Eastern Yangtze Block Research Articles

Sedimentary characteristics and evolution of the Upper Permian Wujiaping Formation in the eastern Yangtze Block

The Upper Permian Wujiaping (WJP) Formation in the Yangtze Block (Southern China) has great potential for shale gas exploration. However, the sedimentary characteristics and environmental evolution of the WJP Formation are poorly understood. Based on 260 hand specimen samples obtained from 18 sections and 1 borehole in the eastern Yangtze Block, petrographic observations reveal that the WJP Formation is composed of 11 rock types. Combined with analyses of geochemical compositions, the sedimentary facies belts of the WJP Formation are divided into shoreland, shallow-water shelf, deep-water shelf shoal, restricted platform, and open platform. Bauxite was developed in the shoreland at the bottom of the WJP Formation, which is attributed to the Dongwu movement. The formation of paleo-uplift exerted a significant effect on paleogeomorphology and led to a major change in the sedimentary model from the underlying stratum to the WJP Formation. During the lower WJP Formation sedimentary period, gradual transgression occurred in the eastern Yangtze Block, and the lithology was mainly composed of clastic rocks. Shoreland, shallow-water shelf, and deep-water shelf were developed northeastward in turn. In the period of the upper WJP Formation, due to durative transgression, the area of the shoreland reduced southwestward. The lithology converted from clastic rocks into carbonates, and the restricted platform and open platform were developed northeastward. A few patched shoals formed in the restricted platform.

Formation of the Banxi Sb deposit in Eastern Yangtze Block: Evidence from individual fluid inclusion analyses, trace element chemistry, and He-Ar-S isotopes

The Banxi Sb deposit (>100,000 t Sb metal with a grade of 0.02%–64.5% Sb (typically 15.3%–25.9% Sb) in the Eastern Yangtze Block is hosted in Neoproterozoic low-grade metamorphic clastic rocks of the Banxi Group. The deposit is characterized by vein-type Sb-only mineralization. Analyses of individual quartz-hosted fluid inclusions obtained by LA-ICP-MS identified Na as the most abundant cation (thousands to tens of thousands of ppm) in the ore-forming fluids of the Banxi Sb deposit, together with variable contents of K, Ca, Al, As, Fe and Sb (tens to thousands of ppm). Multiple chemical criteria, including high Na+/K+ (typically 3–15) and low Mn2+/Na+ (0.003–0.2) and Sr2+/Na+ (0.001–0.01) ratios of individual fluid inclusions, suggest that the fluids responsible for quartz and stibnite precipitation in the Banxi Sb deposit were likely dominated by basinal brines within low-grade metasedimentary rocks. Moreover, the extremely low 3He/4He ratios (0.001–0.048Ra) but relatively high 3He/36Ar (0.5 × 10−5–5.8 × 10−4) and 40Ar/36Ar ratios (323–1111 with an average of 737) of fluid inclusions trapped in ore-stage quartz further support a sedimentary origin of ore-forming fluids in this deposit. The positive and homogeneous sulfur isotopic values (δ34S = 8.0–9.5‰) of stibnite and arsenopyrite are comparable to those of sulfides in Proterozoic metasedimentary rocks (δ34S = 5.6–11.5‰), indicating these metasedimentary rocks may have served as the primary source of S for the Banxi Sb deposit. Combined with geological features and new trace element chemistry data for quartz and stibnite, we envisaged a new formation model for the Banxi Sb deposit that assumes that circulation of weakly alkaline basinal brines through faults under an extensional tectonic setting during the Early Cretaceous (∼130 Ma), mobilization of Sb and S from the underlying and host metasedimentary rocks and ascent of ore- forming fluids along faults, and ultimately precipitation of Sb minerals in or around fault/fracture zones as a result of the decreasing pH of the ore-forming fluids as well as sulfidation during fluid-rock reaction.

Nature and Evolution of Pre‐Neoproterozoic Continental Crust in South China: A Review and Tectonic Implications

AbstractSouth China as an amalgamation of the Yangtze and Cathaysia blocks is composed of Archean to Mesoproterozoic basement overlain by Neoproterozoic and younger cover. Both the constituent Yangtze and Cathaysia blocks contain well‐preserved Neoproterozoic rocks that have been extensively studied in terms of the age and tectonic nature, but less is known about their earlier crustal history due to the incomplete rock record. Recent efforts in investigating the yet survived crustal nature based on isotopic and elemental signatures preserved in igneous and sedimentary rocks have steadily improved our knowledge about the pre‐Neoproterozoic continental crustal evolution in South China. In this paper, we summarize the up‐to‐date pre‐Neoproterozoic records, including petrological, geochronological, geochemical and geophysical data, across South China, and discuss its spatiotemporal patterns of the pre‐Neoproterozoic crust and the relevant tectonic events. While the xenocrystic/inherited and detrital zircon records suggest widespread Archean (mainly ca. 2.5 Ga) crustal components within both the Yangtze and Cathaysia blocks, exposed Archean rocks are only limited to isolated crustal provinces in the Yangtze Block. These Archean rocks are dominated by TTGs (tonalite‐trondhjemite‐granodiorite) with varied ages (3.3–2.5 Ga) and zircon Hf isotopes, indicating a compositionally heterogeneous nature of the Archean Yangtze Block and, by inference, the development of multiple ancient terranes. The early Paleoproterozoic (2.4–2.2 Ga) tectonomagmatic events characterize the western Yangtze Block and are supportive of an east‐west subdivision of the Yangtze basement, whereas the late Paleoproterozoic (2.1–1.7 Ga) orogeneses may have affected a larger area covering both the western and eastern parts of the Yangtze Block, and also the Cathaysia Block. The eastern Yangtze Block with generally northeastward‐younging late Paleoproterozoic magmatism and metamorphism likely experienced a prolonged 2.05–1.75 Ga orogenic process welding the various Archean proto‐continents, consistent with the documentation of a buried late Paleoproterozoic orogenic belt imaged by deep seismic profiling from its central part and of a slightly older ophiolitic mélange in the northern part. The Cathaysia Block was probably involved in a short‐lived 1.9–1.8 Ga orogenic event. The two orogeneses overlapped in time and may have contributed to the cratonization of a possible unified South China, and are referred to be linked with the assembly of the Nuna Supercontinent. The subsequent late Paleoproterozoic to early Mesoproterozoic rift successions and intrusions (1.7–1.5 Ga) in the southwestern Yangtze Block, and the ca. 1.43 Ga rifting in Hainan Island of the Cathaysia Block could be responses to the Nuna break‐up. Late Mesoproterozoic (1.2–1.0 Ga) magmatism of varied age and nature in different localities of the Yangtze Block is reflective of a complex tectonic process in the context of the assembly of the Rodinia Supercontinent. Similar‐aged metamorphism (1.3–1.0 Ga) is recorded in Hainan Island, reflecting the Grenvillian continental collision during the Rodinia assembly, but further studies are necessary to better constrain the late Mesoproterozoic tectonic framework of South China.

Xenocrystic/inherited Precambrian zircons entrained within igneous rocks from eastern South China: Tracking unexposed ancient crust and implications for late Paleoproterozoic orogenesis

South China is characterized by widespread igneous rocks with varied ages and nature in its eastern part, which contain abundant Precambrian xenocrystic/inherited zircons that bear important information of the composition and evolution of the underlying ancient crust. This paper for the first time presents a compiled U-Pb age database of 1416 Precambrian xenocrystic/inherited zircons from igneous rocks in eastern South China, and attempts to provide a constraint on the Precambrian crustal evolution of the constituent Yangtze and Cathaysia blocks. These xenocrystic/inherited zircons, as a conceivable proxy of the unexposed continental crust, document three major tectonothermal events related to continental accretion and subsequent modification that possibly built the continental crust of the eastern Yangtze Block (EYB) at 2.70–2.40 Ga, 2.10–1.55 Ga, and 0.95–0.70 Ga, and the Cathaysia Block (CAB) at 2.70–2.40 Ga, 2.05–1.75 Ga, and 1.10–0.70 Ga, pointing to a complex Precambrian evolutionary history for South China. The EYB zircons are unexpectedly dominated by a 2.10–1.55 Ga age population that shows a multimodal distribution with peaks at 2.05 Ga, 2.0 Ga, 1.90 Ga, 1.85 Ga, and 1.58 Ga, and the CAB zircons are characterized by a unimodal 2.0–1.75 Ga age population that conspicuously peaked at 1.85 Ga, both of which overlap with the tenure of the Nuna supercontinent. These xenocrystic/inherited zircons from both blocks generally have negative εHf(t) values, which in combination with coeval regional magmatic and metamorphic records can assist to trace a possible prolonged (2.05–1.75 Ga) orogenic process in the EYB and a short-lived (1.9–1.8 Ga) orogeny in the CAB. Such orogeneses are proposed to be correlated with the assembly of the Nuna supercontinent.

Provenance investigation for the Cambrian–Ordovician strata from the northern margin of the western Yangtze Block: implications for locating the South China Block in Gondwana

AbstractWe report new U–Pb isotopic data for detrital zircons from Cambrian–Ordovician strata on the northern margin of the western Yangtze Block, which together with published U–Pb isotopic data for coeval strata in the South China Block, provide critical constraints on the provenance of these sediments and further shed light on the early Palaeozoic position of the South China Block in the context of Gondwana. Detrital zircons in this study yield four major age peaks in the early Palaeoproterozoic, early Neoproterozoic, middle Neoproterozoic and late Neoproterozoic – early Palaeozoic. The dominant age population of 900–700 Ma matches well with magmatic ages from the nearby Panxi–Hannan Belt, which indicates that Cambrian–Ordovician sedimentary rocks in the western Yangtze Block were mainly of local derivation. However, compilations of detrital zircon ages for the Cambrian–Ordovician strata from the Cathaysia Block and the eastern Yangtze Block show that both blocks are dominated by late Mesoproterozoic- and early Neoproterozoic-aged detrital zircons, which suggests a remarkable exotic input with typical Gondwana signatures. According to the integrated detrital zircon age spectra of the Cambrian–Ordovician sedimentary rocks from the entire South China Block and palaeocurrent data, the South China Block should have been linked with North India and Western Australia within East Gondwana. Specifically, the Cathaysia Block was located adjacent to Western Australia, while the Yangtze Block was connected with North India.

Late Jurassic high-Mg andesites in the Youjiang Basin and their significance for the southward continuation of the Jiangnan Orogen, South China

The South China Block (SCB) consists of the Yangtze and Cathaysia Blocks which, in the northeast, are separated from each other by the Jiangnan Orogen. The southward continuation of the Jiangnan Orogen into the Youjiang Basin remains much debated. Here, we present detailed petrological, chemical and isotope evidence from the Late Jurassic (~159 Ma) Yangtun and Liuliang high-Mg andesites (HMA), eastern Youjiang Basin, to place important constraints on an ancient subduction event. These HMA are enriched in large ion lithophile elements and depleted in Nb-Ta-Ti, with negative whole-rock εNd(t) and zircon εHf(t) values, similar to Neoproterozoic high-Mg rocks that occur in the eastern Yangtze Block. Considering the distance of our study areas to penecontemporaneous Pacific or Tethyan trenches, we infer that these HMA derived from a metasomatised lithospheric mantle preserved beneath the eastern Yangtze Block, which was associated with a Neoproterozoic subduction event. The Late Jurassic HMA in the eastern Youjiang Basin provide potential clues for the southward continuation of the Jiangnan Orogen into the basin.

Detrital zircon provenance investigation from the Neoproterozoic successions of the South China Block: Paleogeographic implications

We report U-Pb isotopic data for detrital zircons from the Late Neoproterozoic Dengying Formation on the northern margin of the Yangtze Block and the other published Neoproterozoic U-Pb isotopic data in the South China Block (SCB), which provide critical constraints on the provenance of these sediments and further shed light on the Neoproterozoic paleogeographic evolution of the SCB in the context of the Rodinia and the succeeding Gondwana supercontinents. The SCB-northern India provenance linkage has started from the Cryogenian. Single Neoproterozoic zircon population from the Upper Cryogenian suggests local detritus supply and the western margin of the Yangtze Block was possibly on an arc setting. Therefore, we speculate that the SCB was independent or located along the margin of Rodinia supercontinent during the Cryogenian. Detrital zircon age spectrum of this study shows similarity with the Cryogenian samples in the eastern Yangtze Block, but with the addition of ‘Pan-African’ age population, which implies that the SCB-India assemblage had been linked with the Gondwana supercontinent during the Ediacaran. Except for a small amount of recycled materials, the Late Neoproterozoic detritus in part of the western Yangtze Block were predominantly derived from the Panxi-Hannan Belt. Early Paleozoic detrital zircon age spectrum of the Tethyan Himalaya is similar with the Late Neoproterozoic age spectra of the Cathaysia Block, whereas appears to be different from the Late Neoproterozoic age spectra of the western Yangtze Block, which implies that the northern India was probably linked with the southern margin of the SCB during the Ediacaran time interval. Finally, the SCB assembled with the western Australia during the Early Paleozoic.

Different response to middle-Palaeozoic magmatism during intracontinental orogenic processes: evidence from southeastern South China Block

ABSTRACTIntracontinental orogens are major sites to better understand tectonic regime of plate interior. However, deep response to those processes remains poorly constrained due to the rare exposure of magmatic rocks. Middle-Palaeozoic magmatic rocks from the eastern Yangtze Block (EYB) and Cathaysia Block (CB) of the southeastern South China Block (SCB) have markedly unimodal age distribution with peaks at ~424 and ~438 Ma, respectively. The zircon saturation temperatures (T–Zr) range from 699 to 920°C, and increase in granitic rocks of younger age in the CB; however, most of the granitic T–Zr values in EYB are low and relatively uniform (643–855°C, most of them lower than 800°C). In CB, the Nd-Hf isotopic ratios of the granitic rocks (εNd(t) = – 13.2 to + 1.0; magmatic zircon εHf(t) = – 36 to + 12) are variable, nonetheless they clearly show the presence of a juvenile component. On the contrary, Nd isotopic ratios (εNd(t) = – 7.8 to + 1.6) of mafic rocks in EYB abruptly increase in rocks of younger age. Within the long-lived intracontinental orogenic processes of EYB and CB, consistent age peaks between metamorphism and magmatism and the abrupt increase in contributions of juvenile materials suggest that the thickened lithospheric roots were delaminated into asthenospheric mantle as many independent parts below the metamorphic domes in CB. The data suggest that the deep response to the intracontinental orogenic processes was strong non-uniform between EYB and CB in terms of magmatic period and contribution of mantle derived material to granitic magmas (related to the crustal reworking). We deduced that the combined influence of far-field stress transmission (derived from the convergence of Australia and the SCB), local delamination event in CB and different degree of reworking of the crustal sources, is the main reason to the different responses between EYB and CB.

Early crustal evolution of the eastern Yangtze Block: Evidence from detrital zircon U-Pb ages and Hf isotopic composition of the Neoproterozoic Huashan Group in the Dahongshan area

The Yangtze Block is separated into the eastern Yangtze Block (EYB) and western Yangtze Block (WYB) by the Chongqing–Huaying line (CHL). An integrated study of detrital zircon Lu-Hf isotopes and U-Pb ages from two sandstones in the Huashan Group together with previous studies reveals six major tectonothermal events in the EYB occurring at about 2.87Ga, 2.67Ga, 2.5Ga, 2.0Ga, 1.84Ga and 0.8Ga. Combined with paleogeographic map, this study reveals distinct detrital zircon U-Pb ages and Hf isotopic characteristics in the western EYB and eastern EYB. The former has most zircon U-Pb ages grouped at about 2.0Ga, 2.67Ga and 2.87Ga with sporadic 2.5Ga ages. The ∼2.5Ga zircons show negative to slightly positive εHf(t) values. However, the latter has significant ages grouped at ∼2.0Ga and ∼2.5Ga with negligible 2.67Ga and older zircon ages. The ∼2.5Ga zircons show negative to high positive εHf(t) values. The Early Neoarchean (2.7–2.6Ga) was a significant period for production of mature continental crust in the western EYB and the majority of crustal materials were extracted from the depleted mantle until then. Zircon Hf isotopic compositions indicate that the Late Neoarchean to Early Paleoproterozoic event (∼2.5Ga) involved both reworking of old crust and juvenile crust growth. With previous studies of sedimentary facies, lithochemical compositions, zircon morphology and zircon Hf isotopes, it is proposed that there are unexposed ∼2.5Ga basement rocks in the EYB. During the 2.2–1.8Ga, subduction processes, collisional and post-collisional events were recorded in the north and west of the EYB, which can be correlated to the assembly and breakup of the supercontinent Columbia. Two changes in εHf(t) value at ∼840Ma and ∼790Ma have been found from detrital zircons in the northern margin of the EYB, which probably represent arc–continent collision and continent–continent collision events, respectively. This long-term subduction process occurring at 1100–790Ma in the EYB appeals for a peripheral position for the Yangtze Block in the supercontinent Rodinia.

Geochemical and zircon U–Pb–Hf–O isotopic evidence for a coherent Paleoproterozoic basement beneath the Yangtze Block, South China

The early history of the Yangtze Block is not well constrained owing to the scarce outcrops of the Archean to Paleoproterozoic rocks. Here, we report an integrated study of zircon U–Pb and Hf–O isotopes, as well as whole rock major and trace elements and Nd isotopes for the orthogneisses of the Dongling Complex in the eastern Yangtze Block. LA-ICP-MS zircon U–Pb dating yields a 207Pb/206Pb age of ca. 1.85Ga for the protolith age of the orthogneiss, revealing a reliable Paleoproterozoic basement in the eastern Yangtze Block. Zircons from the orthogneiss have εHf(t) values ranging from −13.4 to −9.0, corresponding to THfDM2 model ages of 3.03–2.80Ga. The zircon δ18O values range between 6.5‰ and 8.8‰ with an average of 7.8‰. In addition, the whole rock εNd(t) values of the orthogneisses range from −9.3 to −8.0 and TNdDM2 model ages from 2.98Ga to 3.05Ga. The orthogneisses have relatively high SiO2, K2O+Na2O, Rb, Ga, Zr, Nb, Y and Ce contents and show low CaO, Eu, Ba and Sr concentrations. Their 10,000*Ga/Al ratios range between 3.0 and 3.5. All these geochemical and Nd–Hf–O isotopic signatures suggest A-type granite protolith for orthogneisses in the Dongling Complex. They were generated by partial melting of Archean crustal materials in a post-collisional tectonic setting. When our data is combined with the existing geochronological data in other parts of the Yangtze Block, it demonstrates a coherent Paleoproterozoic basement beneath the Yangtze Block, representing an important part of the Columbia supercontinent. The ca. 1.85Ga A-type magmatism may have led to craton stabilization over all of the Yangtze Block. In addition, a potential Precambrian dual basement structure may exist in the eastern Yangtze Block, which is supported by Paleoproterozoic metamorphic basement and overlaying Neoproterozoic sedimentary cover.

I-type granitoids associated with the early Paleozoic intracontinental orogenic collapse along pre-existing block boundary in South China

The early Paleozoic Wuyi–Yunkai orogeny resulted in extensive magmatism in the Cathaysia and eastern Yangtze blocks, South China. Identifying the nature of related magmatism is essential for understanding the orogeny that remains enigmatic with regard to its tectonic setting and geodynamic driving force. The Zhangjiafang pluton (438±3Ma) in western Jiangxi province is composed of predominant granodiorite with abundant coeval mafic-intermediate microgranular enclaves (MMEs) (~433±5Ma). The granodiorite samples are weakly peraluminous (A/CNK=1.05–1.09) and have low SiO2 (61.9–64.9wt.%) and high Fe2O3 (4.6–5.6wt.%), MgO (2.2–2.8wt.%) and CaO (4.3–4.8wt.%), belonging to I-type suite due to abundant amphibole in the rocks. They exhibit strongly negative whole-rock εNd(t) values (−11 to −9) and zircon εHf(t) values (−14 to −4), similar to the basement of the Cathaysia Block, but distinguishable from simultaneous I-type granites of the Banshanpu and Hongxiaqiao plutons in eastern Yangtze Block in much lower Sr, Ba, Th and U. The MME samples show pronounced negative Nb–Ta–Ti anomalies and have overall less negative whole-rock εNd(t) (−9 to −7) and zircon εHf(t) values (−9 to −4) than the host granodiorite, which are best interpreted as products of mantle-derived melts that mixed insufficiently with crust-derived magma.The block boundary between the eastern Yangtze and Cathaysia blocks should pass through the nearby west area (i.e., Pingxiang City) of the Zhangjiafang pluton. The Early Paleozoic I-type granitic rocks near the block boundary have all negative εNd(t) and εHf(t) values, demonstrating an overall ancient basement of the two blocks prior to the Wuyi–Yunkai orogeny, preferring an intracontinental orogeny model. However, the early Paleozoic mafic rocks and I-type granites with coeval MMEs were frequently present along the Jiangshan–Shaoxing–Pingxiang Fault zone, illustrating widespread modification of the ancient basement adjacent to the block boundary by mantle-derived melt during the orogenic collapse. The pre-existing block boundary might have promoted asthenosphere upwelling and basaltic underplating during the intracontinental orogenic collapse.

Lithospheric electrical structure of South China imaged by magnetotelluric data and its tectonic implications

The region of South China mainly consists of the Yangtze block in the northwest, the Cathaysia block in the southeast and the Jiangnan orogen in between these two major Precambrian continental blocks. The Yangtze block borders the North China Craton in the north and the eastern margin of the Tibetan Plateau in the west. The Cathaysia block adjoins the Pacific tectonic domain in the east. The study of tectonics in this region is of great significance given its important role in understanding the formation of the Asia continent. Under the auspices of SinoProbe Project, new magnetotelluric (MT) data were collected along a ∼1200km long profile starting from central Sichuan Basin near Suining, extending southeastward, passing through the Yangtze Block, Jiangnan Orogen, and terminating within the western Cathaysia Block near Ganzhou. Based on data analysis results, 2D inversions were conducted on the dataset. Resulting model shows that the lithospheric electrical structure of South China is generally resistive which is consistent with the basic feature of stable Precambrian tectonic setting. The resistive western Yangtze block represents the stable, Archean aged cratonic region of the Yangtze basement. While the electrically conductive eastern Yangtze block is characterized by lithospheric shearing of the strike-slip fault system and extensional process that is probably caused by slab roll-back of a flatly subducted plate. The Jiangshao fault performs as a northwestward dipping conductive layer, which indicates the lithospheric underthrusting of Cathaysia block beneath Yangtze block with its frontal edge reaching the area of Jishou in the upper mantle. To the west of Jiangshao fault, eastern flank of the Xuefengshan Mountain marks the overthrusting frontier of the Yangtze block, as well as its southeastern boundary. To the east of Jiangshao fault, the northwestern boundary of the Cathaysia block displays the pattern of wedging tectonics, which is characterized by a conductive layer wedging into the Cathaysia lithosphere at the depth range of Moho.

I-type granitoids in the eastern Yangtze Block: implications for the Early Paleozoic intracontinental orogeny in South China

Abstract The Early Paleozoic intracontinental orogenic belt in the South China Block (SCB) is composed of massive granitoids and high-grade metamorphic rocks. Compared to the widespread distributions of early Paleozoic S-type granites in the eastern SCB, coeval I-type granitoids are rare and consequently receive much less attention. Two spatially associated granodiorite plutons in the northwestern rim of the orogen, namely the Banshanpu pluton and Hongxiaqiao pluton, have been investigated in order to determine how they fit into the geodynamic setting. The Hongxiaqiao pluton shares many lithological similarities with the Banshanpu pluton, except for the presence of abundant mafic microgranular enclaves (MMEs) in the Hongxiaqiao pluton. Zircon U–Pb dating has yielded weighted mean 206Pb/238U ages of 432 ± 3 Ma and 434 ± 3 Ma for the Banshanpu and Hongxiaqiao plutons, respectively, indicating that they were emplaced coevally in the early Silurian. Samples from the two plutons possess similar Nd–Sr isotope compositions (eNdT = − 8.32 to − 6.88; 87Sr/86Sri = 0.7109–0.7169), indicating that they were derived from a similar crustal source. Rocks from the Banshanpu pluton are intermediate- to high-K calc-alkaline and show strongly peraluminous (A/CNK > 1.1), adakite-like characteristics (Sr/Y ratios > 31; Yb

Geochronological and geochemical constraints on the petrogenesis of Middle Paleozoic (Kwangsian) massive granites in the eastern South China Block

To achieve a better understanding of the Kwangsian orogenic event of the eastern South China Block, this paper documents a set of new zircon U–Pb geochronological and Hf isotopic data and whole-rock elemental and Sr–Nd isotopic analytical results for the representative massive granite intrusions across the Jiangshan-Shaoxing fault. The studied samples are classified into two groups, representing the rocks from the Cathaysia Block to the east of the Jiangshan-Shaoxing Fault (Group 1) and those from the eastern Yangtze Block between the Anhua-Luocheng and Jiangshan-Shaoxing faults (Group 2). The Group 1 samples gave the zircon U–Pb ages of 405–454Ma and εHf(t) values of −3.6 to −15.2 with Hf model ages of 1.6–2.4Ga. Group 2 yielded the zircon U–Pb ages of 400–432Ma and εHf(t) values of −0.2 to −12.7 with Hf model ages of 1.3–2.2Ga. Geochemically, the Group 1 samples (A/CNK=1.02–1.43) have relatively lower Al2O3, MgO, CaO, P2O5 and εNd(t) but higher K2O+Na2O than those of Group 2 (A/CNK=0.93–1.44). Both groups show similar chondrite-normalized patterns of rare-earth elements with Eu/Eu* values of 0.15–0.92 and strongly negative Ba, Sr, Nb, P and Ti anomalies in primitive mantle-normalized spider diagrams. Their εNd(t) values range from −11.1 to −8.0 for Group 1, and −8.9 to −5.0 for Group 2, generally similar to those of Precambrian paragneiss and contemporaneous gneissoid granites in the eastern South China Block. Our geochronological results indicate that the Kwangsian massive granites in the eastern South China Block were crystallized between 400Ma and 454Ma, synchronous to the Kwangsian gneissoid granites along the Wugong and Wuyi-Baiyun-Yunkai domains in the eastern South China Block. The synthesis of these whole-rock geochemical and in-situ zircon Hf isotopic data suggests that both the Group 1 and 2 granites across the Jiangshan-Shaoxing Fault were predominantly derived from a crustal source with some proportional metapelitic and metaigneous components from the Precambrian basement. The input of juvenile mantle-derived magma is insignificant. Taking into account other available evidences, it is proposed that the Jiangshan-Shaoxing Fault might be a pre-existing boundary between the Yangtze and Cathaysia Blocks, which was reactivated during the middle Paleozoic (Kwangsian) orogenic event. The massive granitic magmatism was probably resulted from the doubly crustal thickening and subsequent isostatic readjustment in an intracontinental tectonic regime.

Tectonic setting of the South China Block in the early Paleozoic: Resolving intracontinental and ocean closure models from detrital zircon U-Pb geochronology

[1] Zircon U-Pb geochronological data on over 900 zircon grains for Cambrian to Silurian sandstone samples from the South China Block constrain the pre-Devonian tectonic setting of, and the interrelationships between, the constituent Cathaysia and Yangtze blocks. Zircons range in age from 3335 to 465 Ma. Analyses from the Cathaysia sandstone samples yield major age clusters at ∼2560, ∼1850, ∼1000, and 890–760 Ma. Zircons from the eastern and central Yangtze sandstone samples show a similar age distribution with clusters at ∼2550, ∼1860, ∼1100, and ∼860–780 Ma. A minor peak at around 1450 Ma is also observed in the Cathaysia and central Yangtze age spectra, and a peak at ∼490 Ma represents magmatic zircons from Middle Ordovician sandstone in the eastern Yangtze and Cathaysia blocks. The Cambrian and Ordovician strata show a transition from a carbonate-dominated succession in the central Yangtze Block, to an interstratified carbonate-siliciclastic succession in the eastern Yangtze Block, to a neritic siliciclastic succession in the Cathaysia Block. Paleocurrent data across this succession consistently indicate directions toward the W-NNW, from the Cathaysia Block to the Yangtze Block. Our data, together with other geological constraints, suggest that the Cathaysia Block constitutes a fragment on the northern margin of east Gondwana and both Cathaysia and east Gondwana constituted the source for the analyzed early Paleozoic samples. The similar age spectra for the Cambrian to Silurian sandstone samples from the Yangtze and Cathaysia blocks argue against the independent development and spatial separation of these blocks in the early Paleozoic but rather suggest that the sandstone units accumulated in an intracontinental basin that spanned both blocks. Subsequent basin inversion and Kwangsian orogenesis possibly at 400–430 Ma also occurred in an intracontinental setting probably in response to the interaction of the South China Block with the Australian-Indian margin of east Gondwana.

Geochemistry of Cretaceous mafic rocks from the Lower Yangtze region, eastern China: Characteristics and evolution of the lithospheric mantle

Geochemical and isotopic data for Cretaceous mafic rocks (basalt, gabbro, and diorite) from the Lower Yangtze region, northern Yangtze block, constrain the evolution of the lithospheric mantle. The mafic rocks, separated into the northeast and southwest groups, are alkaline and evolved, with low Mg# values (44–58) and variable SiO 2 contents (47.6–57.4 wt%). Enriched LREEs, LILEs, and Pb, together with depleted Nb, Zr, and Ti, suggest that the mantle sources were metasomatized by slab-derived fluid/melt. All samples show high radiogenic 207Pb/ 204Pb( t) (15.41–15.65) and 208Pb/ 204Pb( t) (37.66–38.51) ratios at given 206Pb/ 204Pb( t) (17.65–19.00) ratios, consistent with the mantle sources having been metasomatized by ancient slab-derived material. Mafic rocks of the southwest group show enriched Sr–Nd isotopic characteristics, with 87Sr/ 86Sr( t) ranging from 0.7056 to 0.7071 and ε Nd( t) ranging from −5.3 to −8.3, indicating an origin from enriched lithospheric mantle. Mafic rocks of the northeast group, which record 87Sr/ 86Sr( t) ratios of between 0.7044 and 0.7050 and ε Nd( t) of −2.8 to −0.7, possibly formed by the mixing of melts from isotopically enriched lithospheric mantle and isotopically depleted asthenospheric mantle. Taking into consideration the geochemical and isotopic characteristics of Cretaceous mafic rocks, Cenozoic basalts, and basalt-hosted peridotite xenoliths from the Lower Yangtze region, we propose that an isotopically enriched, subduction-modified lithospheric mantle was replaced by or transformed into an isotopically depleted “oceanic-type” mantle. Such a process appears to have occurred in the eastern North China Craton as well as the eastern Yangtze block, probably in response to subduction of the paleo-Pacific plate beneath East Asia.

Petrogenesis and metallogenesis of the Yanshanian adakite-like rocks in the Eastern Yangtze Block

Petrogenesis and metallogenesis of the Yanshanian adakite-like rocks in the Eastern Yangtze Block

Petrogenesis and metallogenesis of the Yanshanian adakite-like rocks in the Eastern Yangtze Block

Many of the Yanshannian intermediate-acid intrusive rocks related to Cu-Au mineralization in the Eastern Yangtze Block are characterized by high Al2O3, Sr contents, while low in Y, Yb contents, thus with high Sr/Y, and La/Yb ratios, and variational isotope signatures in particular, e.g. eNd(t) = −11.92–1.96, (143Nd/144Nd)i = 0.5120–0.5125, TDM = 0.70–1.71 Ga, (87Sr/86Sr)i = 0.7043 –0.7076. The geochemical characteristics of these rocks suggest that: (1) these rocks are geochemically similar to adakite, which might have been stemmed from the partial melting of thickened basaltic lower crust due to basalt underplating; and (2) the high pressure (1.2–4.0 GPa) and high temperature (850–1150°C) surroundings of the lower crust favor both the fluid and adakite-like magma to generation. Not only can the adakite-like magma carry abundant fluid and Cu-Au ore-froming materials, but also can it bring them to the shallow part with ease and contributes to the Cu-Au mineralization.

Middle Triassic paleomagnetic results from Central Hubei Province, China and their tectonic implications

Twenty one sites consisting of 4 to 7 samples per site were drilled from the redbeds of the Middle Triassic Badong Formation from central Hubei province, China. These samples were subjected to progressive thermal demagnetization which revealed two components of magnetization. The low temperature (A) component was probably acquired at a late stage of folding and is similar in direction to the late Mesozoic overprint widely observed in the eastern Yangtze Block. The high temperature (B) component is prefolding with dual polarity. Unlike the prefolding paleomagnetic directions observed in eastern Sichuan, western Hubei and northwestern Hunan, the B component resolved from central Hubei indicates no significant rotation with respect to other parts of the Yangtze Block. Since one of the two sampled sections is in the strongly deformed foreland fold‐thrust belt of the Qinling Fold Belt, this would seem to negate the possibility that the differential rotations observed in the border area between Sichuan, Hubei, Hunan and Guizhou are associated with the collision of the Yangtze Block with the North China Block.

The osmium isotopic compositions of Cenozoic basalts and peridotite xenoliths from Anhui and Jiangsu provinces

The187Os/186Os ratios of the mantle xenoliths indicate the heterogeneous distribution of siderophile elements Re and Os in the subcontinental lithospheric mantle under the eastern Yangtze Block and the multistage evolution, including depletion and enrichment of the mantle relative to the primitive mantle.