International Journal of Earth Sciences

Thiede, Jörn; Wefer, Gerold

doi: 10.1007/s00531-014-1000-2pmid: N/A

Kryza, Ryszard; Beqiraj, Arjan

doi: 10.1007/s00531-013-0991-4pmid: N/A

Two distinct zircon populations, 1,827 ± 17 and 169 ± 2 Ma in age, have been found in the Rubiku granite dyke in the Middle Jurassic Mirdita ophiolite in central Albania. The old inherited zircons represent a homogeneous population formed during a discrete Palaeoproterozoic, likely magmatic, zircon crystallization event. These older zircons were likely incorporated, in large part, into the granite magma that crystallized broadly at the time of the ophiolite emplacement (around 169 Ma). The limited data available do not allow for the construction of an unequivocal petrogenetic model, though several palaeotectonic scenarios are discussed as possible settings for the granite formation. The models refer to recent findings of old inherited zircons in rocks at recent mid-ocean ridge settings, but also consider likely contributions of crustal materials to primary basic ophiolitic magmas within supra-subduction settings and subsequent accretion/collision circumstances. The presence of old zircons in much younger rocks within ophiolite successions runs counter to geodynamic models of interaction between the oceanic lithosphere and continental crust, but constraining their genesis would require further systematic studies on these old inherited zircons, both in mafic (if present) and in felsic rocks of the ophiolites.

Szczepański, Jacek; Ilnicki, Sławomir

doi: 10.1007/s00531-013-0993-2pmid: N/A

The chemical composition of metamorphosed siliciclastic rocks in the Orlica-Śnieżnik Dome (Bohemian Massif) identifies the main sources for the Neoproterozoic [the Młynowiec Formation (MF)], Early Cambrian [the Stronie Formation (SF)] and Late Cambrian/Early Ordovician [the Goszów quartzites (GQ)] sediments. The MF developed from erosion of a Cadomian magmatic arc along the northern Gondwana margin. The variegated SF, with supra-subduction affinities, shows chemical characteristics pointing to erosion of the freshly exhumed Cadomian orogen and detritus deposition in the back-arc basin. The very different chemical features of the GQ indicate deposition in a basin sited on a passive continental margin. The explanation proposed for the observed changes in chemical composition involves three main stages: (1) The pre ~540 Ma evolution of an active continental margin and related back-arc basin ceased with the collision and accretion of the magmatic arc to the Gondwana margin; (2) Early Cambrian rift to drift transition (540–500 Ma) and development of a depositional basin filled with detritus derived from remnants of the magmatic arc; (3) Peri-Gondwana break-up leading to the formation of shallow-water passive margin depositional basins filled with quartz-rich detritus resembling Early Ordovician Armorican quartzites known from other parts of the Variscan Belt.

Qiu, Liang; Yan, Dan-Ping; Zhou, Mei-Fu; Arndt, Nicholas; Tang, Shuang-Li; Qi, Liang

doi: 10.1007/s00531-013-0996-zpmid: N/A

The Indosinian orogeny is recorded by Triassic angular unconformities in Vietnam and South China and by widely occurring granitoids in the Yunkai-Nanling and the Xuefengshan belts of South China. The Longtan pluton in the northwestern part of the Xuefengshan belt is a typical high-K, calc-alkaline, I-type granitoid, which can shed light on the relationship between the Indosinian tectonic and magmatic activity in the region. Three precise zircon U–Pb ages yielded a mean of 218 ± 0.8 Ma, which is taken as the age of crystallization. The pluton consists of both granodiorite (64.59–68.01 % SiO2 and 3.25–4.22 % K2O) and granite (70.49–71.80 % SiO2 and 4.07–4.70 % K2O). The granodiorites are characterized by relatively high Mg# (54–57), low contents of Na2O (3.2–4.3 wt%), low abundances of incompatible elements (LILE, Nb and P), high initial 87Sr/86Sr (0.7175–0.7184) and negative εNd(t) (−9.98 to −9.72). REE patterns show moderate fractionation ((La/Yb)cn = 8.07–18.80) with negative Eu anomalies (Eu/Eu* = 0.62–0.86). Compared with the granodiorite, the granite has a wider range of Mg# (49–59), lower contents of Na2O (2.8–4.2 wt%), higher initial 87Sr/86Sr (0.7232–0.7243) and more negative εNd(t) (−12.07 to −11.24) values. REE patterns are relatively flat ((La/Yb)cn = 14.73–29.37) with smaller negative Eu anomalies (Eu/Eu* = 0.48–0.63). The granodiorite has lower K2O/Na2O and Al2O3/(MgO + FeOTot) values than the granite. Based on major and trace element geochemistry and Sr–Nd isotopes, we interpret the Longtan granodioritic magma to have been derived by partial melting of interlayered Proterozoic metabasaltic to metatonalitic source rocks, whereas the granite was probably derived from a mixture of Proterozoic metagraywackes and metaigneous rocks. Field, petrographic and geochemical evidence indicate that partial melting and fractional crystallization were the dominant mechanism in the evolution of the pluton. The Longtan granodiorites and granites are petrologically and geochemically similar to typical Indosinian varieties and are considered to have been produced in a similar manner. The Indosinian granitoids in the region show a magmatic peak age of ~238 Ma from the Yunkai-Nanling belt in the southeast and a magmatic peak age of ~218 Ma of the Xuefengshan belt to the northwest. These early and late magmatic episodes of the Indosinian granitoids also display slight variations of regular compositions, εNd(t) values and T DM ages. Thus, we propose a syncollisional extension model that Indosinian granitoids were generated by decompressional partial melting of crustal materials triggered by two extensions during collision of the Indochina and South China blocks. The Longtan pluton in the northwesternmost part of the orogenic belt marks the termination of the Indosinian magmatism and orogenesis.

Kryza, Ryszard; Schaltegger, Urs; Oberc-Dziedzic, Teresa; Pin, Christian; Ovtcharova, Maria

doi: 10.1007/s00531-013-0995-0pmid: N/A

Resolving time differences between successive magmatic pulses in composite granitoid plutons is often a difficult task. High-precision CA-ID-TIMS zircon ages obtained from such a pluton, the Variscan Karkonosze Granite (NE part of the Bohemian Massif), provide evidence that the crystallization of the two main granite facies, porphyritic and equigranular, happened between 312.5 ± 0.3 and 312.2 ± 0.3 Ma, thus unresolvable at the 0.08–0.1 % precision level of a single 206Pb/238U age. This finding is at odds with most other previous dating attempts and asks for a re-evaluation of the previous scattered geochronological data. The main reasons for the scatter of the earlier dates obtained by various techniques can include analytical causes, the presence of older inheritance and disturbance of the U–Pb isotopic system, due to zircon metamictization (enhanced by high-U content in zircon) or late- and post-magmatic alteration.

Biehl, B.; Reuning, L.; Strozyk, F.; Kukla, P.

doi: 10.1007/s00531-014-0999-4pmid: N/A

From salt mine galleries and well data it is known that thick rock salt layers can contain anhydrite and carbonate layers with thicknesses on the millimetre to tens of metre scale. The relatively thick Zechstein 3 anhydrite–carbonate layer in the northern Netherlands has been studied previously using 3-D seismic data. Observations from geophysical well logs in this study reveal the presence of thin sulphate layers on the sub-seismic scale imbedded in the Zechstein 2 (Z2) salt. Core samples, thin sections, seismic data and geochemical measurements were used to determine the mineralogy and origin of one of these Z2 sulphate layers. Bromine analyses show that they mark a freshening event in the Z2 salt, which can be correlated over large distances in the northern Netherlands. Their core-calibrated log signature indicates that the Z2 sulphate layers consist either of pure anhydrite or of anhydrite and polyhalite. The mineralogy and thickness of the sulphate layers are interpreted to vary between synsedimentary morphologic lows (thin anhydrite–polyhalite couplets) and highs (thicker anhydrite layers). Such a combination of core observations and well log analysis is a powerful tool to detect lateral trends in evaporite mineralogy and to reconstruct the environmental setting of their formation. Salt internal geometries can further be used to distinguish between different deformation mechanisms. In our study area, the distribution of sulphate layers within the Z2 salt indicates that subjacent salt dissolution was not the dominant process leading to salt-related deformation.

Qiu, Jun-Ting; Yu, Xin-Qi; Santosh, M.; Li, Peng-Ju; Zhang, De-Hui; Xiong, Guang-Qiang; Zhang, Bin-Yuan

doi: 10.1007/s00531-014-0997-6pmid: N/A

The granitoids and related polymetallic mineralization in the Zhejiang Province at the southeast margin of the Yangtze Block in China provide an important window to evaluate metallogeny associated with convergent margin magmatism. Here, we present geochronological, geochemical, and isotopic data from the granitic rocks of west Zhejiang, to constrain the timing of transformation of the tectonic setting of this region from volcanic arc to intra-plate during Late Mesozoic and its bearing on regional metallogeny. The granitic rocks in west Zhejiang can be geochemically subdivided into two groups. The first group is characterized by relatively steep rare earth element (REE) patterns with slight Eu anomalies, high Sr, low Yb, and negative Nb–Ta–Ti (NTT) anomalies, indicating a volcanic arc environment with a thickened crust in a convergent setting. The second group is featured by flat REE patterns with prominent negative Eu anomalies, low Sr, high Yb, and weak NTT anomalies, suggesting an intra-plate extensional environment with a thin crust. The geochronology of granitic rocks in west Zhejiang, combined with ages of regional tectonic basins and nappe structures, constrains the timing of the tectonic transformation to be in the range from 150 to 140 Ma. Sr–Nd isotopic data and a positive correlation displayed by oxygen fugacity (fO2), and La/Sm and Ba/Th ratios (proxies of subducted sediments and slab dehydration fluids) suggest that the high oxygen fugacity is probably related to the melting of subducted sediments and slab dehydration. From 180 to 80 Ma, due to the increasing dip angle of the subducted Izanagi Plate, the volcanic arc belt migrated oceanward, leaving most of the interior of Zhejiang Province under an intra-plate environment where insufficient subducted components and upwelling mantle generated reduced magmas which were not favorable for Cu–Mo mineralization. Our model provides a plausible explanation for the absence of Cu–Mo porphyry deposits in the adjacent region of Zhejiang, Jiangxi, and Anhui provinces (Zhe-Gan-Wan region) after 140 Ma.

Liu, Dongdong; Guo, Zhaojie; Jolivet, Marc; Cheng, Feng; Song, Yan; Zhang, Ziya

doi: 10.1007/s00531-013-0994-1pmid: N/A

Situated in the southwest of the Central Asian Orogenic Belt (CAOB), the South Tian Shan (STS) Block is a key area for understanding the final accretion of the CAOB. A suite of volcanic rocks interbedded with continental sediments from the Xiaotikanlike Formation lies along the southwestern edge of the Tian Shan orogen. Laser-ablation-inductively coupled plasma-mass spectrometer U–Pb dating provided a crystallization age of 295.0 ± 2.8 Ma (MSWD = 1.3), suggesting an Early Permian magmatic event. The volcanic rocks show a variable composition, with dominant rhyolites and dacites, subordinate basaltic andesites and few basalts. The felsic rocks are enriched in K and exhibit remarkably negative anomalies in Ba, Sr, Eu, P and Ti. These anomalies associated with their high negative ε Nd(t) values and old Nd model ages suggest that they are most likely sourced from ancient lower crustal rocks. The mafic rocks are characterized by high Mg#, Cr, Ni contents, negative Nb, Ta anomalies and pronounced enrichment in light rare earth elements as well as mild enrichment in large-ion lithophile elements. The mafic rocks are thus inferred to derive from enriched subcontinental lithospheric mantle. The petrographic and geochemical characteristics of the Xiaotikanlike Formation volcanic rocks indicate that they were generated under a post-collisional regime. Therefore, the final collision between the Tarim Craton and the Kazakhstan–Yili terrane took place before Early Permian, most probably at Late Carboniferous. Differing from other tectonic units of the CAOB, the recycling of ancient lithospheric crust played a significant role in the continental growth of the STS Block.

Cornamusini, Gianluca; Pascucci, Vincenzo

doi: 10.1007/s00531-014-0998-5pmid: N/A

The Northern Tyrrhenian Sea is located on the collisional zone between the Alpine Corsica and the Northern Apennines and is a key area for gaining a better understanding of the complex relationships between these two systems. The knowledge of the wide offshore part of this zone, located between Corsica (France) and mainland Italy, is based primarily on the analysis of several seismic profiles tied to the outcropping geology and unpublished preliminary reports of few offshore wells. The here presented study of two offshore wells provides a revision of the sedimentology, biostratigraphy and petrography of the thick, mainly siliciclastic, Tertiary successions (about 3,600 m) composing the Elba–Pianosa Ridge (EPR), a structural/morphological high separating the Tuscan Shelf to the east from the Corsica Basin to the west. A comparison with similar deposits cropping out in the surrounding onshore areas (Northern Apennines, Corsica, Tuscan Archipelago, Piedmont Tertiary Basin) provides additional constraints for refinement of the complex geodynamic and regional setting in which the EPR evolved.

Masrouhi, Amara; Bellier, Olivier; Koyi, Hemin

doi: 10.1007/s00531-013-0992-3pmid: N/A

Detailed geologic mapping, structural analysis, field cross-sections, new dating based on planktonic foraminifera, in addition to gravity signature of Lorbeus diapir, are used to characterize polyphase salt diapirism. This study highlights the role of inherited faulting, which controls and influences the piercement efficiency and the style and geometry of the diapir; and also the localization of evaporite early ascent displaying diapiric growth during extension. Salt was extruded along the graben axis developed within extensional regional early Cretaceous tectonic associated with the North African passive margin evolution. Geologic data highlight reactive diapirism during Albian time (most extreme extension period) and passive diapirism during the late Cretaceous post-rift stage. Northeastern Maghreb salt province gives evidences that contractional deformations are not associated with significant diapirism. During shortening, the initial major graben deforms as complex anticlines where diapirs are squeezed and pinched from their feeding.