China block. Table 2. HTC palaeomagnetic data for the Doushantuo Fm Member 3 of Jiulongwan section (JLWS), Yichang (111.069°E, 30.81°N). Results from the Qinglinkou section (QLK) and Zhang et al. (2015) are also listed. Site  Component  N/n  Dg  Ig  Ds  Is  k  α95    HTC  12  74.5  45.4  78.5  39.9  84.4  4.8  JLWS02  HTC  10  60.2  52  66.7  47.6  127.9  4.3  JLWS03  HTC  12  68.6  56  71.3  52.5  54.8  5.9  JLWS04  HTC  9  72.7  54.6  74.8  51  51.2  7.3  JLWS05  HTC  8  70.7  57.6  73.2  54  18.6  13.2  JLWS06a  HTC  7  226.9  −43.4  233  −39.3  154.1  4.9  JLWS07a  HTC  3  329.6  51.8  338  57.6  158.1  9.8  JLWS09a  HTC  6  53.3  43.8  42.7  40.8  57  8.9  JLWS10  HTC  8  85.7  48  74.6  51.5  17.4  13.7  JLWS11  HTC  13  88.8  44.4  79.1  48.5  23.2  8.8  QLK02a  HTC  4  51.2  35.8  27.6  66.4  67  11.3  QLK03a  HTC  11  80.5  6.7  84.8  34.4  24.6  9.4  QLK04  HTC  7  70.9  19.3  76.5  46  60.5  7.8  JLW03b  HTC  5  251.9  −51.8  255.7  −44.4  50.6  7.9  JLW05b  HTC  18  63.3  47.2  69.2  41.8  65.1  4.3  JLW07b  HTC  10  248.7  −50.9  252.3  −44.2  27.8  7.4  JLW12b  HTC  15  71.2  51.3  75.3  44.2  127.9  3.4  ZG01b  HTC  6  78.6  57.6  75.9  51.9  40.2  9.8  ZG02b  HTC  6  75.6  64.7  72.6  58.9  124.6  5.5  ZG03b  HTC  10  64.2  67.0  63.0  61.1  129.8  4.0  ZG04b  HTC  14  82.2  64.1  78.1  58.5  125.7  3.4  Mean of HTC components      15/151  72.9  52.8      43.9  5.8            73.3  59.5  156.6  3.1  Palaeopole (DSTM3)      29.5°N, 177.6°E, dp/dm = 5.5°/8.0°(in situ)          31.3°N, 169.8°E, dp/dm = 3.5°/4.7°(tilt corrected)  Mean of LTC components                  11/111  4.1  41.5  6.9  40  ks/kg  = 110.8/602.8  1.9  Palaeopole        82.2°N, 262.4°E, dp/dm = 1.4°/2.3°(in situ)  Site  Component  N/n  Dg  Ig  Ds  Is  k  α95    HTC  12  74.5  45.4  78.5  39.9  84.4  4.8  JLWS02  HTC  10  60.2  52  66.7  47.6  127.9  4.3  JLWS03  HTC  12  68.6  56  71.3  52.5  54.8  5.9  JLWS04  HTC  9  72.7  54.6  74.8  51  51.2  7.3  JLWS05  HTC  8  70.7  57.6  73.2  54  18.6  13.2  JLWS06a  HTC  7  226.9  −43.4  233  −39.3  154.1  4.9  JLWS07a  HTC  3  329.6  51.8  338  57.6  158.1  9.8  JLWS09a  HTC  6  53.3  43.8  42.7  40.8  57  8.9  JLWS10  HTC  8  85.7  48  74.6  51.5  17.4  13.7  JLWS11  HTC  13  88.8  44.4  79.1  48.5  23.2  8.8  QLK02a  HTC  4  51.2  35.8  27.6  66.4  67  11.3  QLK03a  HTC  11  80.5  6.7  84.8  34.4  24.6  9.4  QLK04  HTC  7  70.9  19.3  76.5  46  60.5  7.8  JLW03b  HTC  5  251.9  −51.8  255.7  −44.4  50.6  7.9  JLW05b  HTC  18  63.3  47.2  69.2  41.8  65.1  4.3  JLW07b  HTC  10  248.7  −50.9  252.3  −44.2  27.8  7.4  JLW12b  HTC  15  71.2  51.3  75.3  44.2  127.9  3.4  ZG01b  HTC  6  78.6  57.6  75.9  51.9  40.2  9.8  ZG02b  HTC  6  75.6  64.7  72.6  58.9  124.6  5.5  ZG03b  HTC  10  64.2  67.0  63.0  61.1  129.8  4.0  ZG04b  HTC  14  82.2  64.1  78.1  58.5  125.7  3.4  Mean of HTC components      15/151  72.9  52.8      43.9  5.8            73.3  59.5  156.6  3.1  Palaeopole (DSTM3)      29.5°N, 177.6°E, dp/dm = 5.5°/8.0°(in situ)          31.3°N, 169.8°E, dp/dm = 3.5°/4.7°(tilt corrected)  Mean of LTC components                  11/111  4.1  41.5  6.9  40  ks/kg  = 110.8/602.8  1.9  Palaeopole        82.2°N, 262.4°E, dp/dm = 1.4°/2.3°(in situ)  aResults excluded in calculating the mean of HTC. bHT components from Zhang et al. (2015) used to calculate the primary remanent magnetization direction. Other abbreviations are the same as Table 1. View Large Table 3. HTC palaeomagnetic data for the Dengying formation of Sanxiarenjia section (SXRJ), Yichang (111.165°E, 30.817°N). Site  Component  N/n  Dg  Ig  Ds  Is  k  α95  SXRJ10  HTC  8  92.5  29  96.1  21.8  60.3  7.2  SXRJ11  HTC  10  81.5  46.6  89  43.7  26.2  9.6  SXRJ12  HTC  8  85.2  36.1  90.6  30.7  98.3  5.6  SXRJ13,14  HTC  7  76.7  39.3  85.3  34.8  50.6  8.6  SXRJ15  HTC  10  79.1  37.1  86.3  30.1  109.4  4.6  SXRJ16  HTC  8  79.1  36.9  87.4  30.4  199.7  3.9  SXRJ17  HTC  11  87.1  34.2  91.7  27.7  111  4.4  SXRJ18  HTC  8  83.3  32.6  88.5  26.4  42.4  8.6  Mean of HTC components                  8/70  83.2  36.6      148.4  4.6            89.5  30.7  129.2  4.9  Palaeopole        15.9°N, 186.6°E, dp/dm = 3.1°/5.4° (in situ)          8.8°N, 187.1°E, dp/dm = 3.0°/5.5° (tilt corrected)  Mean of LTC components                  7/52  357.5  52.6  7.3  61.7  kg/ks = 206.7/148.2  3.9  Palaeopole        86.8°N, 70.0°E, dp/dm = 3.7°/5.4° (in situ)  Site  Component  N/n  Dg  Ig  Ds  Is  k  α95  SXRJ10  HTC  8  92.5  29  96.1  21.8  60.3  7.2  SXRJ11  HTC  10  81.5  46.6  89  43.7  26.2  9.6  SXRJ12  HTC  8  85.2  36.1  90.6  30.7  98.3  5.6  SXRJ13,14  HTC  7  76.7  39.3  85.3  34.8  50.6  8.6  SXRJ15  HTC  10  79.1  37.1  86.3  30.1  109.4  4.6  SXRJ16  HTC  8  79.1  36.9  87.4  30.4  199.7  3.9  SXRJ17  HTC  11  87.1  34.2  91.7  27.7  111  4.4  SXRJ18  HTC  8  83.3  32.6  88.5  26.4  42.4  8.6  Mean of HTC components                  8/70  83.2  36.6      148.4  4.6            89.5  30.7  129.2  4.9  Palaeopole        15.9°N, 186.6°E, dp/dm = 3.1°/5.4° (in situ)          8.8°N, 187.1°E, dp/dm = 3.0°/5.5° (tilt corrected)  Mean of LTC components                  7/52  357.5  52.6  7.3  61.7  kg/ks = 206.7/148.2  3.9  Palaeopole        86.8°N, 70.0°E, dp/dm = 3.7°/5.4° (in situ)  Abbreviations are the same as Table 1. View Large Table 4. ITC palaeomagnetic data for the Doushantuo Fm Member 2 of Jiulongwan section (JLWE), Yichang (111.069°E, 30.81°N). Site  Component  N/n  Dg  Ig  Ds  Is  k  α95  JLWE01  ITC  5  22.2  23.4  25.2  25.8  132.7  6.7  JLWE10  ITC  9  23.9  44.3  34.7  44.2  419.1  2.5  JLWE11  ITC  14  26.2  43.4  37.6  46.0  117.6  3.7  JLWE12  ITC  9  34.8  53.9  34.8  53.2  100.0  5.2  JLWE13  ITC  9  31.2  46.3  35.4  45.5  252.5  3.2  JLWE14  ITC  4  21.6  42.3  35.1  47.9  999.9  2.6  JLWE15  ITC  9  26.0  45.8  31.9  49.6  133.9  4.5  JLWE16  ITC  9  24.8  49.4  32.5  50.5  285.2  3.1  JLWE17  ITC  5  23.4  46.7  32.4  48.7  375.3  4.0  JLWE2,3  ITC  7  28.6  45.6  37.0  45.4  52.0  8.4  JLWE4,5  ITC  7  38.3  35.0  43.9  33.1  44.8  9.1  JLWE6,7  ITC  10  28.0  40.5  35.3  41.0  34.5  8.3  JLWE8,9  ITC  6  23.5  49.0  32.4  48.7  150.1  5.5  Mean of ITC components      12/98  27.6  45.3  35.5  46.2  ks/kg = 200.3/184.3  3.2  Palaeopole        65.6°N, 203.5°E, dp/dm = 2.6°/4.1°(in situ)  Site  Component  N/n  Dg  Ig  Ds  Is  k  α95  JLWE01  ITC  5  22.2  23.4  25.2  25.8  132.7  6.7  JLWE10  ITC  9  23.9  44.3  34.7  44.2  419.1  2.5  JLWE11  ITC  14  26.2  43.4  37.6  46.0  117.6  3.7  JLWE12  ITC  9  34.8  53.9  34.8  53.2  100.0  5.2  JLWE13  ITC  9  31.2  46.3  35.4  45.5  252.5  3.2  JLWE14  ITC  4  21.6  42.3  35.1  47.9  999.9  2.6  JLWE15  ITC  9  26.0  45.8  31.9  49.6  133.9  4.5  JLWE16  ITC  9  24.8  49.4  32.5  50.5  285.2  3.1  JLWE17  ITC  5  23.4  46.7  32.4  48.7  375.3  4.0  JLWE2,3  ITC  7  28.6  45.6  37.0  45.4  52.0  8.4  JLWE4,5  ITC  7  38.3  35.0  43.9  33.1  44.8  9.1  JLWE6,7  ITC  10  28.0  40.5  35.3  41.0  34.5  8.3  JLWE8,9  ITC  6  23.5  49.0  32.4  48.7  150.1  5.5  Mean of ITC components      12/98  27.6  45.3  35.5  46.2  ks/kg = 200.3/184.3  3.2  Palaeopole        65.6°N, 203.5°E, dp/dm = 2.6°/4.1°(in situ)  Abbreviations are the same as Table 1. View Large Table 5. ITC palaeomagnetic data for the Doushantuo Fm Member 3 of Jiulongwan section (JLWS), Yichang (111.069°E, 30.81°N). Site  Component  N/n  Dg  Ig  Ds  Is  k  α95  JLWS01  ITC  13  22.2  43.1  28.7  43.1  317.8  2.3  JLWS02  ITC  11  21.3  43.2  27.9  43.3  864.3  1.6  JLWS03  ITC  13  18.9  43.5  22.5  42.6  146.6  3.4  JLWS04  ITC  14  22.1  44.5  24.4  45.3  78.5  4.5  JLWS05  ITC  8  21.6  46.7  25.6  45.7  55.8  7.5  JLWS06  ITC  8  16.6  44.6  24.5  44.3  999.9  1.8  JLWS07  ITC  10  18.8  43.5  26.3  43.0  528.7  2.1  JLWS08  ITC  8  24.0  47.3  35.8  46.4  243.3  3.6  JLWS09  ITC  10  23.4  44.1  15.9  36.0  212.4  3.3  JLWS10  ITC  11  23.6  49.1  15.7  42.6  127.1  4.1  JLWS11  ITC  16  25.6  48.5  17.6  42.3  298.4  2.1  Mean of ITC components                  11/122  21.6  45.3  23.9  43.3  ks/kg  = 245/789.2  1.6  Palaeopole        70.7°N, 207.6°E, dp/dm = 1.3°/2.0°(in situ)  QLK02  ITC  7  17.9  44.7  335.7  55.2  151.1  4.9  QLK03  ITC  13  26.6  46.8  350.0  66.1  43.4  6.4  QLK04  ITC  9  16.4  49.3  337.9  64.9  105.6  5.0  Site  Component  N/n  Dg  Ig  Ds  Is  k  α95  JLWS01  ITC  13  22.2  43.1  28.7  43.1  317.8  2.3  JLWS02  ITC  11  21.3  43.2  27.9  43.3  864.3  1.6  JLWS03  ITC  13  18.9  43.5  22.5  42.6  146.6  3.4  JLWS04  ITC  14  22.1  44.5  24.4  45.3  78.5  4.5  JLWS05  ITC  8  21.6  46.7  25.6  45.7  55.8  7.5  JLWS06  ITC  8  16.6  44.6  24.5  44.3  999.9  1.8  JLWS07  ITC  10  18.8  43.5  26.3  43.0  528.7  2.1  JLWS08  ITC  8  24.0  47.3  35.8  46.4  243.3  3.6  JLWS09  ITC  10  23.4  44.1  15.9  36.0  212.4  3.3  JLWS10  ITC  11  23.6  49.1  15.7  42.6  127.1  4.1  JLWS11  ITC  16  25.6  48.5  17.6  42.3  298.4  2.1  Mean of ITC components                  11/122  21.6  45.3  23.9  43.3  ks/kg  = 245/789.2  1.6  Palaeopole        70.7°N, 207.6°E, dp/dm = 1.3°/2.0°(in situ)  QLK02  ITC  7  17.9  44.7  335.7  55.2  151.1  4.9  QLK03  ITC  13  26.6  46.8  350.0  66.1  43.4  6.4  QLK04  ITC  9  16.4  49.3  337.9  64.9  105.6  5.0  Abbreviations are the same as Table 1. View Large Table 6. ITC palaeomagnetic data for the Dengying formation of Sanxiarenjia section (SXRJ), Yichang (111.165°E, 30.817°N). Site  Component  N/n  Dg  Ig  Ds  Is  k  α95  SXRJ10  ITC  5  32.6  54.8  48.3  56.3  278.8  4.6  SXRJ11  ITC  3  18.7  53.6  27.8  58.9  239.5  8.0  SXRJ12  ITC  7  28.6  50.7  40.3  53.9  119.9  5.5  SXRJ16  ITC  7  25.3  52.9  44.5  58.0  156.2  4.8  SXRJ17  ITC  4  24.3  53.2  37.1  56.2  272.8  5.6  SXRJ18  ITC  6  23.9  49.1  36.8  53.3  158.4  5.3  Mean of ITC components                  6/32  25.5  52.5  39.2  56.3  kg/ks = 533.4/332  2.9  Palaeopole        68.3°N, 188.4°E, dp/dm = 2.7°/4° (in situ)  Site  Component  N/n  Dg  Ig  Ds  Is  k  α95  SXRJ10  ITC  5  32.6  54.8  48.3  56.3  278.8  4.6  SXRJ11  ITC  3  18.7  53.6  27.8  58.9  239.5  8.0  SXRJ12  ITC  7  28.6  50.7  40.3  53.9  119.9  5.5  SXRJ16  ITC  7  25.3  52.9  44.5  58.0  156.2  4.8  SXRJ17  ITC  4  24.3  53.2  37.1  56.2  272.8  5.6  SXRJ18  ITC  6  23.9  49.1  36.8  53.3  158.4  5.3  Mean of ITC components                  6/32  25.5  52.5  39.2  56.3  kg/ks = 533.4/332  2.9  Palaeopole        68.3°N, 188.4°E, dp/dm = 2.7°/4° (in situ)  Abbreviations are the same as Table 1. View Large Table 7. Known palaeopoles of the South China block from the Neoproterozoic to the Devonian. Pole name  Formation  Age (dating method)  1  2  3  4  5  6  7  Q  Plat (N)  Plong (E)  A95  References  Xiaofeng Dyke  Xiaofeng Dyke  802 ± 10 Ma (SHRIMP U-Pb zircon)  y  y  y  n  y  y  y  6  13.5  91  11  Li et al. (2004)  Liantuo  Liantuo Fm  720 ± 10 Ma (SIMS-U-Pb zircon)  y  y  y  n  y  y  y  6  13.2  155.2  5.3  Jing et al. (2015)                              Lan et al. (2015)  Nantuo  Nantuo Fm  636.3 ± 4.9 Ma (SHRIMP U-Pb zircon)  y  y  y  y  y  y  y  7  9.3  165.9  4.3  Zhang et al. (2008b, 2013)  DST  Doushantuo Fm  635–550 Ma (TIMS U-Pb zircon)  n  y  y  y  y  n  n  4  0.6  196.9  7.1  Macouin et al. (2004)                              Condon et al. (2005)  DS3  Doushantuo Fm  595 ± 22 Ma (Re–Os isochron age)  y  y  y  n  y  y  y  6  25.9  185.5  6.7  Zhang et al. (2015)                              Zhu et al. (2013)  DSTM3  Doushantuo Fm  595 ± 22 Ma (Re–Os isochron age)  n  y  y  y  y  y  y  6  31.3  169.8  4.1  This study                              Zhu et al. (2013)  Є2  Douposi Fm  510 ± 11 Ma (stratigraphy and fossils)  n  y  y  y  y  y  y  6  −51.3  166  6.8  Yang et al. (2004)  O3  Pagoda Fm  458 ± 6 Ma (stratigraphy and fossils)  n  y  y  y  y  y  y  6  −45.8  191.3  3.2  Han et al. (2015)  Sx  Xiushan Fm  430 ± 2 Ma (stratigraphy and fossils)  n  y  y  y  y  n  y  5  4.9  194.7  5.6  Opdyke et al. (1987)  Sc1  Rongxi, Huixingshao, Niugundang Fm  430 ± 7 Ma (stratigraphy and fossils)  n  y  y  y  y  n  y  5  14.9  196.1  5.1  Huang et al. (2000)  D1–2  Longdongshui Fm  419–382 Ma (International Chronostratigraphic Chart, 2016/04)  n  y  y  n  y  y  y  5  36.1  231.4  12.1  Zhang et al. (2001)    Jipao Fm et al.                            Pole name  Formation  Age (dating method)  1  2  3  4  5  6  7  Q  Plat (N)  Plong (E)  A95  References  Xiaofeng Dyke  Xiaofeng Dyke  802 ± 10 Ma (SHRIMP U-Pb zircon)  y  y  y  n  y  y  y  6  13.5  91  11  Li et al. (2004)  Liantuo  Liantuo Fm  720 ± 10 Ma (SIMS-U-Pb zircon)  y  y  y  n  y  y  y  6  13.2  155.2  5.3  Jing et al. (2015)                              Lan et al. (2015)  Nantuo  Nantuo Fm  636.3 ± 4.9 Ma (SHRIMP U-Pb zircon)  y  y  y  y  y  y  y  7  9.3  165.9  4.3  Zhang et al. (2008b, 2013)  DST  Doushantuo Fm  635–550 Ma (TIMS U-Pb zircon)  n  y  y  y  y  n  n  4  0.6  196.9  7.1  Macouin et al. (2004)                              Condon et al. (2005)  DS3  Doushantuo Fm  595 ± 22 Ma (Re–Os isochron age)  y  y  y  n  y  y  y  6  25.9  185.5  6.7  Zhang et al. (2015)                              Zhu et al. (2013)  DSTM3  Doushantuo Fm  595 ± 22 Ma (Re–Os isochron age)  n  y  y  y  y  y  y  6  31.3  169.8  4.1  This study                              Zhu et al. (2013)  Є2  Douposi Fm  510 ± 11 Ma (stratigraphy and fossils)  n  y  y  y  y  y  y  6  −51.3  166  6.8  Yang et al. (2004)  O3  Pagoda Fm  458 ± 6 Ma (stratigraphy and fossils)  n  y  y  y  y  y  y  6  −45.8  191.3  3.2  Han et al. (2015)  Sx  Xiushan Fm  430 ± 2 Ma (stratigraphy and fossils)  n  y  y  y  y  n  y  5  4.9  194.7  5.6  Opdyke et al. (1987)  Sc1  Rongxi, Huixingshao, Niugundang Fm  430 ± 7 Ma (stratigraphy and fossils)  n  y  y  y  y  n  y  5  14.9  196.1  5.1  Huang et al. (2000)  D1–2  Longdongshui Fm  419–382 Ma (International Chronostratigraphic Chart, 2016/04)  n  y  y  n  y  y  y  5  36.1  231.4  12.1  Zhang et al. (2001)    Jipao Fm et al.                            1–7 and Q: reliability criteria of Van der Voo (1990). 1–Well-dated rock age; 2–sufficient number of samples; 3–adequate demagnetization; 4–field tests; 5–structural control and tectonic coherence with craton or block involved; 6–presence of reversals; 7–no resemblance to palaeopoles of younger age. Plat: latitude of the palaeopoles; Plong: longitude of the palaeopoles; A95: radius of circle with 95 per cent confidence about the palaeopole. View Large Our study demonstrates that a combination of rock magnetic, palaeomagnetic and SEM analyses can be used to determine the different magnetization characteristics of different carbonate lithologies. The application of this approach provides a new perspective on the resolution of the issue of multi-phase magnetization in South China. The proposed mechanism of multiple magnetizations may explain the pervasive Silurian and Jurassic-Cretaceous remagnetization events in the stable Yangtze block. In addition, the new DSTM3 pole can be used as a reliable Ediacaran pole of South China to constrain its position in the reconstruction of supercontinents (e.g. Rodinia or Gondwana). 6 CONCLUSIONS Detailed palaeomagnetic and rock magnetic studies reveal multiple magnetizations in the Ediacaran strata in South China. These secondary remagnetizations seem to be characteristic of the Ediacaran and lower Palaeozoic strata across a large region of the Central Yangtze. Several lines of evidence, including magnetic studies, SEM observations, clay mineral analysis and geochemical and isotope analysis, suggest that the remagnetization events were induced by multiple generations of oil and gas formation in the Ediacaran and Lower Cambrian strata. In addition, our study demonstrates that reliable palaeomagnetic results can be obtained from the relatively massive Ediacaran limestone of the South China block at localities where only minor hydrocarbon circulation has occurred and that future work should focus on strata of low permeability and with minimal organic matter content. This new Ediacaran palaeopole was proved by the fold and reversal tests, and petrographic observation, indicating a reliable pole for the South China block. The pole can be used for the reconstruction of South China in Rodinian and Gondwanan continents. Acknowledgements We gratefully acknowledge the careful and constructive comments of Andrew Biggin, Eduard Petrovsk and an anonymous reviewer who considerably improved the manuscript. Meanwhile, we are particularly grateful to Dr Pengju Liu for guidance in the field. This work was funded by the Chinese National Natural Science Foundation (Grant No. 41230208). REFERENCES Abrajevitch A., Van der Voo R., 2010. Incompatible Ediacaran paleomagnetic directions suggest an equatorial geomagnetic dipole hypothesis, Earth planet. Sci. Lett. , 293, 164– 170. https://doi.org/10.1016/j.epsl.2010.02.038 Google Scholar CrossRef Search ADS   An Z., Jiang G., Tong J., Tian L., Ye Q., Song H., Song H., 2015a. Stratigraphic position of the Ediacaran Miaohe biota and its constrains on the age of the upper Doushantuo δ 13 C anomaly in the Yangtze Gorges area, South China, Precambrian Res. , 271, 243– 253. https://doi.org/10.1016/j.precamres.2015.10.007 Google Scholar CrossRef Search ADS   An Z., Tong J., Ye Q., Tian L., Song H., Zhao X., 2015b. Neoproterozoic stratigraphic sequence and sedimentary evolution at Qinglinkou section, east Yangtze Gorges area, J. China Univ. Geosci.-Earth Sci. , 39( 7), 795– 806. Bristow T.F., Kennedy M.J., Derkowski A., Droser M.L., Jiang G., Creaser R.A., 2009. Mineralogical constraints on the paleoenvironments of the Ediacaran Doushantuo Formation, Proc. Natl. Acad. Sci. USA , 106, 13 190–13 195. https://doi.org/10.1073/pnas.0901080106 Google Scholar CrossRef Search ADS   Bristow T.F., Bonifacie M., Derkowski A., Eiler J.M., Grotzinger J.P., 2011. A hydrothermal origin for isotopically anomalous cap dolostone cements from south China, Nature , 474, 68– 71. https://doi.org/10.1038/nature10096 Google Scholar CrossRef Search ADS PubMed  Cogné J.P., 2003. PaleoMac: A Macintosh (TM) application for treating paleomagnetic data and making plate reconstructions, Geochem. Geophys. Geosyst. , 4( 1), 1– 8. Google Scholar CrossRef Search ADS   Condon D., Zhu M., Bowring S., Wang W., Yang A., Jin Y., 2005. U-Pb ages from the Neoproterozoic Doushantuo Formation, China, Science , 308, 95– 98. https://doi.org/10.1126/science.1107765 Google Scholar CrossRef Search ADS PubMed  Derkowski A., Bristow T.F., Wampler J., Środoń J., Marynowski L., Elliott W.C., Chamberlain C.P., 2013. Hydrothermal alteration of the Ediacaran Doushantuo Formation in the Yangtze Gorges area (South China), Geochim. Cosmochim. Acta , 107, 279– 298. https://doi.org/10.1016/j.gca.2013.01.015 Google Scholar CrossRef Search ADS   Dunlop D., Özdemir Ö., Clark D., Schmidt P., 2000. Time–temperature relations for the remagnetization of pyrrhotite (Fe 7 S 8) and their use in estimating paleotemperatures, Earth planet. Sci. Lett. , 176, 107– 116. https://doi.org/10.1016/S0012-821X(99)00309-X Google Scholar CrossRef Search ADS   Elmore R.D., Engel M.H., Crawford L., Nick K., Imbus S., Sofer Z., 1987. Evidence for a relationship between hydrocarbons and authigenic magnetite, Nature , 325, 428– 430. https://doi.org/10.1038/325428a0 Google Scholar CrossRef Search ADS   Elmore D.R., Kelley J., Evans M., Lewchuk M.T., 2001. Remagnetization and orogenic fluids: testing the hypothesis in the central Appalachians, Geophys. J. Int. , 144, 568– 576. https://doi.org/10.1111/j.1365-246X.2001.00349.x Google Scholar CrossRef Search ADS   Elmore R.D., Foucher J.L.-E., Evans M., Lewchuk M., Cox E., 2006. Remagnetization of the Tonoloway Formation and the Helderberg Group in the Central Appalachians: testing the origin of syntilting magnetizations, Geophys. J. Int. , 166, 1062– 1076. https://doi.org/10.1111/j.1365-246X.2006.02875.x Google Scholar CrossRef Search ADS   Evans D.A., 1998. True polar wander, a supercontinental legacy, Earth planet. Sci. Lett. , 157, 1– 8. https://doi.org/10.1016/S0012-821X(98)00031-4 Google Scholar CrossRef Search ADS   Evans D.A., 2003. True polar wander and supercontinents, Tectonophysics , 362, 303– 320. https://doi.org/10.1016/S0040-1951(02)000642-X Google Scholar CrossRef Search ADS   Fisher R., 1953. Dispersion on a sphere, Proc. R. Soc. A , 217, 295– 305. https://doi.org/10.1098/rspa.1953.0064 Google Scholar CrossRef Search ADS   Font E., Trindade R., Nédélec A., 2006. Remagnetization in bituminous limestones of the Neoproterozoic Araras Group (Amazon craton): Hydrocarbon maturation, burial diagenesis, or both?, J. geophys. Res. , 111( B06204), 1– 17. https://doi.org/10.1029/2005JB004106 Google Scholar PubMed  Halls H.C., Lovette A., Hamilton M., Söderlund U., 2015. A paleomagnetic and U–Pb geochronology study of the western end of the Grenville dyke swarm: Rapid changes in paleomagnetic field direction at ca. 585 Ma related to polarity reversals?, Precambrian Res. , 257, 137– 166. https://doi.org/10.1016/j.precamres.2014.11.029 Google Scholar CrossRef Search ADS   Han Z., Yang Z., Tong Y., Jing X., 2015. New paleomagnetic results from Late Ordovician rocks of the Yangtze Block, South China, and their paleogeographic implications, J. geophys. Res. , 120, 4759– 4772. https://doi.org/10.1002/2015JB012005 Google Scholar CrossRef Search ADS   Hao F., Guo T., Zhu Y., Cai X., Zou H., Li P., 2008. Evidence for multiple stages of oil cracking and thermochemical sulfate reduction in the Puguang gas field, Sichuan Basin, China, AAPG Bull. , 92, 611– 637. https://doi.org/10.1306/01210807090 Google Scholar CrossRef Search ADS   Huang B., Zhou Y., Zhu R., 2008. Discussions on Phanerozoic evolution and formation of continental China, based on paleomagnetic studies, Earth Sci. Frontiers , 15, 348– 359. https://doi.org/10.1016/S1872-5791(08)60039-1 Google Scholar CrossRef Search ADS   Huang K.N., Opdyke N.D., Zhu R.X., 2000. Further paleomagnetic results from the Silurian of the Yangtze Block and their implications, Earth planet. Sci. Lett. , 175, 191– 202. https://doi.org/10.1016/S0012-821X(99)00302-7 Google Scholar CrossRef Search ADS   Jackson M., 1990. Diagenetic sources of stable remanence in remagnetized Paleozoic cratonic carbonates: A rock magnetic study, J. geophys. Res. , 95, 2753– 2761. https://doi.org/10.1029/JB095iB03p02753 Google Scholar CrossRef Search ADS   Jackson M., McCabe C., Ballard M.M., Van der Voo R., 1988. Magnetite authigenesis and diagenetic paleotemperatures across the northern Appalachian basin, Geology , 16, 592– 595. https://doi.org/10.1130/0091-7613(1988)0160592:MAADPA2.3.CO;2 Google Scholar CrossRef Search ADS   Jiang G., Shi X., Zhang S., Wang Y., Xiao S., 2011. Stratigraphy and paleogeography of the Ediacaran Doushantuo Formation (ca. 635–551 Ma) in South China, Gondwana Res. , 19, 831– 849. https://doi.org/10.1016/j.gr.2011.01.006 Google Scholar CrossRef Search ADS   Jing X.-Q., Yang Z., Tong Y., Han Z., 2015. A revised paleomagnetic pole from the mid-Neoproterozoic Liantuo Formation in the Yangtze block and its paleogeographic implications, Precambrian Res. , 268, 194– 211. https://doi.org/10.1016/j.precamres.2015.07.007 Google Scholar CrossRef Search ADS   Katz B., Elmore R., Cogoini M., Ferry S., 1998. Widespread chemical remagnetization: Orogenic fluids or burial diagenesis of clays?, Geology , 26, 603– 606. https://doi.org/10.1130/0091-7613(1998)0260603:WCROFO2.3.CO;2 Google Scholar CrossRef Search ADS   Katz B., Elmore R.D., Cogoini M., Engel M.H., Ferry S., 2000. Associations between burial diagenesis of smectite, chemical remagnetization, and magnetite authigenesis in the Vocontian trough, SE France, J. geophys. Res. , 105, 851– 868. https://doi.org/10.1029/1999JB900309 Google Scholar CrossRef Search ADS   Kent D.V., Zeng X., Wen Y.Z., Opdyke N.D., 1987. Widespread late Mesozoic to Recent remagnetization of Paleozoic and lower Triassic sedimentary rocks from South China, Tectonophysics , 139, 133– 143. https://doi.org/10.1016/0040-1951(87)90202-2 Google Scholar CrossRef Search ADS   Kirschvink J.L., 1980. The least-squares line and plane and the analysis of paleomagnetic data, Geophys. J. R. astr. Soc. , 62, 699– 718. https://doi.org/10.1111/j.1365-246X.1980.tb02601.x Google Scholar CrossRef Search ADS   Klein R., Salminen J., Mertanen S., 2015. Baltica during the Ediacaran and Cambrian: A paleomagnetic study of Hailuoto sediments in Finland, Precambrian Res. , 267, 94– 105. https://doi.org/10.1016/j.precamres.2015.06.005 Google Scholar CrossRef Search ADS   Kruiver P.P., Dekkers M.J., Heslop D., 2001. Quantification of magnetic coercivity components by the analysis of acquisition curves of isothermal remanent magnetisation, Earth planet. Sci. Lett. , 189, 269– 276. https://doi.org/10.1016/S0012-821X(01)00367-3 Google Scholar CrossRef Search ADS   Lan Z., Li X.-H., Zhu M., Zhang Q., Li Q.-L., 2015. Revisiting the Liantuo Formation in Yangtze Block, South China: SIMS U–Pb zircon age constraints and regional and global significance, Precambrian Res. , 263, 123– 141. https://doi.org/10.1016/j.precamres.2015.03.012 Google Scholar CrossRef Search ADS   Li Z., Evans D., Zhang S., 2004. A 90 spin on Rodinia: possible causal links between the Neoproterozoic supercontinent, superplume, true polar wander and low-latitude glaciation, Earth planet. Sci. Lett. , 220, 409– 421. https://doi.org/10.1016/S0012-821X(04)00064-0 Google Scholar CrossRef Search ADS   Lin J.-L., 1983. The Apparent Polar Wander Paths for the North and South China Blocks , Universyty of California Santa Barbara. Lin J.-L., Fuller M., Zhang W.-Y., 1985a. Paleogeography of the North and South China blocks during the Cambrian, J. Geodyn. , 2, 91– 114. https://doi.org/10.1016/0264-3707(85)90003-1 Google Scholar CrossRef Search ADS   Lin J.L., Fuller M., Zhang W.Y., 1985b. Preliminary Phanerozoic polar wander paths for the North and South China blocks, Nature , 313, 444– 449. https://doi.org/10.1038/313444a0 Google Scholar CrossRef Search ADS   Liu Q.et al.  , 2013. TSR versus non-TSR processes and their impact on gas geochemistry and carbon stable isotopes in Carboniferous, Permian and Lower Triassic marine carbonate gas reservoirs in the Eastern Sichuan Basin, China, Geochim. Cosmochim. Acta , 100, 96– 115. https://doi.org/10.1016/j.gca.2012.09.039 Google Scholar CrossRef Search ADS   Lowrie W., 1990. Identification of ferromagnetic minerals in a rock by coercivity and unblocking temperature properties, Geophys. Res. Lett. , 17, 159– 162. https://doi.org/10.1029/GL017i002p00159 Google Scholar CrossRef Search ADS   Loyd S.J., Corsetti F.A., Eagle R.A., Hagadorn J.W., Shen Y., Zhang X., Bonifacie M., Tripati A.K., 2015. Evolution of Neoproterozoic Wonoka–Shuram Anomaly-aged carbonates: evidence from clumped isotope paleothermometry, Precambrian Res. , 264, 179– 191. https://doi.org/10.1016/j.precamres.2015.04.010 Google Scholar CrossRef Search ADS   Lu G., Marshak S., Kent D.V., 1990. Characteristics of magnetic carriers responsible for Late Paleozoic remagnetization in carbonate strata of the mid-continent, USA, Earth planet. Sci. Lett. , 99, 351– 361. https://doi.org/10.1016/0012-821X(90)90139-O Google Scholar CrossRef Search ADS   Ma Y., Zhang S., Guo T., Zhu G., Cai X., Li M., 2008. Petroleum geology of the Puguang sour gas field in the Sichuan Basin, SW China, Mar. Pet. Geol. , 25, 357– 370. https://doi.org/10.1016/j.marpetgeo.2008.01.010 Google Scholar CrossRef Search ADS   Macouin M., Besse J., Ader M., Gilder S., Yang Z., Sun Z., Agrinier P., 2004. Combined paleomagnetic and isotopic data from the Doushantuo carbonates, South China: implications for the ‘snowball Earth’ hypothesis, Earth planet. Sci. Lett. , 224, 387– 398. https://doi.org/10.1016/j.epsl.2004.05.015 Google Scholar CrossRef Search ADS   Macouin M., Ader M., Moreau M.-G., Poitou C., Yang Z., Sun Z., 2012. Deciphering the impact of diagenesis overprint on negative δ 13 C excursions using rock magnetism: case study of Ediacaran carbonates, Yangjiaping section, South China, Earth planet. Sci. Lett. , 351, 281– 294. https://doi.org/10.1016/j.epsl.2012.06.057 Google Scholar CrossRef Search ADS   Manning E.B., Elmore R., 2015. An integrated paleomagnetic, rock magnetic, and geochemical study of the Marcellus shale in the Valley and Ridge province in Pennsylvania and West Virginia, J. geophys. Res. , 120, 705– 724. https://doi.org/10.1002/2014JB011418 Google Scholar CrossRef Search ADS   McCabe C., Elmore R.D., 1989. The occurrence and origin of Late Paleozoic remagnetization in the sedimentary rocks of North America, Rev. Geophys. , 27, 471– 494. https://doi.org/10.1029/RG027i004p00471 Google Scholar CrossRef Search ADS   McCabe C., Van der Voo R., Peacor D.R., Scotese C.R., Freeman R., 1983. Diagenetic magnetite carries ancient yet secondary remanence in some Paleozoic sedimentary carbonates, Geology , 11, 221– 223. https://doi.org/10.1130/0091-7613(1983)11221:DMCAYS2.0.CO;2 Google Scholar CrossRef Search ADS   McCausland P.J., Van der Voo R., Hall C.M., 2007. Circum-Iapetus paleogeography of the Precambrian–Cambrian transition with a new paleomagnetic constraint from Laurentia, Precambrian Res. , 156, 125– 152. https://doi.org/10.1016/j.precamres.2007.03.004 Google Scholar CrossRef Search ADS   McCausland P.J., Hankard F., Van der Voo R., Hall C.M., 2011. Ediacaran paleogeography of Laurentia: Paleomagnetism and 40Ar–39Ar geochronology of the 583 Ma Baie des Moutons syenite, Quebec, Precambrian Res. , 187, 58– 78. https://doi.org/10.1016/j.precamres.2011.02.004 Google Scholar CrossRef Search ADS   McElhinny M.W., 1964. Statistical significance of the fold test in palaeomagnetism, Geophys. J. R. astr. Soc. , 8, 338– 340. https://doi.org/10.1111/j.1365-246X.1964.tb06300.x Google Scholar CrossRef Search ADS   McFadden K.A., Huang J., Chu X., Jiang G., Kaufman A.J., Zhou C., Yuan X., Xiao S., 2008. Pulsed oxidation and biological evolution in the Ediacaran Doushantuo Formation, Proc. Natl. Acad. Sci. USA , 105, 3197– 3202. https://doi.org/10.1073/pnas.0708336105 Google Scholar CrossRef Search ADS   McFadden P., 1990. A new fold test for palaeomagnetic studies, Geophys. J. Int. , 103, 163– 169. https://doi.org/10.1111/j.1365-246X.1990.tb01761.x Google Scholar CrossRef Search ADS   McFadden P., McElhinny M., 1990. Classification of the reversal test in palaeomagnetism, Geophys. J. Int. , 103, 725– 729. https://doi.org/10.1111/j.1365-246X.1990.tb05683.x Google Scholar CrossRef Search ADS   Meert J.G., 2014. Ediacaran–Early Ordovician paleomagnetism of Baltica: a review, Gondwana Res. , 25, 159– 169. https://doi.org/10.1016/j.gr.2013.02.003 Google Scholar CrossRef Search ADS   Meert J.G., Van der Voo R., Powell C.M., Li Z.-X., McElhinny M.W., Chen Z., Symons D., 1993. A plate-tectonic speed limit?, Nature , 363, 216– 217. https://doi.org/10.1038/363216a0 Google Scholar CrossRef Search ADS   Opdyke N.D., Huang K., Xu G., Zhang W., Kent D.V., 1987. Paleomagnetic results from the Silurian of the Yangtze paraplatform, Tectonophysics , 139, 123– 132. https://doi.org/10.1016/0040-1951(87)90201-0 Google Scholar CrossRef Search ADS   Peters C., Dekkers M., 2003. Selected room temperature magnetic parameters as a function of mineralogy, concentration and grain size, Phys. Chem. Earth, Parts A/B/C , 28, 659– 667. https://doi.org/10.1016/S1474-7065(03)00120-7 Google Scholar CrossRef Search ADS   Peters K.E., Cunningham A.E., Walters C.C., Jigang J., Zhaoan F., 1996. Petroleum systems in the Jiangling-Dangyang area, Jianghan basin, China, Organic Geochem. , 24, 1035– 1060. https://doi.org/10.1016/S0146-6380(96)00080-0 Google Scholar CrossRef Search ADS   Roberts A.P., Chang L., Rowan C.J., Horng C.S., Florindo F., 2011. Magnetic properties of sedimentary greigite (Fe3S4): an update, Rev. Geophys. , 49( 1), 1– 46. https://doi.org/10.1029/2010RG000336 Google Scholar CrossRef Search ADS   Sawaki Y.et al.  , 2010. The Ediacaran radiogenic Sr isotope excursion in the Doushantuo Formation in the three Gorges area, South China, Precambrian Res. , 176, 46– 64. https://doi.org/10.1016/j.precamres.2009.10.006 Google Scholar CrossRef Search ADS   Schmidt P.W., 2014. A review of Precambrian palaeomagnetism of Australia: palaeogeography, supercontinents, glaciations and true polar wander, Gondwana Res. , 25, 1164– 1185. https://doi.org/10.1016/j.gr.2013.12.007 Google Scholar CrossRef Search ADS   Schmidt P.W., Williams G.E., 2010. Ediacaran palaeomagnetism and apparent polar wander path for Australia: no large true polar wander, Geophys. J. Int. , 182, 711– 726. https://doi.org/10.1111/j.1365-246X.2010.04652.x Google Scholar CrossRef Search ADS   Suk D., Peacor D., Van der Voo R., 1990a. Replacement of pyrite framboids by magnetite in limestone and implications for palaeomagnetism, Nature , 345, 611– 613. https://doi.org/10.1038/345611a0 Google Scholar CrossRef Search ADS   Suk D., Van Der Voo R., Peacor D.R., 1990b. Scanning and transmission electron microscope observations of magnetite and other iron phases in Ordovician carbonates from east Tennessee, J. geophys. Res. , 95, 12327– 12336. https://doi.org/10.1029/JB095iB08p12327 Google Scholar CrossRef Search ADS   Suk D., Van der Voo R., Peacor D.R., 1991. SEM/STEM observations of magnetite in carbonates of eastern North America: evidence for chemical remagnettzation during the Alleghenian Orogeny, Geophys. Res. Lett. , 18, 939– 942. https://doi.org/10.1029/91GL00916 Google Scholar CrossRef Search ADS   Suk D., Van Der Voo R., Peacor D.R., 1993. Origin of magnetite responsible for remagnetization of early Paleozoic limestones of New York State, J. geophys. Res. , 98, 419– 434. https://doi.org/10.1029/92JB01323 Google Scholar CrossRef Search ADS   Sun W.-W., Jackson M., 1994. Scanning electron microscopy and rock magnetic studies of magnetic carriers in remagnetized early Paleozoic carbonates from Missouri, J. geophys. Res., 99, 2935–2942. Tian H., Xiao X., Wilkins R.W., Tang Y., 2008. New insights into the volume and pressure changes during the thermal cracking of oil to gas in reservoirs: Implications for the in-situ accumulation of gas cracked from oils, AAPG Bull. , 92, 181– 200. https://doi.org/10.1306/09210706140 Google Scholar CrossRef Search ADS   Tohver E., Weil A., Solum J., Hall C., 2008. Direct dating of carbonate remagnetization by 40 Ar/39 Ar analysis of the smectite–illite transformation, Earth planet. Sci. Lett. , 274, 524– 530. https://doi.org/10.1016/j.epsl.2008.08.002 Google Scholar CrossRef Search ADS   Van der Voo R., 1990. The reliability of paleomagnetic data, Tectonophysics , 184, 1– 9. https://doi.org/10.1016/0040-1951(90)90116-P Google Scholar CrossRef Search ADS   Vernhet E., Reijmer J.J., 2010. Sedimentary evolution of the Ediacaran Yangtze platform shelf (Hubei and Hunan provinces, Central China), Sedimentary Geol. , 225, 99– 115. https://doi.org/10.1016/j.sedgeo.2010.01.005 Google Scholar CrossRef Search ADS   Wang L., Shi X., Jiang G., 2012. Pyrite morphology and redox fluctuations recorded in the Ediacaran Doushantuo Formation, Palaeogeog. Palaeoclimat. Palaeoecol. , 333, 218– 227. https://doi.org/10.1016/j.palaeo.2012.03.033 Google Scholar CrossRef Search ADS   Weil A.B., Van der Voo R., 2002. Insights into the mechanism for orogen-related carbonate remagnetization from growth of authigenic Fe-oxide: a scanning electron microscopy and rock magnetic study of Devonian carbonates from northern Spain, J. geophys. Res. , 107( B4), 1– 14. https://doi.org/10.1029/2001JB000200 Google Scholar CrossRef Search ADS   Woods S.D., Elmore R., Engel M., 2002. Paleomagnetic dating of the smectite-to-illite conversion: Testing the hypothesis in Jurassic sedimentary rocks, Skye, Scotland, J. geophys. Res. , 107( B5), 1– 10. https://doi.org/10.1029/2000JB000053 Google Scholar CrossRef Search ADS   Wu F., Van der Voo R., Liang Q., 1988. Reconnaissance magnetostratigraphy of the Precambrian-Cambrian boundary section at Meishucun, southwest China, Cuadernos de geología ibérica, J. Iberian Geol. , 12, 205– 222. Xiao D., Luo G., Bo Z., 1965. Primary disscussion on the Neo-structure activities near Yichang, West of Hubei Province, J. Nanjing Univ. (Nat. Sci.) , 9( 1), 133– 150. Yang Z., Sun Z., Yang T., Pei J., 2004. A long connection (750–380 Ma) between South China and Australia: paleomagnetic constraints, Earth planet. Sci. Lett. , 220, 423– 434. https://doi.org/10.1016/S0012-821X(04)00053-6 Google Scholar CrossRef Search ADS   Zechmeister M., Pannalal S., Elmore R., 2012. A multidisciplinary investigation of multiple remagnetizations within the Southern Canadian Cordillera, SW Alberta and SE British Columbia, Geol. Soc., London, Spec. Publ. , 371, SP371. 311, 123– 144. https://doi.org/10.1144/SP371.11 Google Scholar CrossRef Search ADS   Zhang H., 1994. Paleomagnetism of Proterozoic rock in Hunan and Guangxi provinces, South China, in Proceedings of the Annual Meeting of the Chinese Geophysics Society , Seismology Press, Beijing, pp. 333– 334. (in Chinese). Zhang H., Zhang W., Li P., 1983. Palaeomagnetism of the Sinian System of Eastern Yangzi Gorges in China, Bull. Tianjin Inst. Miner. Res. , 6, 57– 68. Zhang Q.-R., Li X.-H., Feng L.-J., Huang J., Song B., 2008a. A new age constraint on the onset of the Neoproterozoic glaciations in the Yangtze Platform, South China, J. Geol. , 116, 423– 429. https://doi.org/10.1086/589312 Google Scholar CrossRef Search ADS   Zhang S., Zhu H., Meng X., 2001. New paleomagnetic results from the Devonian-Carboniferous successions in the Southern Yangtze Block and their paleogeographic implications, Acta Geol. Sin. , 75, 303– 313. Zhang S., Jiang G., Han Y., 2008b. The age of the Nantuo Formation and Nantuo glaciation in South China, Terra Nova , 20, 289– 294. https://doi.org/10.1111/j.1365-3121.2008.00819.x Google Scholar CrossRef Search ADS   Zhang S.et al.  , 2013. Paleomagnetism of the late Cryogenian Nantuo Formation and paleogeographic implications for the South China Block, J. Asian Earth Sci. , 72, 164– 177. https://doi.org/10.1016/j.jseaes.2012.11.022 Google Scholar CrossRef Search ADS   Zhang S.et al.  , 2015. New paleomagnetic results from the Ediacaran Doushantuo Formation in South China and their paleogeographic implications, Precambrian Res. , 259, 130– 142. https://doi.org/10.1016/j.precamres.2014.09.018 Google Scholar CrossRef Search ADS   Zhao Z., Xing Y., Ma G., Chen Y., 1985. Biostratigraphy of the Yangtze Gorge Area,(1) Sinian , Geological Publishing House, Beijing, 1, 143. Zhao Z.-Q.et al.  , 1988. The Sinian System of Hubei , China University of Geosciences Press, Wuhan, p. 205. Zhou C., Tucker R., Xiao S., Peng Z., Yuan X., Chen Z., 2004. New constraints on the ages of Neoproterozoic glaciations in South China, Geology , 32, 437– 440. https://doi.org/10.1130/G20286.1 Google Scholar CrossRef Search ADS   Zhu B., Becker H., Jiang S.-Y., Pi D.-H., Fischer-Gödde M., Yang J.-H., 2013. Re–Os geochronology of black shales from the Neoproterozoic Doushantuo Formation, Yangtze platform, South China, Precambrian Res. , 225, 67– 76. https://doi.org/10.1016/j.precamres.2012.02.002 Google Scholar CrossRef Search ADS   Zhu G., Zhang S., Liang Y., Dai J., Li J., 2005. Isotopic evidence of TSR origin for natural gas bearing high H2S contents within the Feixianguan Formation of the northeastern Sichuan Basin, southwestern China, Sci. China D , 48, 1960– 1971. https://doi.org/10.1360/082004-147 Google Scholar CrossRef Search ADS   Zhu G., Zhang S., Huang H., Liu Q., Yang Z., Zhang J., Wu T., Huang Y., 2010. Induced H 2 S formation during steam injection recovery process of heavy oil from the Liaohe Basin, NE China, J. Pet. Sci. Eng. , 71, 30– 36. https://doi.org/10.1016/j.petrol.2010.01.002 Google Scholar CrossRef Search ADS   Zhu G., Wang T., Xie Z., Xie B., Liu K., 2015. Giant gas discovery in the Precambrian deeply buried reservoirs in the Sichuan Basin, China: Implications for gas exploration in old cratonic basins, Precambrian Res. , 262, 45– 66. https://doi.org/10.1016/j.precamres.2015.02.023 Google Scholar CrossRef Search ADS   Zhu M., Zhang J., Yang A., 2007. Integrated Ediacaran (Sinian) chronostratigraphy of South China, Palaeogeog. Palaeoclimat. Palaeoecol. , 254, 7– 61. https://doi.org/10.1016/j.palaeo.2007.03.025 Google Scholar CrossRef Search ADS   SUPPORTING INFORMATION Supplementary data are available at GJI online. Table S1. Results of the coercivity based on IRM component analysis (Kruiver et al. 2001). In the columns, the three distinguished components and their contributions are shown: saturation IRM (SIRM), the field at which half of the SIRM is reached (B1/2) and the dispersion parameter (DP) represents one standard deviation. Please note: Oxford University Press is not responsible for the content or functionality of any supporting materials supplied by the authors. Any queries (other than missing material) should be directed to the corresponding author for the paper. © The Authors 2017. Published by Oxford University Press on behalf of The Royal Astronomical Society.

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