Spatial–temporal relationships of Mesozoic volcanic rocks in NE China: Constraints on tectonic overprinting and transformations between multiple tectonic regimes

LA-ICP-MS zircon U–Pb ages and geochemical data are presented for the Mesozoic volcanic rocks in northeast China, with the aim of determining the tectonic settings of the volcanism and constraining the timing of the overprinting and transformations between the Paleo-Asian Ocean, Mongol–Okhotsk, and circum-Pacific tectonic regimes. The new ages, together with other available age data from the literature, indicate that Mesozoic volcanism in NE China can be subdivided into six episodes: Late Triassic (228–201 Ma), Early–Middle Jurassic (190–173 Ma), Middle–Late Jurassic (166–155 Ma), early Early Cretaceous (145–138 Ma), late Early Cretaceous (133–106 Ma), and Late Cretaceous (97–88 Ma). The Late Triassic volcanic rocks occur in the Lesser Xing’an–Zhangguangcai Ranges, where the volcanic rocks are bimodal, and in the eastern Heilongjiang–Jilin provinces where the volcanics are A-type rhyolites, implying that they formed in an extensional environment after the final closure of the Paleo-Asian Ocean. The Early–Middle Jurassic (190–173 Ma) volcanic rocks, both in the Erguna Massif and the eastern Heilongjiang–Jilin provinces, belong chemically to the calc-alkaline series, implying an active continental margin setting. The volcanics in the Erguna Massif are related to the subduction of the Mongol–Okhotsk oceanic plate beneath the Massif, and those in the eastern Jilin–Heilongjiang provinces are related to the subduction of the Paleo-Pacific Plate beneath the Eurasian continent. The coeval bimodal volcanic rocks in the Lesser Xing’an–Zhangguangcai Ranges were probably formed under an extensional environment similar to a backarc setting of double-direction subduction. Volcanic rocks of Middle–Late Jurassic (155–166 Ma) and early Early Cretaceous (145–138 Ma) age only occur in the Great Xing’an Range and the northern Hebei and western Liaoning provinces (limited to the west of the Songliao Basin), and they belong chemically to high-K calc-alkaline series and A-type rhyolites, respectively. Combined with the regional unconformity and thrust structures in the northern Hebei and western Liaoning provinces, we conclude that these volcanics formed during a collapse or delamination of a thickened continental crust related to the evolution of the Mongol–Okhotsk suture belt. The late Early Cretaceous volcanic rocks, widely distributed in NE China, belong chemically to a low- to medium-K calc-alkaline series in the eastern Heilongjiang–Jilin provinces (i.e., the Eurasian continental margin), and to a bimodal volcanic rock association within both the Songliao Basin and the Great Xing’an Range. The volcanics in the eastern Heilongjiang–Jilin provinces formed in an active continental margin setting related to the subduction of the Paleo-Pacific Plate beneath the Eurasian continent, and the bimodal volcanics formed under an extensional environment related either to a backarc setting or to delamination of a thickened crust, or both. Late Cretaceous volcanics, limited to the eastern Heilongjiang–Jilin provinces and the eastern North China Craton (NCC), consist of calc-alkaline rocks in the eastern Heilongjiang–Jilin provinces and alkaline basalts in the eastern NCC, suggesting that the former originated during subduction of the Paleo-Pacific Plate beneath the Eurasian continent, whereas the latter formed in an extensional environment similar to a backarc setting. Taking all this into account, we conclude that (1) the transformation from the Paleo-Asian Ocean regime to the circum-Pacific tectonic regime happened during the Late Triassic to Early Jurassic; (2) the effect of the Mongol–Okhotsk suture belt on NE China was mainly in the Early Jurassic, Middle–Late Jurassic, and early Early Cretaceous; and (3) the late Early Cretaceous and Late Cretaceous volcanics can be attributed to the subduction of the Paleo-Pacific Plate beneath the Eurasian continent.     

Seismic structure of the Helan–Liupan–Ordos western margin tectonic belt in North-Central China and its geodynamic implications

We study high-resolution three-dimensional P-wave velocity (Vp) tomography and anisotropic structure of the crust and uppermost mantle under the Helan–Liupan–Ordos western margin tectonic belt in North-Central China using 13,506 high-quality P-wave arrival times from 2666 local earthquakes recorded by 87 seismic stations during 1980–2008. Our results show that prominent low-velocity (low-V) anomalies exist widely in the lower crust beneath the study region and the low-V zones extend to the uppermost mantle in some local areas, suggesting that the lower crust contains higher-temperature materials and fluids. The major fault zones, especially the large boundary faults of major tectonic units, are located at the edge portion of the low-V anomalies or transition zones between the low-V and high-V anomalies in the upper crust, whereas low-V anomalies are revealed in the lower crust under most of the faults. Most of large historical earthquakes are located in the boundary zones where P-wave velocity changes drastically in a short distance. Beneath the source zones of most of the large historical earthquakes, prominent low-V anomalies are visible in the lower crust. Significant P-wave azimuthal anisotropy is revealed in the study region, and the pattern of anisotropy in the upper crust is consistent with the surface geologic features. In the lower crust and uppermost mantle, the predominant fast velocity direction (FVD) is NNE–SSW under the Yinchuan Graben and NWW–SEE or NW–SE beneath the Corridor transitional zone, Qilian Orogenic Belt and Western Qinling Orogenic Belt, and the FVD is NE–SW under the eastern Qilian Orogenic Belt. The anisotropy in the lower crust may be caused by the lattice-preferred orientation of minerals, which may reflect the lower-crustal ductile flow with varied directions. The present results shed new light on the seismotectonics and geodynamic processes of the Qinghai–Tibetan Plateau and its northeastern margin.     

Jurassic–Cretaceous tectonic evolution of Southeast China: geochronological and geochemical constraints of Yanshanian granitoids

We report results of laser ablation inductively coupled plasma-mass spectrometry-based dating, as well as the analysis of bulk-rock major and trace elements, and Sr–Nd isotopes to address the genesis and tectonic settings of the Yanshanian granitoids in neighbouring sections of Zhejiang, Jiangxi, and Anhui provinces (the WZG region) within the Yangtze block. Geochronological results indicate that intense magmatic activity took place during Jurassic to Cretaceous time in the WZG region. Three episodes can be clearly distinguished by their bulk-rock geochemistry. (1) Early–Middle Jurassic granitoids (180–170 Ma) have high Sr and low Yb content, high ?_Nd(t) and low initial ⁸⁷Sr/⁸⁶Sr ratios, and weakly negative Eu anomalies. These granitoids are strongly enriched with LREE, Rb, K, and Th but are depleted of HREE, Nb, and Ta. (2) Late Jurassic to Early Cretaceous granitoids (165–140 Ma) have relatively low Sr and low Yb contents, as well as low ?_Nd(t) and high initial ⁸⁷Sr/⁸⁶Sr ratios, with characteristics similar to those of the Early–Middle Jurassic granitoids in terms of the rare earth element and trace element patterns. (3) Early Cretaceous granitoids (140–120 Ma) have extremely low Sr and high Yb concentrations, as well as high SiO₂ but low MgO, CaO, and Al₂O₃ content, with strong negative anomalies in Eu, Ba, Sr, P, and Ti. These characteristics indicate that the WZG Jurassic granitoids were related to northwestward subduction of the Izanagi plate, whereas the Early Cretaceous granitoids formed in a within-plate extensional setting. The time of transition between the two tectonic environments can be constrained to ～140 Ma. This tectonic transition may be attributed to progressive slab roll-back of the Izanagi plate. The presence of two A-type granite belts in the WZG region probably reflects lithospheric thinning. The NE trend of the A-type granite belts indicates that this extension in Southeast China was controlled by underflow of the Izanagi plate.     

Biogenic sedimentation factors of mineralization in the Neoproterozoic strata of the Baikal–Patom region

The formation environments of stratiform ore deposits in the Neoproterozoic Baikal–Patom region (BPR) have been considered. A model for the formation of the Sukhoi Log gold ore deposit in the Bodaibo zone has been put forward. The first stage is gold concentration by a chemolithotrophic bacterial community. Independently established facts suggest that bacterial communities may also have contributed to initial metal accumulation in the sediments of the Kholodnaya Pb–Zn deposit. The ore beds occur in the high-carbon sediments of the side and trough of a back-arc basin. Sedimentation (Dal’nyaya Taiga and Zhuya regional horizons) took place during the “back-arc basin–foreland basin” transition. This transition is characterized by increased sediment bioproductivity, which is clearly evidenced from the increased biophile-element content and taxonomic diversity of organic remains. Hundreds of microfossil sites in the Neoproterozoic BPR host littoral benthos (cyanobacteria and brown algae) and plankton (green algae). Most microfossils in the outer shelf, on the basin side, and in its trough belong to chemolithotrophic bacteria. These bacteria are assumed to have accumulated metals in the vent field of the back-arc basin. Studies showed the ability of microorganisms (bacteria, algae, fungi, etc.) to accumulate Fe, Mn, Au, Pb, Zn, and other metals. Bacterial communities are particularly important for metal accumulation in the vent fields of rift zones and areas of arc volcanism. All these conditions were observed in the Neoproterozoic BPR.     

Sedimentary characteristics of Cenozoic strata in central-southern Ningxia,NW China: Implications for the evolution of the NE Qinghai–Tibetan Plateau

Cenozoic sedimentary deposits in central-southern Ningxia province, NW China are an important record of Tertiary tectonic events along the evolving Qinghai–Tibetan Plateau’s northeast margin. Shortly after the onset of the Indo-Eurasia collision to the south, a thrust belt and adjoining foreland basin began to form during 40–30 Ma. The Eocene Sikouzi Formation developed in a distal setting to this basin, in normal fault-bound basins that may have formed in a forebulge setting. Subsequent deposition of the Oligocene Qingshuiying Formation occurred during a phase of apparently less intense tectonism and the previous underfilled foreland basin became overfilled. During the Early Miocene, contractional deformation was mainly distributed to the west of the Liupan Shan. This resulted in deformation of the Qingshuiying Formation as indicated by an unconformity with the overlying Miocene Hongliugou Formation. The unconformity occurs proximal to the Haiyuan Fault suggesting that the Haiyuan Fault may have begun movement in the Early Miocene. In the Late Miocene, thrusting occurred west of the southern Helan Shan and an unconformity developed between the Hongliugou and Qingshuiying Formations proximal to the the Cha-Gu Fault. Relationships between the Miocene stratigraphy and major faults in the region imply that during the Late Miocene the deformation front of the Qinghai–Tibetan Plateau had migrated to the Cha-Gu Fault along the western Ordos Margin, and the Xiang Shan was uplifted. Central-southern Ningxia was then incorporated into the northeast propagating thrust wedge. The driving force for NE propagation of the thrust wedge was most likely pronounced uplift of the northeastern plateau at the same time. Analysis of the sedimentary record coupled with consideration of the topographic evolution of the region suggests that the evolving fold-and-thrust belt experienced both forward-breaking fold-and-thrust belt development, and out-of-sequence fault displacements as the thrust wedge evolved and the foreland basin became compartmentalised. The documented sedimentary facies and structural relationship also place constraints on the Miocene-Recent evolution of the Yellow River and its tributaries.     

The Neoproterozoic–Early Paleozoic tectonic evolution of the South China Block: An overview

This paper gives a brief review of what I consider as the state of the art regarding the largely accepted data and ideas concerning the Proterozoic to Early Paleozoic tectonic evolution of South China. The South China craton was built by the welding of the Yangtze and Cathaysia blocks, with a different previous history giving a different pre-Neoproterozoic basement composition, due to the Jiangnan (Jinning, Sibao) orogeny. This Jiangnan orogeny was a collisional event, induced by the consumption of an intervening oceanic domain by subduction beneath the Yangzte plate. The evolution involved a volcanic arc on the Yangtze active margin, active from ca. 980 Ma to ca. 850 Ma, the subsequent collision beginning at around 870–860 Ma and responsible for the emplacement of thrust sheets of ophiolitic mélange (dated around 1000–900 Ma) and blueschists (900–870 Ma), followed by late- to post-collisional granitic plutonism (840–800 Ma). The newly amalgamated South China craton suffered from rifting, starting around 850 Ma, marked by mafic–ultramafic magmatism until ca. 750 Ma. The Nanhua rift basin evolved with a thick sedimentation in its middle part until the Ordovician. South China was affected by the Early Paleozoic orogeny (mainly Silurian), characterized by a strong quasi-symmetrical intracontinental shortening, involving the sedimentary cover of the rift and its margins as well as the basement, leading to crustal thickening. This crustal thickening induced an important anatexis and emplacement of peraluminous granites during the Silurian. Unlike the Jiangnan orogeny, which was of collisional type, the Early Paleozoic one was a bit similar to a Pyrenean intracontinental type.Some pending problems need further research for clarification, for example: the location and timing of integration of South China within Rodinia, the triggering factor of the Early Paleozoic orogeny, the mapping of the contacts bounding the Lower Paleozoic thrust sheets responsible for the crustal thickening.     

Regional sandstone-type uranium mineralization rooted in Oligo–Miocene tectonic inversion in the Songliao Basin,NE China

The tectonic inversion of the Songliao Basin during the Oligo–Miocene may have played an important role in controlling the development of sandstone-type uranium deposits (SUDs). Here we investigate drill holes along a southeast to northwest section in this basin based on apatite fission-track (AFT) and zircon fission-track (ZFT) techniques. We present 50 data from 15 deep boreholes at different depths between 665 and 3956 m and different structural units including grabens and horsts formed in the Early Cretaceous beneath the basin. The results of the effective AFT ages are 100 ± 11 to 2.3 ± 0.4 Ma (P(x²) > 5%) and ZFT ages are 97.5–20.4 Ma (including binomial peak ages). These results reveal that the basin underwent two distinct stages of rapid cooling after Late Cretaceous. In the first stage, during the Late Cretaceous–Early Paleogene (~80–50 Ma), tectonic uplift occurred in all of the structural units including grabens and horsts, which was marked by an unconformity between the latest Cretaceous Mingshui and the Eocene Yi'an formations. In the second stage, during the Oligo–Miocene (~40–10 Ma), tectonic uplift occurred mainly in the grabens but not in the horsts, corresponding with a few sediments of the Neogene Da'an and Taikang formations. We propose that the folds and the thrust faults mostly characterize in the second stage indicating a major tectonic inversion in the basin. The shifting of the two stages was probably in response to differences in the subduction angles and directions of motion of the Paleo-Pacific Plate from the southeast. Combined with previous information, it was demonstrated that most of the U mineralization ages are younger than 40 Ma, with a peak in the Miocene or later (<20 Ma). We thus propose that the SUDs have been redistributed and redeposited locally in successive stages during and after the Oligo–Miocene tectonic inversion.     

Petrogenesis and tectonic implications of early Devonian mafic dike–granite association in the northern West Junggar,NW China

Mafic dike–granite associations are common in extensional tectonic settings and important and pivotal in reconstructing crust–mantle geodynamic processes. We report results of zircon U–Pb and hornblende ⁴⁰Ar-³⁹Ar ages and major-element and trace-element data for mafic dike–granite association from the northern West Junggar, in order to constrain their ages, petrogenesis, and geodynamic process. The mafic dike–granite association was emplaced in the early Devonian. The Xiemisitai monzogranites have high SiO₂ contents and low MgO, Cr, and Ni concentrations, suggesting that they were mainly derived from crustal sources and were probably generated by partial melt of the juvenile mid-lower crust. The mafic dikes have low Mg^# and Cr and Ni abundances, suggesting that they have experienced significant fractional crystallization. The Xiemisitai mafic dikes contain hornblende and biotite and display negative Nb–Ta–Ti anomalies, enrichment of LREEs and LILEs, and depletion of HREEs and HFSEs, consistent with an origin from a lithospheric mantle metasomatized by subducted slab-derived fluids. In addition, the Xiemisitai mafic dikes are plotted within melting trends with little to no garnet (Cpx: Grt = 6:1) in their source. The La/Yb versus Tb/Yb plot also indicates the presence of less than 1% residual garnet in the source region for the Xiemisitai mafic dikes. Therefore, it can be inferred that the Xiemisitai mafic dikes were generated at a correspondingly shallow depth, mostly within the spinel stability field. The Xiemisitai mafic dikes were most probably generated by the partial melting of the metasomatized lithospheric mantle at relatively shallow depths (<80 km). The Xiemisitai mafic dike–granite association could have been triggered by asthenospheric upwelling as a result of the rollback of the subducted Irtysh–Zaysan oceanic lithosphere.     

Zircon U–Pb ages and tectonic implications of Paleozoic plutons in northern West Junggar,North Xinjiang,China

North Xinjiang, Northwest China, is made up of several Paleozoic orogens. From north to south these are the Chinese Altai, Junggar, and Tian Shan. It is characterized by widespread development of Late Carboniferous–Permian granitoids, which are commonly accepted as the products of post-collisional magmatism. Except for the Chinese Altai, East Junggar, and Tian Shan, little is known about the Devonian and older granitoids in the West Junggar, leading to an incomplete understanding of its Paleozoic tectonic history. New SHRIMP and LA-ICP-MS zircon U–Pb ages were determined for seventeen plutons in northern West Junggar and these ages confirm the presence of Late Silurian–Early Devonian plutons in the West Junggar. New age data, combined with those available from the literature, help us distinguish three groups of plutons in northern West Junggar. The first is represented by Late Silurian–Early Devonian (ca. 422 to 405 Ma) plutons in the EW-striking Xiemisitai and Saier Mountains, including A-type granite with aegirine–augite and arfvedsonite, and associated diorite, K-feldspar granite, and subvolcanic rocks. The second is composed of the Early Carboniferous (ca. 346 to 321 Ma) granodiorite, diorite, and monzonitic and K-feldspar granites, which mainly occur in the EW-extending Tarbgatay and Saur (also spelled as Sawuer in Chinese) Mountains. The third is mainly characterized by the latest Late Carboniferous–Middle Permian (ca. 304 to 263 Ma) granitoids in the Wuerkashier, Tarbgatay, and Saur Mountains.As a whole, the three epochs of plutons in northern West Junggar have different implications for tectonic evolution. The volcano-sedimentary strata in the Xiemisitai and Saier Mountains may not be Middle and Late Devonian as suggested previously because they are crosscut by the Late Silurian–Early Devonian plutons. Therefore, they are probably the eastern extension of the Early Paleozoic Boshchekul–Chingiz volcanic arc of East Kazakhstan in China. It is uncertain at present if these plutons might have been generated in either a subduction or post-collisional setting. The early Carboniferous plutons in the Tarbgatay and Saur Mountains may be part of the Late Paleozoic Zharma–Saur volcanic arc of the Kazakhstan block. They occur along the active margin of the Kazakhstan block, and their generation may be related to southward subduction of the Irtysh–Zaysan Ocean between Kazakhstan in the south and Altai in the north. The latest Late Carboniferous–Middle Permian plutons occur in the Zharma–Saur volcanic arc, Hebukesaier Depression, and the West Junggar accretionary complexes and significantly postdate the closure of the Irtysh–Zaysan Ocean in the Late Carboniferous because they are concurrent with the stitching plutons crosscutting the Irtysh–Zaysan suture zone. Hence the latest Late Carboniferous–Middle Permian plutons were generated in a post-collisional setting. The oldest stitching plutons in the Irtysh–Zaysan suture zone are coeval with those in northern West Junggar, together they place an upper age bound for the final amalgamation of the Altai and Kazakhstan blocks to be earlier than 307 Ma (before the Kaslmovian stage, Late Carboniferous). This is nearly coincident with widespread post-collisional granitoid plutons in North Xinjiang.     

The late Mesozoic–Cenozoic tectonic evolution of the South China Sea: A petrologic perspective

This paper presents a review of available petrological, geochonological and geochemical data for late Mesozoic to Recent igneous rocks in the South China Sea (SCS) and adjacent regions and a discussion of their petrogeneses and tectonic implications. The integration of these data with available geophysical and other geologic information led to the following tectono-magmatic model for the evolution of the SCS region. The geochemical characteristics of late Mesozoic granitic rocks in the Pearl River Mouth Basin (PRMB), micro-blocks in the SCS, the offshore continental shelf and Dalat zone in southern Vietnam, and the Schwaner Mountains in West Kalimantan, Borneo indicate that these are mainly I-type granites plus a small amount of S-type granites in the PRMB. These granitoids were formed in a continental arc tectonic setting, consistent with the ideas proposed by Holloway (1982) and Taylor and Hayes, 1980, Taylor and Hayes, 1983, that there existed an Andean-type volcanic arc during later Mesozoic era in the SCS region. The geochonological and geochemical characteristics of the volcanics indicate an early period of bimodal volcanism (60–43 Ma or 32 Ma) at the northern margin of the SCS, followed by a period of relatively passive style volcanism during Cenozoic seafloor spreading (37 or 30–16 Ma) within the SCS, and post-spreading volcanism (tholeiitic series at 17–8 Ma, followed by alkali series from 8 Ma to present) in the entire SCS region. The geodynamic setting of the earlier volcanics was an extensional regime, which resulted from the collision between India and Eurasian plates since the earliest Cenozoic, and that of the post-spreading volcanics may be related to mantle plume magmatism in Hainan Island. In addition, the nascent Hainan plume may have played a significant role in the extension along the northern margin and seafloor spreading in the SCS.     

Provenance analyses of early Mesozoic sediments in the Ningwu basin: Implications for the tectonic–palaeogeographic evolution of the northcentral North China Craton

This paper reports results from detrital zircon U–Pb geochronology, Hf isotopic geochemistry, sandstone modal analysis, and palaeocurrent analysis of the early Mesozoic strata within the Ningwu basin, China, with the aims of constraining the depositional ages and sedimentary provenances and shedding new light on the Mesozoic tectonic evolution of the northcentral North China Craton (NCC). The zircons from early Mesozoic sandstones are characterized by three major populations: Phanerozoic (late Palaeozoic and early Mesozoic), late Palaeoproterozoic (with a peak at approximately 1.8 Ga), and Neoarchaean (with a peak at approximately 2.5 Ga). Notably, three Phanerozoic zircons in the Early Triassic Liujiagou Formation were found to have positive ε_Hf(t) values and characteristics typical of zircons from the Central Asian Orogenic Belt (CAOB). Therefore, the CAOB began to represent the provenance of sediment in the sedimentary basins in the northern NCC no later than the Early Triassic (261 Ma), implying that the final amalgamation of the NCC and CAOB occurred before the Early Triassic. The U–Pb geochronologic and Hf isotopic results show that the Lower Middle Triassic sediments were mainly sourced from the Yinshan–Yanshan Orogenic Belt (YYOB), and that a sudden change in provenances occurred, shifting from a mixed YYOB and CAOB source in the Middle Jurassic to a primarily YYOB source in the Late Jurassic. The results of the sandstone modal analysis suggest that the majority of the samples from the Lower Middle Jurassic rocks were derived from either Continental Block or Recycled Orogen sources, whereas all the samples from the Upper Jurassic rocks were derived from Mixed sources. The change in source might be ascribed to the southward subduction and closure of the Okhotsk Ocean and the resulting intense uplift of the YYOB during the Late Jurassic. This uplift likely represents the start of the Yanshan Orogeny.     

Geochemistry,geochronology and Sr–Nd–Pb–Hf isotopic compositions of Middle to Late Jurassic syenite–granodiorites–dacite in South China: Petrogenesis and tectonic implications

In situ zircon U–Pb ages and Hf isotope data, major and trace elements and Sr–Nd–Pb isotopic compositions are reported for coeval syenite–granodiorites–dacite association in South China. The shoshonitic syenites are characterized by high K₂O contents (5.9–6.1 wt.%) and K₂O/Na₂O ratios (1.1–1.2), negative Eu anomalies (Eu/Eu* = 0.65 to 0.77), enrichments of Rb, K, Nb, Ta, Zr and Hf, but depletion of Sr, P and Ti. The adakitic granodiorite and granodiorite porphyry intrusions are characterized by high Al₂O₃ contents (15.0–16.8 wt.%), enrichment in light rare earth elements (LREEs), strongly fractionated LREEs (light rare earth elements) to HREEs (heavy rare earth elements), high Sr (438–629 ppm), Sr/Y (29.2–53.6), and low Y (11.7–16.8 ppm) and HREE contents (e.g., Yb = 1.29–1.64 ppm). The calc-alkaline dacites are characterized by LREE enrichment, absence of negative Eu anomalies, and enrichment of LILEs such as Rb, Ba, Th, U and Pb, and depletion of HFSEs such as Nb, Ta, P and Ti.Geochemical and Sr–Nd–Hf isotopic compositions of the syenites suggest that the shoshonitic magmas were differentiated from parental shoshonitic melts by fractional crystallization of olivine, clinopyroxene and feldspar. The parent magmas may have originated from partial melting of the lithospheric mantle with small amount contribution from crustal materials. The adakitic granodiorite and granodiorite porphyry have Sr–Nd–Pb isotopic compositions that are comparable to that of the mafic lower crust. They have low Mg# and MgO, Ni and Cr contents, abundant inherited zircons, low ε_Nd(t) and ε_Hf(t) values as well as old whole-rock Nd and zircon Hf model ages. These granodiorites were likely generated by partial melting of Triassic underplated mafic lower crust. The Hf isotopic compositions of the dacites are relatively more depleted than the Cathaysia enriched mantle, suggesting those magmas were derived from the partial melting of subduction-modified mantle sources. The coeval shoshonitic, high-K calc-alkaline and calc-alkaline rocks in Middle to Late Jurassic appear to be associated with an Andean-type subduction. This subduction could have resulted in the upwelling of the asthenosphere beneath the Cathaysia Block, which induced partial melting of the mantle as well as the mafic lower crust, and formed an arc regime in the coastal South China during Middle to Late Jurassic.     

40Ar–39Ar age and geochemistry of subduction-related mafic dikes in northern Tibet,China: petrogenesis and tectonic implications

The early Permian Xiaomiao mafic dike swarm in the East Kunlun orogenic belt (EKOB) provides an excellent opportunity to study the petrogenesis of such swarms developed in supra-subduction zone environments, and to investigate the early plate tectonic history of the Palaeo-Tethyan Ocean. Hornblende ⁴⁰Ar–³⁹Ar dating results indicate that the mafic dikes formed in the early Permian (277.76 ± 2.72 Ma). The Xiaomiao mafic hypabyssals have the following compositional range: SiO₂ = 46.55–55.75%, MgO = 2.80–7.38%, Mg^# = 36–61, and (Na₂O + K₂O) = 2.87–4.95%. Chemically, they display calc-alkali affinities, ranging in composition from gabbro to gabbroic diorite. All analysed dikes are enriched in light rare earth elements and large-ion lithophile elements (e.g. Rb and Ba), but are depleted in heavy rare earth elements and high field strength elements (e.g. Nb, Ta, and Ti). Their I_Sr and ?_Nd(t) values range from 0.707 to 0.715 and –2.60 to +2.91, respectively. They are geochemically similar to subduction-related basaltic rocks (e.g. island arc basalt), but differ from E-MORB and N-MORB. Petrographic and major element data reveal that fractional crystallizations of clinopyroxene, olivine, hornblende, and Fe–Ti oxides may have occurred during magma evolution, but that crustal contamination was minor. Based on geochemical and Sr–Nd isotopic bulk-rock compositions, we suggest that the mafic dikes were likely generated by 10–20% partial melting of a spinel + minor garnet lherzolite mantle source metasomatized by subducted, slab-derived fluids, and minor sediments. Based on our results, we propose that the early evolution of the Palaeo-Tethyan Ocean involved the spreading and initial subduction of the Carboniferous to early Permian ocean basin followed by late Permian subduction, which generated the magmatic arc.     

Structure and tectonic evolution of the Late Jurassic–Early Cretaceous Wandashan accretionary complex,NE China

The Late Jurassic–Early Cretaceous Wandashan accretionary complex (AC) in NE China is a key region for constraining the subduction and accretion of the Palaeo-Pacific Ocean; however, the protoliths and structure of the region remain poorly understood, resulting in debates regarding crustal growth mechanisms and subduction-related accretionary processes in Northeast China. In this contribution, we integrate detailed field observations, ocean plate stratigraphy (OPS) reconstruction, and associated geological data to determine the structure and tectonic evolution of the Wandashan AC. The Wandashan AC formed through the progressive incorporation of OPS units along an oceanic trench. The observed OPS comprises, in ascending order, Permian basalt and limestone, Middle Triassic–Early Jurassic chert, Middle Jurassic siliceous shale and mudstone, and Late Jurassic–Early Cretaceous turbidite. Numerous NNE–SSW-striking thrust faults have segmented the OPS into a series of bedding-parallel tectonic slices that were successively thrust over the Jiamusi massif along a basal thrust (the Yuejinshan Fault), producing a large-scale imbricate thrust system. The Wandashan AC underwent oceanward accretion via multiple deformational processes. The OPS units were detached and rearranged along or within a decollement through offscraping, underplating, thrusting, and duplexing. The units were then emplaced over the Jiamusi massif along the basal thrust. The timing of accretion and thrusting is constrained to the latest Middle Jurassic to earliest Early Cretaceous (ca. 167–131 Ma). Reconstructed accretion-related structural lines within the Wandashan AC trend dominantly NE–SW, close to the direction of Jurassic extension at the eastern Asian continental margin. Large-scale left-lateral strike-slip movement on the Dunmi Fault during the late Early Cretaceous resulted in the folding of structural lines within the Wandashan AC, producing their present-day westward-convex orientation.     

P–T evolution and tectonic significance of lawsonite-bearing schists from the eastern segment of the southwestern Tianshan,China

Robust quantification of pressure (P)–temperature (T) paths for subduction-related HP/UHP metamorphic rocks is fundamental in recognizing spatial changes in both the depth of detachment from the down-going plate and the thermal evolution of convergent margin sutures in orogenic belts. Although the Chinese southwestern (SW) Tianshan is a well-known example of an accretionary metamorphic belt in which HP/UHP metabasites occur in voluminous host metasedimentary schists, information about the P–T evolution of these rocks in the eastern segment is limited, precluding a full understanding of the development of the belt as a whole. In this study at Kekesu in the eastern segment of the SW Tianshan, we use microstructural evidence and phase equilibrium modelling to quantify the peak and retrograde P–T conditions from two lawsonite-bearing micaschists and an enclosed garnet–epidote blueschist; for two of the samples we also constrain the late prograde P–T path. In the two micaschist samples, relics of prograde lawsonite are preserved in quartz inclusions in garnet, whereas in the metabasite, polymineralic aggregates included in garnet are interpreted as pseudomorphs after lawsonite. For garnet micaschist TK21, which is mainly composed of garnet, phengite/paragonite, albite, chlorite, quartz and relict lawsonite, with accessary rutile, titanite and ilmenite, the maximum P–T conditions for the peak stage are 18.0–19.0 kbar at 480–485°C. During initial exhumation, the retrograde P–T path passed through metamorphic conditions of 15.0–17.0 kbar at 460–500°C. For garnet–glaucophane micaschist TK33, which is mainly composed of garnet, glaucophane, phengite/paragonite, albite, chlorite, quartz, relict lawsonite and minor epidote, with accessary titanite, apatite, ilmenite and zircon, the maximum P conditions for the peak stage are >24.0 kbar at 400–500°C. During exhumation, the P–T path passed through metamorphic conditions of 17.5–18.5 kbar at 485–495°C and 14.0–17.5 kbar at 460–500°C. For garnet–epidote blueschist TK37, which is mainly composed of garnet, glaucophane, epidote, phengite, chlorite, albite and quartz, with accessary titanite, apatite, ilmenite, zircon and calcite, the prograde evolution passed through metamorphic conditions of ~20.0 kbar at ~445°C to P_max conditions of ~21.5 kbar at 450–460°C and T_max conditions of 19.5–21.0 kbar at 490–520°C. During exhumation, the rock passed through metamorphic conditions of 17.5–19.0 kbar at 475–500°C, before recording P–T conditions of <17.5 kbar at <500°C. These results demonstrate that maximum recorded pressures for individual samples vary by as much as 6 kbar in the eastern segment of the SW Tianshan, which may suggest exhumation from different depths in the subduction channel. Furthermore, the three samples record similar P–T paths from ~17.0 to 15.0 kbar, which suggests they were juxtaposed at a similar depth along the subduction interface. We compare our new results with published information from eclogites in the same area before considering the wider implications of these data for the orogenic development of the belt as a whole.     

Source and possible tectonic driver for Jurassic–Cretaceous gold deposits in the West Qinling Orogen,China

The West Qinling Orogen (WQO) in Central China Orogenic Belt contains numerous metasedimentary rock-hosted gold deposits (>2000 t Au), which mainly formed during two pulses: one previously recognized in the Late Triassic to Early Jurassic (T3–J1) and one only recently identified in the Late Jurassic to Early Cretaceous (J3–K1). Few studies have focused on the origin and geotectonic setting of the J3–K1 gold deposits.Textural relationships, LA-ICP-MS trace element and sulfur isotope compositions of pyrites in hydrothermally altered T3 dykes within the J3–K1 Daqiao deposit were used to constrain relative timing relationships between mineralization and pyrite growth in the dykes, and to characterize the source of ore fluid. These results are integrated with an overview of the regional geodynamic setting, to advance understanding of the tectonic driver for J3–K1 hydrothermal gold systems. Pyrite in breccia- and dyke-hosted gold ores at Daqiao have similar chemical and isotopic compositions and are considered to be representative of J3–K1 gold deposits in WQO. Co/Ni and sulfur isotope ratios suggest that ore fluids were derived from underlying Paleozoic Ni- and Se-rich carbonaceous sedimentary rocks. The geochemical data do not support the involvement of magmatic fluids. However, in the EQO (East Qinling Orogen), J3–K1 deposits are genetically related to magmatism. Gold mineralization in WQO is contemporaneous with magmatic deposits in the EQO and both are mainly controlled by NE- and EW-trending structures produced by changes in plate motion of the Paleo-Pacific plate as it was subducted beneath the Eurasian continent. We therefore infer that the J3–K1 structural regime facilitated the ascent of magma in the EQO and metamorphic fluids in the WQO with consequent differences in the character of contemporaneous ore deposits. If this is correct, then the far-field effects of subduction along the eastern margin of NE Asia extended 1000's of km into the continental interior.     

Geodynamics and stress–strain patterns in different tectonic settings

Deformation patterns in subduction zones, feeder systems of volcanoes, and rifts are compared and investigated in terms of relations among elastoplastic strain, rheology, pore fluids, and temperature. Regional-scale subduction processes have been explored in segments of the Kuriles–Kamchatka, Izu-Bonin, and Mariana zones. Slab geometry constraints from the 3D velocity structure are used to model the balance of forces in the three subduction zones and to distinguish the regions of predominant push or pull. Stress and strain variations in suprasubduction crust are considered for the case of magma sources beneath the Klyuchevskoy group of volcanoes. Time-lapse (4D) seismic tomography shows crustal magma reservoirs to appear and disappear rapidly as the volcanoes become active or dormant, respectively. This behavior is due to rapid strain changes which cause fast flow of fluids and the ensuing decrease or increase of melting temperature in the magma reservoirs. In addition to subduction zones, stress–strain patterns are modeled for collisional (compressive) settings, with the example of the Altai–Sayan area and the Caucasus, and for the conditions of rifting (extension), in the case of the Vilyui basin. As the modeling shows, formation of a superdeep basin does not necessarily require the crust to stretch twice or more: only 20% stretching in the necking region is enough to produce a 10–15 km deep basin.     

Evolution process of the Late Silurian–Late Devonian tectonic environment in Qimantagh in the western portion of east Kunlun,China: Evidence from the geochronology and geochemistry of granitoids

Journal of Earth System Science - The East Kunlun Orogenic Belt has undergone a composite orogenic process consisting of multiple orogenic cycles and involving many types of magmatic rocks spread...