International Geology Review

Luo, Wenjuan; Hou, Tong; Santosh, M.; Wen, Shihua; Zhang, Zhaochong

doi: 10.1080/00206814.2012.751177pmid: N/A

Volcanic rocks in the Middle–Lower Yangtze River Valley (MLYRV) constitute a bimodal magmatic suite, with a significant compositional gap (between 50% and 63% SiO2) between the mafic and felsic members. The suite is characterized by a relatively wide spectrum of rock types, including basalts, trachytes, and rhyolites. The basaltic rocks have low-to-moderate SiO2 contents of 46.00–50.01%, whereas the trachytes and rhyolites possess SiO2 contents in the range of 63.08–77.61%. Rocks of the bimodal suite show moderate enrichment of LILEs, negative Nb, Ta, and Ti anomalies, and are significantly enriched in LREEs. The basalts were most likely generated by parental mafic magmas derived from enriched lithospheric mantle with minor assimilation of crustal materials involving coeval crystal fractionation during magma evolution. The results of energy-constrained assimilation and fractional crystallization simulations demonstrate that the felsic magma was produced by the mixing of 5–20% lower crustal anatectic melts with an evolved mafic magma (∼48% SiO2) and accompanied by extensive clinopyroxene, plagioclase, biotite, and Fe–Ti oxide fractionation. Our model for the genesis of felsic rocks in bimodal suites is different from the traditional models of crustal melting and fractional crystallization or assimilation–fractional crystallization of basaltic liquids.

Wu, Yue; Zhang, Changqing; Mao, Jingwen; Ouyang, Hegeng; Sun, Jia

doi: 10.1080/00206814.2012.753177pmid: N/A

The Chipu Mississippi Valley-type (MVT) deposit is located on the southwest (SW) margin of the Sichuan Basin. Occurrence of plentiful organic matter (bitumen) at this deposit and abundant hydrocarbon reservoirs in the SW Sichuan Basin implies a link between lead–zinc mineralization and hydrocarbon systems in this area. The high δ34S values of most metal sulphides from the different ore stages suggest that H2S-bearing gases and/or thermochemical sulphate reduction (TSR) by organic matter could have been the source of reduced sulphur involved in ore formation. Sulphides with small positive to negative δ34S values can be attributed to organically bound sulphur at the Chipu deposit. Carbon and oxygen isotopic compositions from sparry carbonates suggest mixing of organic carbon with seawater-derived carbon in the mineralization process. From the early to the later ore stages, δ13CPDB values of ore-hosting carbonates are increasingly more negative, which indicates strengthening of the TSR role during mineralization. Hydrogen and oxygen isotopes in fluid inclusions in the quartz gangue indicate the provenance of the ore-forming fluids in the hydrocarbons. Moreover, some extremely low hydrogen isotope values suggest the addition of hydrogen from the same source. The low H/C ratios and high non-hydrocarbon component of the bitumen (Zhang et al. 2010) also suggest that the organic matter may have been involved in TSR and subjected to a strong oxidation by ore-bearing fluids. This study attempts to explain the lead–zinc mineralization process and the role of organic matter in it. As there is a demonstrable relationship between the evolution of the hydrocarbons and regional lead–zinc mineralization along the SW edge of the Sichuan Basin, we propose a possible model in which the MVT mineralization coincided with the degradation of hydrocarbon reservoirs due to the large-scale migration of basinal fluids, most likely driven by the late Indosinian orogeny in response to the closure of the Palaeo-Tethys Ocean.

Jia, Ru-Ya; Jiang, Yao-Hui; Liu, Zheng; Zhao, Peng; Zhou, Qing

doi: 10.1080/00206814.2012.755766pmid: N/A

The western Kunlun orogen occupies a key position along the tectonic junction between the Pan-Asian and Tethyan domains, reflecting Proto- and Palaeo-Tethys subduction and terrane collision during early Palaeozoic to early Mesozoic time. We present the first detailed zircon U–Pb chronology, major and trace element, and Sr–Nd–O–Hf isotope geochemistry of the Qiukesu pluton and its microgranular enclaves from this multiple orogenic belt. SHRIMP zircon U–Pb dating shows that the Qiukesu pluton was emplaced in the early Silurian (ca. 435 Ma). It consists of weakly peraluminous high-K calc-alkaline monzogranite and syenogranite, with initial 87Sr/86Sr ratios of 0.7131–0.7229, ϵNd(T) of –4.1 to –5.7, δ18O of 8.0–10.8‰, and ϵHf(T) (in situ zircon) of –4.9. Elemental and isotopic data suggest that the granites formed by partial melting of lower-crustal granulitized metasedimentary-igneous Precambrian basement triggered by underplating of coeval mantle-derived enclave-forming intermediate magmas. Fractional crystallization of these purely crustal melts may explain the more felsic end-member granitic rocks, whereas such crustal melts plus additional input from coeval enclave-forming intermediate magma could account for the less felsic granites. The enclaves are intermediate (SiO2 57.6–62.2 wt.%) with high K2O (1.8–3.6 wt.%). They have initial 87Sr/86Sr ratios of 0.7132–0.7226, ϵNd(T) of –5.0 to –6.0, δ18O of 6.9–9.9‰, and ϵHf(T) (in situ zircon) of –8.1. We interpret the enclave magmas as having been derived by partial melting of subduction-modified mantle in the P–T transition zone between the spinel and spinel-garnet stability fields. Our new data suggest that subduction of the Proto-Tethyan oceanic crust was continuous to the early Silurian (ca. 435 Ma); the final closure of the Proto-Tethys occurred in the middle Silurian.

Liao, Shi-Yong; Yin, Fu-Guang; Sun, Zhi-Ming; Wang, Dong-Bing; Tang, Yuan; Sun, Jie

doi: 10.1080/00206814.2012.758354pmid: N/A

The time of final closure of the Palaeo-Tethys and the Sibumasu-Indochina collision in Southeast Asia represents a major unresolved geologic problem. Here, we present zircon chronology, whole-rock elemental, Sr–Nd, and zircon Hf isotopic geochemistry for newly discovered mafic dikes from the northern segment of the Sibumasu terrane, to provide constraints on this issue. Zircon U–Pb data indicate that the dikes were emplaced at 240 ± 3 Ma. These are the earliest Mesozoic magmatic rocks reported so far in the Sibumasu terrane, the late Palaeozoic passive margin of the Palaeo-Tethys. They are subalkaline tholeiites, showing geochemical characteristics similar to those of enriched mid-ocean ridge basalts (E-MORBs). They have 87Sr/86Sr(t) ratios of 0.703161–0.703826, ϵNd(t) of +4.8 to +7.5, and zircon ϵHf(t) of +9.2 to +13.3, implying strong mantle depletion. They were derived by partial melting of asthenospheric mantle and underwent subsequent fractional crystallization and lithospheric assimilation. The geologic–petrologic evidence suggests that the mafic dikes were generated in a collisional setting, when suturing of the Baoshan and Simao subterranes (the two subterranes are part of the Sibumasu and Indochina terranes, respectively) occurred. These early Middle Triassic mafic dikes provide an upper limit for Sibumasu–Indochina collision. In conjunction with previous work, we conclude that the final closure of the Palaeo-Tethys and collision of the Sibumasu and Indochina terranes took place during the late Permian to Early Triassic.

Koroneos, Antonios; Kilias, Adamantios; Avgerinas, Asterios

doi: 10.1080/00206814.2012.758830pmid: N/A

We studied the structural, petrological, and geochemical characteristics, as well as the geotectonic setting, of the Hercynian (300 ± 3 Ma) Voras plutonic rocks intrusive into the lower part of the Pelagonian nappe pile (East Pelagonian Zone, EPZ). These rocks are compared with the neighbouring Hercynian Varnountas and Kastoria plutons intruding the tectonic upper Pelagonian part (Korabi West Pelagonian Zone, KoWPZ). Based on modal and chemical compositions, four rock-types can be distinguished for the Voras ploutonic rocks: (1) hornblende-biotite granodiorite to granite (HbBtGrd), (2) biotite granite (BtGr), (3) leucogranite (LGr), and (4) mafic microgranular enclaves (MMEs) occurring typically in HbBtGrd and BtGr. Aplites intrude HbBtGrd and BtGr whereas xenoliths are rare. The MMEs are metaluminous, while all the other rock-types are slightly peraluminous. Crystallization pressures range from 2.4 to 2.9 kbar for the HbBtGrd and 3.0 kbar for the MME. Field observations, chemical, mineralogical, and petrographical data suggest that theMME and HbBtGrd + BtGr formed as a result of a two-step evolution process. In the first step, a mantle-sourced basic magma with composition similar to the more basic MMEs fractionated (F = 0.40, 60% crystallization) and concurrently mixed with an acid magma, of composition similar to the more acid BtGr. The crystallized mineral assemblage is Qtz38.27Pl27.83Hb14.84Bt10.61Kf6.13Ap2.30Zrn0.02. The process evolved with a low r-value (r = 0.3) giving the more basic HbBtGrd. In the second step, evolved magma (more basic HbBtGrd) fractionated, while simultaneously mixing with the same acid magma, but with higher r-value (r = 0.8), giving the more evolved HbBtGrd and the BtGr after 50% crystallization (F = 0.50) to the phase assemblage Qtz30.07Pl27.75Hb5.59Bt3.33Kf23.26Tit5.86Ap1.91Zrn0.04Mt2.19. We interpret the evolution of the LGr through fractional crystallization; it formed by partial melting of gneisses or felsic charnokites and granoulites. The less evolved MME (basic end-member) had a mantle origin, whereas for the more evolved BtGr (acid end-member) we favour partial melting of gneisses or mafic charnokites. Detailed structural analysis shows a strong, polyphase Alpine deformation which affected the Hercynian Voras ploutonic rocks and is thoroughly imprinted on the host Pelagonian metamorphic basement rocks. No evidence of relict Hercynian structures has been recognized, possibly due to intense reworking by the younger Alpine deformation. We have identified five tectonic events (D1–D5) from the Late Jurassic to recent, that evolved progressively from ductile, synmetamorphic (D1, D2) to semi-ductile (D3), and finally brittle (D4, D5) conditions. Voras plutonic rocks in the EPZ, and Varnountas and Kastoria plutons in the KoWPZ, have similar major and trace element geochemistries, as well as structural evolution; they coevally intruded (Carboniferous) the Pelagonian continent as a single unit. They show similar crystallization pressures and mantle contributions for their genesis; both are related to a volcanic arc geotectonic setting, associated with the subduction of the Palaeotethys Ocean beneath the Pelagonian continental fragment, the latter possibly of Gondwana origin.

Zhang, Hong-Rui; Yang, Tian-Nan; Hou, Zeng-Qian; Song, Yu-Cai; Ding, Yan; Cheng, Xian-Feng

doi: 10.1080/00206814.2012.759669pmid: N/A

The Palaeo-Tethyan tectonic evolution of central Tibet remains a topic of controversy. Two Permian to Late Triassic arc-like volcanic suites have been identified in the eastern Qiangtang (EQ) block of north-central Tibet. Three competing models have been proposed to explain the formation of these volcanic suites, with two models involving a single stage of long-lived subduction but with opposing subduction polarities, while the other model involves a two-stage subduction process. Here, we present new whole-rock geochemistry, including Sr–Nd isotope data, for late Permian felsic volcanics of the Zaduo area. These volcanics are mainly low to middle K calc-alkaline felsic tuffs and rhyolites with SiO2 concentrations up to 73 wt.%. In primitive mantle-normalized diagrams, the volcanics are typified by large ion lithophile element enrichment and high-field-strength element (e.g. Nb, Ta, P, and Ti) depletion, with slightly negative Eu anomalies. They have initial Sr ratios (87Sr/86Sr) i of 0.70319–0.70547, and ϵNd(t) values of +3.4 to +3.5, suggesting derivation by the partial melting of a depleted mantle wedge, followed by assimilation of crustal material. The available geochemical data indicate the presence of two distinctive igneous evolution trends within the Permian to Late Triassic volcanics of the EQ block, consistent with a two-stage subduction model. Permian to Early Triassic arc-like volcanics are formed during northward (present-day orientation) subduction, whereas the Late Triassic volcanics are related to southward (present-day orientation) subduction of mafic crust of the Garze–Litang Ocean.

Gou, Jun; Sun, De-You; Liu, Yong-Jiang; Ren, Yun-Sheng; Zhao, Zhong-Hua; Liu, Xiao-Ming

doi: 10.1080/00206814.2013.771949pmid: N/A

We have undertaken major and trace element analyses of volcanic rocks in Northeast China, as well as U–Pb dating and Hf isotopic analysis of their zircons, in order to determine the petrogenesis and tectonic setting of the volcanics. Mesozoic volcanism in the southern Manzhouli area occurred in two stages: Middle to Late Jurassic (164–147 Ma) and Early Cretaceous (142–123 Ma). The first stage is represented by the Tamulangou, Jixiangfeng, and Qiyimuchang formations. The Jixiangfeng Formation (162–156 Ma) is a rhyolite–trachyte dominated unit that lies between two basalt units, namely the underlying Tamulangou (164–160 Ma) and overlying Qiyimuchang (151–147 Ma) formations. The second igneous stage is dominated by rhyolitic lavas and tuffs of the Shangkuli Formation and basaltic rocks of the Yiliekede Formation, and they yield zircon U–Pb ages of 142–125 and 135–123 Ma, respectively. Basaltic rocks of the Tamulangou and Yiliekede formations have a wide range of MgO contents (1.64–9.59 wt%), but are consistently depleted of Nb and Ta and enriched with incompatible trace elements such as large ion lithophile elements (LILEs) and light rare earth elements (LREEs). Trachytes and rhyolites of the Jixiangfeng and Shangkuli formations are characterized by enrichment in LILEs and LREEs relative to HFSEs and HREEs, and with negative Nb, Ta, P, and Ti anomalies and positive ϵ Hf(t) values (3.49–9.98). These data suggest that basaltic volcanic rocks in southern Manzhouli were generated by fractional crystallization of a common parental magma, which was derived by partial melting of metasomatized (enriched) lithospheric mantle, whereas the trachytic and rhyolitic magmas were produced by the melting of lower crustal mafic and felsic granulites, respectively. Geochronological data indicate that Mesozoic volcanism in southern Manzhouli was initiated in the Middle to Late Jurassic and continued into the Early Cretaceous. It was mainly induced by lithospheric extension after the closure of the Mongol–Okhotsk Ocean.