Orogeny In South China Research Articles

The origin and tectonic implications of the ca. 406 Ma magmatic activity in the western Yangtze Block, South China

In this study, we report for the first time a new gabbro diorite, forming in an intraplate extensional environment during the Early Devonian, in the Shimian of the western margin of the Yangtze Block. Zircon SIMS U–Pb dating indicates that the gabbro diorite formed at ca. 406 Ma. This gabbro diorite belongs to low-K tholeiite and has enriched in Rb, Ba, K, Pb, negative anomalies of Nb, Ta, depletion of flat LREE patterns, negative εHf(t) (−1.1 to −11.7), and high δ18O (8.83 ± 0.21‰) ratios, showing that the magma formed by partial melting of N-MROB-type mantle and indicating that the magma underwent crustal contamination during emplacement. The magma is formed in the extensional environment, which provides evidence for the extensional environment in the western margin of the Yangtze. The geochemical characteristics and emplacement time of the rocks are similar to those in the early Paleozoic post-orogenic mafic rocks in South China, suggesting that the gabbro diorite may be influenced by the early Paleozoic orogeny in South China.

Early Paleozoic metamorphic P–T–t evolution of the northern Cathaysia Block (SE China): Implications for thermal history and geodynamic setting

The tectono-thermal evolution of the Early Paleozoic orogeny in South China has been debated. In this contribution, detailed studies of petrography, mineral chemistry, phase equilibria modeling, conventional geothermobarometry, and zircon–monazite geochronology were conducted on metamorphic rock samples from the Zhoutan, Longyou, and Chencai groups in the northern Cathaysia Block, South China. All rocks record clockwise P–T paths, including the Pmax (pre-Tmax), Tmax, and post-peak retrograde stages. Sillimanite-garnet-mica gneiss from the Zhoutan Group is characterized by decompression and heating from the Pmax (0.97 GPa/710 °C) to the Tmax stage (0.70–0.85 GPa/795–815 °C). Garnet-amphibole gneiss from the Longyou Group and sillimanite-garnet-biotite gneiss from the Chencai Group experienced Tmax metamorphism at 0.90–1.00 GPa/865–880 °C and 0.90–1.00 GPa/805–820 °C, respectively. LA-ICP-MS zircon–monazite UPb dating yields 453–446 Ma for the retrograde metamorphism of the Zhoutan Group, ∼457 Ma and ∼432 Ma for the peak and retrograde metamorphism, respectively, of the Longyou Group, and 441–436 Ma for the peak metamorphism of the Chencai Group. This indicates that the Early Paleozoic crustal thickening event may have occurred 10–20 Ma earlier in the western region than in the eastern region of the northern Cathaysia Block. Integrated Early Paleozoic P–T–t data from the Cathaysia Block provide medium-pressure facies series conditions without the products of high-pressure facies series metamorphism, which are common in subduction-to-collision orogens. Based on the current metamorphic characteristics and other geological data for Early Paleozoic orogeny in South China, we infer that the current tectonic domains developing metamorphism–magmatism–deformation may not be the “first scene” of subduction-to-collision. The stress that caused the Early Paleozoic orogeny likely originated from the ancient plate boundaries of South China.

The early Paleozoic Wuyi–Yunkai orogeny in South China: a collisional orogeny with a major lag in time between onset of collision and peak metamorphism in subducted continental crust

Abstract Collisional orogeny is characterized by deep subduction of continental crust and major crustal thickening, leading to high-pressure/high-temperature metamorphism and anataxis of the subducted continental crust. Since conductive heating of large slabs of cold, subducted continental crust is a slow process, heating to a temperature that is high enough to generate significant partial melting can take tens of millions of years. Where the spatial and temporal relationships are obscured due to later modification (e.g. post-collisional rifting), the peak metamorphism and magmatism may be interpreted as an orogeny that is separate from the collision, or may be interpreted as an intraplate orogeny as no contemporaneous arcs or ophiolite may be present. We propose here that this is the case for the early Paleozoic orogeny in South China. In our model, the West Cathaysia terrane of South China was part of a continent (possibly Australia) on the lower plate and collided with another continent (possibly India) in the Cambrian–Ordovician, at a late stage of Gondwana assembly, and the late Ordovician–Silurian Wuyi–Yunkai orogeny, characterized by amphibolite–granulite facies metamorphism and extensive anataxis, was a continuation of the Cambrian–Ordovician collisional orogeny. In this interpretation, the Wuyi–Yunkai orogen was part of the Kuunga orogen before Gondwana break-up.

Episodic water-fluxed anatexis recorded by migmatites from the Paleozoic Wuyi-Yunkai Orogeny in South China

The Wuyi-Yunkai Orogeny (WYO) in South China occurred during the assembly of Gondwana, and has abundant granites and migmatites which have been mostly interpreted as products of dehydration anatexis. However, the early Paleozoic migmatites in the Cathaysia Block, investigated in this study, instead document water-fluxed anatexis during the orogeny. The quartzo-feldspathic migmatites are moderately peraluminous, and have ca. 700 Ma and ca. 1800 Ma inherited zircon sub-populations, indicating that they were derived from Middle Neoproterozoic paragneisses. Titanite is common, while other anhydrous peritectic minerals such as garnet and pyroxene are absent in the migmatites and their leucosomes. Both mineral assemblages and microstructures indicate that the rocks were melted through reactions that include quartz + plagioclase + biotite + H2O = titanite + opaque + melt, K-feldspar + plagioclase + quartz + H2O = melt, and K-feldspar + biotite + quartz + H2O = melt. U-Pb ages of the newly-formed zircon and titanite display at least four age populations, recording episodic melting mainly at ca. 466, 437, 425 and 412 Ma, with evidence of local anatexis extending back to at least 480 Ma. Multiple stages of anatexis with short time intervals may have resulted from water-present melting due to episodic fluid influx. Zircon εHf(t) values are negative (-5 to -15) in the 700 Ma and 1800 Ma inherited zircons to nearly zero (0 to -5) in 470–400 Ma newly-formed zircons, and may have resulted from dissolution of the Lu-rich apatite during water-fluxed anatexis. The newly-formed zircons also have high δ18O values (9–12 ‰). These lines of evidence suggest that fluid flux-influenced melting was induced by metamorphic fluids that were released from polyphase metamorphism during crustal thickening and extension in the WYO, and channelized through regional shear zones. This highlights the extent and significance of water-fluxed anatexis and its role in the differentiation of the continental crust during orogeny.

Early Paleozoic oceanic slab subduction in South China: Evidence from adakite‐like granodiorite and high‐Mg diorite from Puyang pluton in the Wuyi orogenic belt

AbstractThe mechanism of Caledonian orogeny in South China is still controversial. The main argument focuses on the issue that whether there existed oceanic subduction. To answer this question, the complex Puyang pluton in the central part of Wuyi orogenic belt was selected for zircon U–Pb dating, in‐situ Lu–Hf isotopic analysis and geochemical testing. The Puyang pluton is mainly composed of granodiorite and diorite. The results of geochronology indicate that the granodiorite and diorite emplaced at 450 ± 3.9 Ma and 443 ± 4.0 Ma. Their emplacement time were well corresponding to the subduction stage of the Yunkai orogeny in the southwestern part of Cathaysia block during the Early Paleozoic (460–440 Ma). The Puyang adakite‐like granodiorite shows enrichments in Sr, but depletions in Y and Yb contents, high in Na2O, K2O and Al2O3, with εHf(t) values range from −10.5 to −7.4 (mean of −9.0) and two‐stage Hf model ages range from 1.91 to 2.10 Ga. These characteristics indicate that the magmas were generated by partial melting of subducted oceanic crust mixed with melts from the above wedgy mantle peridotite. The Puyang high‐Mg diorite shows enrichments in Sr and Ba, depletions in Rb, Y and Yb contents, and strongly high in MgO and Al2O3 contents, with εHf(t) values of −2.9 to 0 (mean of −1.4) and two‐stage Hf model ages of 1.43 to 1.61 Ga. These indicate that the magmas were mainly generated by partial melting of the wedgy mantle peridotite, mixed by adakitic melts from subducted oceanic slab. Comprehensive analysis shows that Puyang adakite‐like granodiorite and high‐Mg diorite were formed in fore‐arc setting, where the mantle and crustal magmas mixed during the oceanic subduction initiation period. By extension, this study offered important evidences to support the point that the Caledonian orogeny in South China was related to oceanic subduction which initiated prior to ~450 Ma.

The Origin of the Black Shale Series and Bentonites from the Wufeng Formation in the Southwestern Upper Yangtze: Implications for the Convergence of the Yangtze and Cathaysia Blocks in the Late Ordovician

AbstractEarly Paleozoic black organic sediments and bentonites occur widely in the craton basin within the Yangtze block and are generally believed to be genetically related to a specific tectonic setting on the cratonic boundary. However, the intimate relationship between their origins and the dynamic mechanisms are unclear, as exemplified by the genesis of the black shale series and bentonites from the Wufeng Formation during the Ordovician–Silurian transition (OST). In order to reveal the relationship between the Wufeng Formation and the convergence of the Yangtze and Cathaysia blocks (i.e., the intracontinental Kwangsian Orogeny), two stratigraphic sections respectively in Zhaotong area (Northeast Yunnan) and Puyi area (Northwestern Guizhou) that were located in the semi‐restricted inner Yangtze Sea during the OST were systematically studied, on the basis of whole‐rock geochemical composition, pyrite δ34S (δ34Spy), total organic carbon (TOC), stable Sr isotope, pyrite framboid size distribution and zircon U‐Pb age, trace elements. The evidence shows that the paleo‐oceanic environment changed significantly at the turn of the early–late Katian and formed the black shale series in the Wufeng Formation. These acritarch assemblages were formed in the transition process of the Upper Yangtze Basin from passive continental margin basin to foreland basin during this interval. Based on previous research on the genetic relationship between black shale series and plate tectonic movement, a basin‐mountain evolution model suitable for South China in the Late Ordovician is presented. The two bentonites in the Wufeng Formation with U‐Pb ages of 445.5 ± 0.8 Ma and 441.9 ± 2.4 Ma primarily originated from the intermediate–acid volcanic eruption during the collision and convergence between the Yangtze and Cathaysia blocks in the Late Ordovician, the provenance region probably being located in the Jiangnan orogenic belt. Thus, we believe that the appearance of the black shale series and bentonite in the Wufeng Formation at the turn of the early–late Katian may represent the initiation of basin‐mountain transformation and the Kwangsian Orogeny in South China, which provides important evidence for the collision and convergence of the Yangtze and Cathaysia blocks in the Late Ordovician.

Detrital zircon U–Pb ages and trace elements indicate the provenance of Early Carboniferous Li-rich claystone from central Guizhou, South China

Lithium is of great importance for green energy technology, and most industrial Li is sourced from brine lakes and pegmatites. Recently, strong Li enrichment has been reported in the late Palaeozoic Jiujialu Formation (C1jj) in Guizhou, South China; however, the source and the sedimentary processes that formed the Jiujialu Formation remain unclear. We present the U–Pb ages and trace element compositions of detrital zircon grains from the Jiujialu Formation and underlying dolomite of the Loushanguan Group (Cam2–3ls), which indicate the grains in the Jiujialu Formation on the margin of the Central Guizhou Uplift and in a depression in southeast Guizhou have three primary age peaks (1000–900, ∼760, and ∼530 Ma). These ages are similar to those of Ordovician strata, indicating a contribution from the Ordovician strata to the Jiujialu Formation in these areas. Detrital zircon grains from the Jiujialu Formation in the core of the Central Guizhou Uplift and the Loushanguan Group have two primary age peaks at ∼760 and ∼530 Ma, which suggests that the underlying Loushanguan Group supplied most of the material to the Jiujialu Formation in this area. The age distribution of detrital zircon grains indicates that sediment erosion from the sedimentary stratal transport was limited, and deposition occurred near the direct source. In addition, the trace element compositions of the detrital zircon grains indicate that their original source was magmatic rocks of the South China Block. An early Palaeozoic intracontinental orogeny in South China uplifted subduction-related magmatic rocks (1000–900 Ma) in the northeast of the South China Block, exposing them to weathering and providing material for the Ordovician strata. This suggests that the weathering of sedimentary strata can form clay-type Li deposits in areas with carbonate bedrock, and that the provenance of Cambrian–Ordovician sedimentary strata in the Yangtze Block was controlled by an early Palaeozoic intracontinental orogeny.

Nearshore warm-water biota development in the aftermath of the Late Ordovician Mass Extinction in South China

The Late Ordovician Mass Extinction (LOME) occurred between two significant biotic diversifications: the Great Ordovician Biodiversification Event and the Silurian–Devonian Nekton Revolution. Upper Hirnantian carbonate deposits therefore are critical archives for understanding warm-water biotas, biodiversity resurgence and trophic chain development in the immediate aftermath of the LOME. In this study, we use microfacies analysis to investigate composition and biodiversity of the upper Hirnantian carbonates developed in northeastern Guizhou Province of China. These carbonates lay along the northern margins of the Dianqiangui landmass and are composed of intraclastic pack- to grainstone, mud- to wackestone, and bioclastic wacke- to packstone. Depositional age of the carbonates is estimated to be late Hirnantian, chiefly indicated by their coincidence with the Hirnantian Carbon Isotope Excursion and the brachiopods Cathaysiorthis and Eospirigerina. The development of carbonates containing diverse warm-water fossils at this transitional time reflects glacio-eustatic changes in the aftermath of the maximum Hirnantian glaciation and LOME, and the consequence of regional Kwangsian Orogeny in South China. Supposed autotrophic organisms in the carbonates are represented by calcimicrobes Allonema and Girvanella, and calcareous algae Amsassia, Dasyporella and Dimorphosiphon. Heterotrophic organisms are also abundant, including bryozoans (Atactoporella, Dekayia, Eridotrypa, Hallopora, Moorephylloporina and Ptilodictya?), trilobites (Dicranopeltis, Eoleonaspis, Mucronaspis (Songxites), Niuchangella and an aulacopleurid-like trilobite), rugose corals (Axiphoria, Eurogrewingkia, Meitanolasma and Grewingkia), tabulate corals (auloporid coral, Catenipora, Halysites, Paleofavosites and Propora), stromatoporoids (Camptodictyon, Ecclimadictyon and Cystostroma), conodonts (Ozarkodina and Walliserodus), benthic graptolites (Desmograptus), gastropods (at least two genera), and ostracods. Such a high biodiversity is evidence of a complex trophic web after the extinction pulses in South China. The biotic development of the warm-water benthic community might have been triggered by a short-lived localized favourable environment that facilitated the survival of warm-water biota during the Hirnantian, which is postulated to have pioneered further Silurian reef recovery in South China.

Polyphase deformation in the Badu complex: Insights into Triassic intraplate orogeny in South China

The occurrence of Triassic high-pressure eclogite- and high-temperature granulite-facies metamorphism in the Badu complex contrasts with regional greenschist-to amphibolite-facies metamorphism in most of South China, hence calling for unraveling related structural and geodynamic processes within a regional intraplate-orogenic setting. We analyzed the geometry, kinematics, and overprinting relationships of structures in the central Badu complex, and dated the structural evolution via geo-thermochronology. We identified deformation episodes D1 to D4: D1 features rootless, tight-isoclinal folds (F1), and an initially likely gentle-dipping foliation (S1) with unclear kinematics, with the majority of structures modified/erased by later deformations; D2 comprises a gentle-dipping foliation (S2), and a ∼N-trending lineation (L2), with dominant top-to-the-south shear subparallel or slightly oblique to the strike of the regional Triassic orogen; D3 includes open to tight, upright folds with subhorizontal axes reflecting orogen-normal, ∼E-W shortening; D4 features ∼ N-trending reverse faults. D1 is bracketed between ∼245 and ∼235 Ma and occurred at temperatures >600 °C, following regional high-pressure metamorphism (251‒245 Ma). D2 and D3 occurred between ∼235 and ∼221 Ma under retrograde, amphibolite-to greenschist-facies metamorphic conditions, with cooling rates of ∼17–21 °C/Myr. By combining our new data with published work on the Badu complex, we trace a >120-km-long, ∼25-km-wide, N- to NNE-striking, high-grade metamorphic and intense deformation zone that represents a localized but distinct anomaly in the vast (∼1300-km-wide) Triassic structural belt in South China. The high-pressure metamorphism, the orogen-subparallel shearing, and the orogen-normal shortening suggest localized (oblique) underthrusting of the Coastal terrane beneath the Wuyishan terrane during the Triassic intraplate orogeny within South China.

Linking ocean subduction with early Paleozoic intracontinental orogeny in South China: Insights from the Xiaying complex in eastern Guangxi Province

The tectonic setting of the early Paleozoic orogeny in the South China Block (SCB) has been hotly controversial. Although most studies have ascribed it to an intracontinental orogeny, there are various views on the geodynamic drivers of the intracontinental orogeny. This study reports on the Ordovician Xiaying igneous complex in the eastern part of Guangxi province, southern South China. This complex is composed of granodiorite, monzogranite and micro-granular diorite enclosed by granodiorite. The zircon U–Pb dating suggests that these three types of rocks emplaced synchronously at around 464–442 Ma, significantly older than those S- and I-type granites (450–400 Ma) distributed in the interior of the SCB. One monzogranite collected in the border of the complex has an older age (480 Ma). The Xiaying hornblende–bearing granodiorites have low SiO2 (61.8–68.7 wt%), P2O5 (0.07–0.10 wt%), Rb/Sr (0.52–1.30) and Rb/Ba (0.13–0.34) and high Fe2O3t (3.84–5.57 wt%), MgO (1.48–2.35 wt%), CaO (2.28–4.54 wt%) and Na2O/K2O (0.90–1.91), with metaluminous to weakly peraluminous features (most A/CNK = 0.87–1.07), showing typical I-type granitoid characteristics. The Nd–Hf isotopic model ages (ca 1.6–1.8 Ga) indicate that the Xiaying granodiorites were probably formed by partial melting of a late Paleoproterozoic basement in the deep crust. The micro–granular dioritic enclaves (MMEs) have similar formation ages and Nd–Hf isotopic compositions to the host granodiorites, and show geochemical evolution relation with granodiorites, suggesting that they resulted from the mixing of mafic and granodioritic magmas, which derived from a depleted mantle and a Paleoproterozoic crust, respectively. Based on their similar formation ages and zircon Hf-isotope compositions and the assimilation and fractional crystallization (AFC) modelling, it is suggested that major monzogranites were generated from the granodioritic magma through the AFC process. The rock associations and arc-related geochemical characteristics of the granodiorites and dioritic MMEs suggest that they probably formed in a subduction-related tectonic setting. Combined with the spatial distribution of the rock types and formation ages of these early Ordovician to early Devonian magmatic rocks in the SCB from southeast to northwest, it is inferred that there was an Ordovician northwestward subduction of the proto–South China Ocean occurring to the south of the SCB, which may connect with the Tam Ky–Phuoc Son Suture Zone in the eastern Indochina Block. It was the subduction of the proto–South China Ocean and subsequent collision between the southern SCB and an unknown continent that triggered early Ordovician to early Devonian intracontinental orogeny in the interior of the SCB.

Geochemical characteristics of organic-rich shale, Upper Yangtze Basin: Implications for the Late Ordovician–Early Silurian orogeny in South China

The tectonic–sedimentary environment of the Upper Yangtze Basin in China underwent significant changes from the Late Ordovician to Early Silurian. The widely distributed shale formed during this interval has recorded key information about paleo-ocean conditions. Analysis of major, trace and rare earth elements was conducted on 135 shale samples collected from five wells in the study area. The covariation of the MoU enrichment factors indicates that the Upper Yangtze Basin in the Wufeng stage was characterized by sedimentary differentiation. The southern basin was dominated by a reducing bottom water environment under euxinic conditions. The northern and central basins were dominated by a suboxic–anoxic environment. Up to the early Longmaxi stage, the Upper Yangtze Basin formed a relatively uniform sedimentary environment with gradually increasing oxygen content in the bottom water. The paleoproductivity index indicates that marine organisms flourished in the Wufeng to early Longmaxi stages, making an important contribution to the siliceous content in organic-rich shale; subsequently, terrestrial siliciclastics gradually dominated. Analysis of paleogeography, rare earth elements, and weathering indicates that Chuanzhong and Qianzhong uplifts in the Wufeng stage were the provenance area of the Upper Yangtze Basin, with a relatively high weathering degree. Weathering decreased in the beginning of the Longmaxi stage and then gradually increased, with the Xuefeng Uplift becoming the main provenance areas after the early Longmaxi stage. Controlled by stress transfer from the continuous syn–Caledonian Kwangsian Orogeny in eastern South China, the Upper Yangtze Basin and Xiangzhong Basin were squeezed and pushed up successively. The uplift separated the open shallow sea and formed a relatively limited deep-water shelf environment, resulting in a relatively euxinic bottom water environment, which was favorable for organic matter accumulation. The tectonic–sedimentary evolution of the Upper Yangtze Basin in the Late Ordovician to Early Silurian proposed in this study indicates that the formation of organic-enriched shale was not only related to glacio-eustatic change but also strongly controlled by tectonic activities in South China.

Origin of the Heping granodiorite pluton: Implications for syn-convergent extension and asthenosphere upwelling accompanying the early Paleozoic orogeny in South China

We present geochemical and geochronological data of host granodiorites and mafic microgranular enclaves (MMEs) from the Heping pluton, which is situated in the central part of the Wuyi-Yunkai Orogen (WYO) in the South China Block (SCB), and reveal syn-convergent extension and asthenosphere upwelling during the early Paleozoic intracontinental orogeny. Two host and two MME samples from the Heping pluton yield LA-ICP-MS zircon UPb ages of ca. 445 Ma, coincident with the peak magmatism-metamorphism of the WYO. The host granodioritic samples are metaluminous (A/CNK = 0.68–0.91) and have abundant amphibole and low-moderate SiO2 (59.1 to 67.3 wt%), belonging to high-K calc-alkaline I-type granite, whereas the MMEs are more basic with SiO2 of 56.3 to 59.6 wt%. Both the host and MME samples display weakly negative Eu anomalies and distinctly relative depletions of Ta, Nb, and Ti, and most of them share similar initial 87Sr/86Sr ratios of 0.7089–0.7112 and εNd(t) values of −6.52 to −7.12, except one MME sample with a lower initial 87Sr/86Sr of 0.70758 and a higher εNd(t) of −4.33. The zircon εHf(t) values exhibit a wide range from −13.68 to −0.87. Petrological, geochemical, and chronological data suggest that the Heping pluton were generated by mingling of mafic and felsic magmas. The mafic endmember magma was originated mainly from the enriched subcontinental lithosphere mantle at the spinel–garnet transitional zone (~70 km), subsequently underplated/intraplated into the lower-middle crust resulting in the melting of the intermediate rock at pressure < 8 kbar to produce the felsic endmember magma. We proposed a new tectonic model “syn-convergent extension and asthenosphere upwelling during the intracontinental orogeny”. The syn-convergent extension zones, which include a series of NW-trending transverse faults and the NE-trending reactivated pre-exsiting suture and rift zone (i.e. Shaoxing-Jiangshan -Pingxiang- Chenzhou fault) within the WYO, are in favor of asthenosphere upwelling leading to intensive crust-mantle interaction.

The depositional mechanism of organic-rich siliceous shales in Upper Yangtze area: Response to the Kwangsian Orogeny in South China

The organic-rich siliceous shale in Upper Ordovician is the major source rock for shale gas in the South China, while the deposition mechanism of these rocks is still under debated. Here we present systematic geochemical and petrographic characteristics of the Upper Ordovician sediments to evaluate the redox conditions, paleo-productivity, terrestrial input, and to investigate the depositional mechanisms of these organic rich siliceous shales in the Wufeng Formation. In shallow water areas, the Linxiang Formation is dominantly characterized by grey thick Limestone. The Upper Daduhe Formation is composed of calcareous mudstone intercalated with argillaceous limestone, while the Lower Daduhe Formation consists of limestone interbedded with mudstone. At deep water area, the Linxiang Formation is dominated by limestone and muddy limestone. The lower and upper Wufeng Formation are mainly characterized by grey argillaceous shale and organic-rich siliceous shale respectively. It is suggested that the lithological and environmental changes in the Yangtze area were the response to the development of the Kwangsian Orogeny. During the late Ordovician, the Yangtze basement constantly descended and the sea level rose relatively due to the collision between the Yangtze and Cathaysia blocks. Thus, the oxygen concentration decreased with the rising sea level according to the UEF values and Ce/Ce* ratios, which resulted in anoxic water condition and promoted the preservation and enrichment of organic matter in marine sediments. Meanwhile, uplifts (e.g., Jiangnan - Xuefeng, Qianzhong and Chuanzhong) constantly expanded around the Sichuan Basin, leading to high terrigenous input that is reflected in geochemical and Nd isotopic compositions. Additionally, primary productivity gradually increased, because the Si/Al ratios exhibit an increasing trend in both shallow and deep water areas, and it may be related to the high clastic influx which provided adequate nutrients for siliceous organisms. Overall, anoxic water condition and high biogenic Si influx caused by Kwangsian Orogeny are considered as the most important factors controlling the organic-rich siliceous shale in the Wufeng Formation.

Three-stage Mesozoic intracontinental tectonic evolution of South China recorded in an overprinted basin: evidence from stratigraphy and detrital zircon U–Pb dating

AbstractThe NE–NNE-trending Yuan-Ma Basin in central South China, an overprinted basin, is important for understanding the transition in Mesozoic intracontinental deformation in South China from compressional to extensional settings. A detailed sedimentary and structural cross-section across the basin reveals the Upper Triassic – Lower Jurassic black coal-bearing shale, greyish-green sandstone and brick-red claystone, and the Middle Jurassic brick-red sandstone, pebbly sandstone and conglomerate in the eastern segment of the basin. The Lower Cretaceous brick-red coarse sandstone, pebbly sandstone and siltstone occurred in the western and central segments, as well as fault breccia and Lower Cretaceous sandstone at the western margin of the basin. Detrital zircon U–Pb dating by laser ablation inductively coupled plasma mass spectrometry shows that the magmatic and metamorphic zircons yield significant age clusters at 900–700, 500–350 and 300–150 Ma, as well a minor age cluster at 120–100 Ma. Synthesizing the stratigraphic sequences, structures, isotopic dating results and palaeocurrent data, we infer that the Yuan-Ma Basin experienced three evolutionary stages and tectonic settings: (1) during Late Triassic – Early Jurassic time, the Yuan-Ma Basin was related to the diachronous progressive intracontinental deformation as a result of the early Mesozoic Xuefeng intracontinental orogeny in South China; (2) during Middle–Late Jurassic time, the Yuan-Ma Basin was related to intracontinental compression in South China; and (3) during late Early Cretaceous time, the Yuan-Ma Basin was constrained by the intracontinental extension that occurred in eastern China. These three stages, a result of various tectonic regimes, caused the intracontinental deformation that was controlled by the evolution of the continents and their margins.

Discovery of Middle–Late Devonian and Early Permian magmatic events in East Asia and their implication for the Indosinian orogeny in South China: Insights from the sedimentary record

AbstractWhether the driver of the Indosinian orogeny in the South China block was related to the evolution of the Paleotethyan Ocean or the Paleo-Pacific Ocean has been a point of much debate. We applied detrital zircon U-Pb dating to Permian–Triassic sedimentary rocks from South China to trace sediment provenance and to further test these models. Our results, combined with other published data from the Pingxiang, Youjiang, Yong’an, and Yongding Basins, show that 400–350 Ma and 300–260 Ma zircon grains are ubiquitous throughout the entirety of southern South China. This indicates regional magmatic events as potential sources. The discovery of Middle–Late Devonian and Early Permian igneous rocks, tuffs, and volcaniclastic rocks in Southeast Asia and Hainan Island implies the presence of two magmatic events (400–350 Ma and 300–260 Ma) within or beyond the southern margin of South China. This information, together with the mostly negative εHf(t) values of 400–350 Ma and 300–260 Ma zircon grains, arc-like geochemical signatures of the possible source rocks, and the regional geology of East Asia, suggests that they originated from sources related to Paleotethyan and even Proto-Tethyan subduction. Thus, Permian–Triassic sedimentation and the Indosinian orogeny in South China were largely controlled by the evolution of the Tethyan Ocean.

Two kinds of authigenic xenotime overgrowths in response to an Early Paleozoic tectonothermal event in South China

This study has documented two kinds of xenotime overgrowths from the Cryogenian-Ediacaran sedimentary rocks in South China. The identical tetragonal dipyramidal structures prompted one type of xenotime to overgrow detrital zircon grains, whereas the other type coexists with rutile crystals. SIMS Pb-Pb, NanoSIMS, EPMA, and TEM imaging analyses suggest these xenotime overgrowths, together with their rutile substrate could be of hydrothermal origin; formed by means of percolating fluids leaching ilmenite and REE(Y) bearing silicate minerals. Iron, Ti, and REE(Y) ions leached in this way were combined with O, S, and P anions to precipitate rutile, xenotime, iron oxides, and pyrite in other pore spaces. Such fluid induced activities were probably induced by the ca. 460–390 Ma Wuyi-Yunkai Orogeny in South China. The most important finding is that the rutile-xenotime assemblage was precipitated almost simultaneously, rendering a dissolution-reprecipitation process impossible. Absence of dissolution margins on detrital zircons suggest detrital Fe-Ti oxides are more prone to be leached by fluids than zircon grains, probably because the latter has more stable chemical properties.

Geochronology and geochemistry of Early Silurian felsic volcanic rocks in the Dabaoshan ore district, South China: Implications for the petrogenesis and geodynamic setting

The Early Paleozoic orogeny in South China has remained controversial in the long run with debate over intracontinental orogeny versus plate subduction. Limited studies have focused on the Early Paleozoic volcanic rocks in South China, which is crucial for us to understand the tectonic evolution of the South China Block. The Dabaoshan Fe–Cu–Pb–Zn–Mo–W orefield, one of the largest polymetallic deposits in South China, is hosted by Paleozoic volcanic‐sedimentary rocks and Mesozoic granitic porphyry. In this study, zircon U–Pb dating yielded ages of 439.4 ± 2.7 Ma, 439.3 ± 3.2 Ma, and 442.0 ± 2.2 Ma for the Dabaoshan dacite. These ages are all in good agreement within errors and indicate that the dacite formed during the early Silurian (ca. 440 Ma). The dacites are slightly peraluminous, calc‐alkaline with relatively high K2O + Na2O, FeOt, TiO2, CaO/Na2O ratios, and small negative Eu anomalies. They show enrichment in LILEs and LREE and depletion in Ba, Nb, Sr, P, and Ti. It has high initial 87Sr/86Sr values (0.70983–0.71140) and negative εNd(t) values ranging from −7.6 to −10 with T2DM ages of 1.80–1.99 Ga, εHf(t) values from −4.95 to −15.48 with T2DM ages of 1.72 Ga to 2.38 Ga. The overlapped Nd–Hf two‐stage model ages range from 1.89 Ga to 1.99 Ga, which indicates that the Dabaoshan dacite originated from a Paleoproterozoic source. Combining these geochemical and in situ zircon Hf isotopic data, we propose that the Dabaoshan dacite may have originated from the partial melting of mixed source of Paleoproterozoic metasedimentary and metaigneous rocks as a result of underplating of mafic magma during the post‐orogenic collapse and extension followed by lithospheric thining and asthenospheric mantle upwelling in an intraplate setting. Based on field observation and the geochronology results, we propose that the stratiform Cu–Pb–Zn mineralization may be related with the early Silurian volcanic rocks.

Early Paleozoic tectonic reactivation of the Shaoxing-Jiangshan fault zone: Structural and geochronological constraints from the Chencai domain, South China

The Shaoxing-Jiangshan fault zone (SJFZ), as a fundamental Neoproterozoic block boundary that separates the Yangtze Block from the Cathaysia Block, is the key to understanding the evolution of South China from Neoproterozoic block amalgamation to early Paleozoic crustal reworking. New structural observations coupled with geochronological ages from the Chencai domain indicate that intense ductile deformation and metamorphism along the SJFZ occurred at ∼460-420 Ma, in response to the early Paleozoic orogeny in South China. To the east of the SJFZ, the deformation involves widespread generations of NE-striking foliation, intrafolial folds, and local development of sinistral-oblique shear zones. The shearing deformation occurred under amphibolite facies conditions at temperatures of >550 °C (locally even >650 °C). To the west of the SJFZ, the deformation corresponds to sinistral-oblique shearing along NE-striking, steep-dipping zones under greenschist facies conditions at temperatures of 400–500 °C. These deformation styles, as typical mid-crustal expressions of continental reworking, reflect tectonic reactivation of the pre-existing, deeply rooted Neoproterozoic block boundary in the early Paleozoic. We infer that the tectonic reactivation, possibly induced by oblique underthrusting of north Cathaysia, facilitated ductile shearing and burial metamorphic reactions, giving rise to the high-strain zones and high-grade metamorphic rocks. With respect to pre-existing mechanical weakness, our work highlights the role of tectonic reactivation of early structures in localizing later deformation before it propagates into yet undeformed domains.

Thermal gradient and geochronology of a Paleozoic high-grade terrane in the northeastern Cathaysia block, South China

The Chencai terrane in the northeastern Cathaysia Block forms part of the northeast extension of the Paleozoic orogeny in South China. Recent field investigations have revealed the occurrence of high grade rocks in this terrane including granulite facies metapelites (khondalites) and orthopyroxene-bearing granulites. However, the thermal gradients and metamorphic history of this region have not been well constrained, thus impeding a better understanding of the geodynamic history of the Paleozoic orogeny in South China. Here we report results from a systematic study on the metamorphic PT conditions and geochronology of the orthopyroxene granulites and metasedimentary rocks (metapelites) from the Chencai area. The orthopyroxene granulites preserve granulite facies peak assemblage of Opx+Grt+Pl+Kfs+Qtz and a retrograde P-T trajectory. Peak PT conditions are 845–860°C and 0.83–0.86Gpa, corresponding to a thermal gradient of ~30°C/km which is much hotter than the normal thermal gradient of continental crust. Mineral compositions of the metasedimentary rocks re-equilibrated during retrograde metamorphism so that the peak PT conditions recorded by these rocks are much lower than those obtained from the orthopyroxene granulite. Peak PT conditions are 710–720°C, 0.57–0.58Gpa for garnet-biotite gneiss and 760–780°C, 0.60–0.62Gpa for biotite garnetite. Zircon U-Pb dating shows that this episode of granulite facies metamorphism occurred at 430–450Ma and the detrital population shows minor or no contribution from the basement rocks of the Cathaysia Block. The Neoproterozoic rocks occurring along the Jiangnan belt are considered to have provided the sedimentary source as indicated by the two main age peaks of 0.79Ga and 0.83Ga from the detrital zircon cores. Lu-Hf isotope data on the zircons indicates extensive alteration and little new zircon growth during the Paleozoic metamorphism. The εHf(t) values and model ages (TDM1) of zircons suggest that the source rocks involved juvenile crustal components with Hf model ages of 1.1–1.5Ga. Sedimentation of such a continental shelf sequence as well as the high thermal gradient of ~30°C/km confirm the previously proposed continental arc formation and maturation model in the Chencai region.

Mesozoic gold mineralization in Hainan Province of South China: Genetic types, geological characteristics and geodynamic settings

Numerous gold deposits and occurrences in Hainan Province of South China, predominantly of Mesozoic age account for more than 143t of proven gold reserves. The Au mineralization occurs either along a group of NE-, NNW- and WNW-trending shear zones or within NW- to NNW-trending intraformational detachment faults which are closely related to folding, shearing and transpressional to transtensional deformation. Combined with the Mesozoic tectonics and associated magmatism, the ore geology, fluid inclusion geochemistry, C-H-O-S-(Pb) isotopes and geochronology consistently indicate that there are at least two gold deposit-types in Hainan Island, i.e., orogenic-type and intrusion-related.The predominant orogenic gold mineralization, which produces more than 95% of gold metal reserves in Hainan Island, formed in the Early Mesozoic (ca. 228–224Ma). This ore deposit-type, represented by the Baolun, Gezhen, and Wangxia deposits, is generally hosted by metamorphosed volcaniclastic sedimentary rocks of the Paleo- to Mesoproterozoic, the Silurian and the Permian ages, and is closely associated with brittle-ductile shearing. These deposits, with native gold as main gold occurrence and low sulfide abundance (<5% by volume), are derived from the CO2-rich (mainly 4.8–16.8mol%), near neutral (pH=∼7), low-salinity (generally 3–10.5wt.% NaCleq.), and intermediate-temperature ore fluids. In combination with the paleomagnetism, sedimentation, and petrographical and geochemical features of the Late Paleozoic to Early Mesozoic granitoids, the orogenic gold mineralization is considered to be formed in a post-collisional tectonic setting, in response to the Indosinian orogeny in South China triggered by the closure of the Paleotethys Ocean. Abundant Bi-As-Te-Mo-S phases, which grew synchronously with the gold minerals, suggest a possible involvement of magmatic fluids, especially for the large-scale, high-grade Baolun deposit.The Fuwen Au-dominated Au-Ag deposit, which is hosted by Lower Cretaceous continental clastic rocks, was interpreted as an intrusion-related deposit and most likely formed in the Late Cretaceous. This deposit is endowed with high gold grade (average 28–95g/t Au), extremely high sulfide abundance (>50% by volume), Au-Ag-Cu-Pb-Zn metal association, and small amounts of alteration minerals (pyrite, quartz, sericite, chlorite and calcite). The rare isotopic data of O-H-S-Pb, and the ore occurrences mainly in the intraformational detachment fault zones and subordinately within the Late Cretaceous adakite-like granitoids (zircon U-Pb age of ca. 100Ma), suggest that the Fuwen deposit had an intimate genetic link to the arc-related extension-type magmatism, due to asthenospheric upwelling triggered by the slab roll-back of the subducted Paleo-Pacific plate beneath the South China Block.