Southern Qinling Research Articles

Meso–Cenozoic Exhumation in the South Qinling Shan (Central China) Recorded by Detrital Apatite Fission-Track Dating of Modern River Sediments

The Qinling Shan is located between the North China Craton and the South China Block. Not only is investigating the exhumation process of the Qinling Shan beneficial for comprehending the tectonic collision history of mainland China but also for enhancing our understanding of the development of the Yellow and Yangtze Rivers. Previous studies have predominantly focused on bedrock analysis in the Qinling Shan. However, modern fluvial detrital samples offer a more extensive range of thermal history information. Therefore, we gathered modern fluvial debris samples from the Hanjiang River, which is the largest river in the South Qinling Shan. Subsequently, we conducted apatite fission-track analysis using the laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) method. A total of 214 valid track ages were obtained, with an age distribution ranging from 9.5 to 334.0 Ma. The Density Plotter software was employed to decompose the data and generate four prominent age peaks: 185, 103, 69, 35, and 12 Ma. The exhumation events of the Early Jurassic (185 Ma) and Cretaceous (103–69 Ma) in the Southern Qinling Shan were strongly influenced by the collision between the South China Block and the North China Craton, as well as the subduction of the West Pacific Plate, respectively. The far-field effect of the collision between the Indian Plate and the southern Asian continent influenced the exhumation of the South Qinling Shan during the Late Eocene (35 Ma) and Middle Miocene (12 Ma), respectively. In conjunction with the reported findings, we comprehensively analyzed the geological implications of the Mesozoic and Cenozoic exhumations of the Qinling Shan. The Qinling Shan emerged as a watershed between the Ordos and Sichuan Basins in the early Mesozoic and Cenozoic, respectively. However, the exhumation and expansion of the Tibetan Plateau has forced the Yangtze River to flow eastward, resulting in its encounter with the South Qinling Shan in the late Cenozoic. The exhumation of the Qinling Shan has resulted in fault depression in the southern Ordos Basin. This geological process has also contributed to the widespread arid climatic conditions in the basin. During the Miocene, the Yellow River experienced limited connectivity due to a combination of structural and climatic factors. As a result, the Qinling Shan served as an obstacle, dividing the connected southern Yangtze River from the northern segment of the Yellow River during the late Cenozoic era.

Mineralogy and trace element geochemistry of the early Cambrian black shale-hosted Zhongcun vanadium deposit, southern Qinling, China

Mineralogy and trace element geochemistry of the early Cambrian black shale-hosted Zhongcun vanadium deposit, southern Qinling, China

Deformation of the Qinling belt revealed byP-wave velocity and azimuthal anisotropy tomography

SUMMARYThe Qinling belt is a transitional zone lying among three units: the North China block (NCB), the South China block (SCB) and the northeastern Tibetan Plateau (NETP). Owing to the interaction of these units, complex deformation has occurred in the Qinling belt. Although many studies have been conducted to understand the deformation mechanism in the Qinling belt, some key issues are still under debate, such as whether middle-lower crustal flow exists beneath the western Qinling belt (WQB). High-resolution images of subsurface structures are essential to shed light on the deformation mechanism. In this paper, high-resolution images of the velocity structure and azimuthal anisotropy beneath the Qinling belt are obtained by using an eikonal equation-based traveltime tomography method. Our seismic tomography inverts 38 719 high-quality P-wave first arrivals from 1697 regional earthquakes recorded by 387 broad-band seismic stations. In the WQB, our tomography results show low-velocity anomalies but relatively weak anisotropy in the middle-lower crust. These features suggest that middle-lower crustal flow may not exist in this area. In the central Qinling belt (CQB), we find low-velocity anomalies in the middle-lower crust; however, the fast velocity directions no longer trend E–W but vary from NNE–SSW to N–S. These characteristics can be ascribed to the convergence and collision between the NCB and the SCB. In addition, we find strong low-velocity anomalies in the uppermost mantle beneath the CQB, which may indicate delamination of the lower crust. In the southern Qinling belt, we observe significant high-velocity anomalies in the upper crust beneath the Hannan–Micang and Shennong–Huangling domes. These high-velocity anomalies indicate a mechanically strong upper crust, which is responsible for the arc-shaped deformation process of the Dabashan fold. Based on the P-wave velocity and azimuthal anisotropic structures revealed by the inversion of high-quality seismic data, the deformation of the Qinling belt is affected mainly by the convergence between the NCB and the SCB rather than by the middle-lower crustal flow from the Tibetan Plateau.

Late Ordovician-Early Silurian extension of the northern margin of the Upper Yangtze Platform (South China) and its impact on organic matter accumulation

Breakthroughs in shale gas exploration in the Upper Ordovician-Lower Silurian strata of the Upper Yangtze Platform have attracted interest in its sedimentary-tectonic evolution, but the tectonic background of the northern margin of the Upper Yangtze Platform remains unclear. In this paper, the Wufeng-Longmaxi formations on the northern margin of the Upper Yangtze Platform were investigated. Based on geochemical and mineralogical analyses of the tuffs/K-bentonites of the Wufeng Formation and the barite in the Longmaxi Formation, as well as previous research results, it was concluded that the northern margin of the Upper Yangtze Platform was in an extensional tectonic background during the Late Ordovician-Early Silurian. Detailed analysis revealed that, (1) the U–Pb zircon age of the tuff in the Bajiaokou section in South Qinling is 443.91 ± 0.92 Ma. The Zr/TiO2–Nb/Y diagram of the tuffs/K-bentonites indicates that their protoliths were alkaline-subalkaline basalt and andesite series rock. Based on the Th–Hf/3-Ta, Th–Tb*3-Ta*2, and TiO2–Nb/3-Th diagrams, there are undiscovered intraplate tension calc-alkaline basalts in the northern Yangtze Platform or the southern Qinling region, which provided volcanic clastic materials to the Ziyang, Lan'gao, Chengkou, Yichang and other regions. (2) Scanning electron microscopy revealed that the barite crystals in the Longmaxi Formation exhibit dissolution features and have a large particle size. Energy spectrum analysis of these barite crystals revealed that they have C, O, S, and Ba contents of 8.48 wt%, 22.98 wt%, 13.09 wt% and 55.44 wt%, so they are speculated to have been formed via cold methane seep genesis in a weak extensional tectonic setting. The 87Sr/86Sr ratios of the barite revealed that different types of barite were simultaneously formed in this area under the influences of hydrothermal and cold methane seeps. (3) The analysis of the heavy minerals in the Lower Silurian strata in the Bajiaokou section revealed that the provenance in the South Qinling area changed significantly during the late Early Silurian. Based on the above analyses, the northern margin of the Upper Yangtze Platform was in an extensional tectonic setting during the Late Ordovician-Early Silurian. The distribution of the total organic carbon content indicated that the extensional tectonic background provided good conditions for the enrichment and preservation of organic matter. The results of this study provide an understanding of the regional sedimentary-tectonic pattern and evolution of the Yangtze Platform during this period, as well as a reference for future shale gas exploration in this region.

Tectonic evolution of the Qinling Orogenic Belt, Central China – new evidence from geochemical, zircon (U-pb) geochronology and HF isotopes

The paper deals with a review of tectonic evolution in different regions of China with the help of different techniques and models. Tectonic evolution shows that in the shallow layers of China the structural impact is not solid, and huge structural zones are deficient within the list and incline zones where the principal wretchedness and delicate slant zones framed. The Qinling Orogenic Belt (QOB) situated between the North China Craton (NCC) and the Yangtze Craton (YZC) is made from the Northern Qinling Belt (NQB) and the Southern Qinling Belt (SQB). The Hf isotopic creations of zircons from the different rocks recommend that the NQB most likely created on the cellar of the southern NCC. The stones in the SQB show zircon age spectra and Hf-isotope structures like those in the northern YZC, recommending a nearby proclivity. We thusly decipher the SQB to have created on the cellar of the northern YZC. Incorporating the new information in this investigation with those from past works, we propose another structural model for the development and advancement of the QOB during late Mesoproterozoic to early Paleozoic including the accompanying significant occasions: (1) Late Mesoproterozoic to early Neoproterozoic (Grenvillian) toward the north subduction of the Songshugou Ocean; Early-center Neoproterozoic (870-800 Ma) bidirectional subduction and impact; Middle Neoproterozoic (∼800-710 Ma) post-crash expansion; Middle-late Neoproterozoic (710-600 Ma) inside plate augmentation; Late Neoproterozoic-early Paleozoic (600-520 Ma) opening of the Shangdan Ocean; and Early Paleozoic (520-420 Ma) subduction-crash. We accordingly follow in any event two unmistakable Wilson cycles in the QOB.

Origin of the Tudigou intrusion and associated porphyry Cu mineralization in the southern Qinling, Central China: Implication for regional tectonic setting

The Tudigou porphyry Cu deposit is located in the central part of the southern Qinling belt. Copper mineralization is closely associated with monzogranite porphyry. Zircon LA–ICP–MS U–Pb dating of monzogranite porphyry suggests that the ages of magmatism at Tudigou vary between 150 ± 1 and 148 ± 1 Ma and overlap with the molybdenite Re–Os age of 150–148 Ma, indicating that the Tudigou intrusion and associated mineralization formed during the Late Jurassic. The enrichment in large‐ion lithophile elements, depletion in high‐field‐strength elements, high Ba/Th and La/Yb ratios, and negative correlation between Dy/Yb and La/Sm ratios, together with narrow ranges of εHf(t) values (εHf(t) = −4.0 to +1.4) suggest monzogranite porphyry was derived from a metasomatized lithospheric mantle with minor crustal components contribution, which is also evidenced by the lead isotopic result. The La/Yb ratios of 30 Mesozoic plutons in the southern Qinling belt were used to calculate the Moho depth, which suggests that the magmatism and mineralization in the region occurred during lithosphere thinning in response to the tectonic transition from compression to extension transitional regime. The calculated oxygen fugacity of the Late Mesozoic porphyry deposits in the Qinling–Dabie orogen have identical ΔFMQ values (averaging +1.4 to +2.4). However, the V/Sc ratios (the proxy for water content) are distinct among different deposit sizes, indicating that the dissolved water in the magma plays a critical role in porphyry‐type mineralization.

Lithospheric electrical structure and its implications for the evolution of the middle Qinling Orogenic Belt, Central China: Constraints from 3-D magnetotelluric imaging

The Qinling Orogenic Belt is located at the central part of the China Central Orogenic Belt and links the Qilian and Kunlun Orogenic Belts to the west with the Dabie-Sulu Orogenic Belt to the east, which is sandwiched between the South China Block and North China Block. There have been several geodynamic models developed to explain the evolution process of the Qinling Orogenic Belt, but they remain controversial. The magnetotelluric (MT) method can study subsurface electrical structures. In order to have a better understanding of the tectonic evolution of the Qinling Orogenic Belt, a study of broadband and long period MT profile across the middle Qinling Orogenic Belt is carried out. The MT profile starts from the northeastern margin of the Sichuan basin, crosses the Qinling, and finally reaches the Weihe basin at about 108.5°E. Based on data analysis results, we have finally obtained a lithospheric electrical resistivity model of the middle Qinling Orogenic Belt through three-dimensional (3-D) inversion of magnetotelluric data. The MT result reveals that the Sichuan and Weihe basins may have remnants of the ancient craton basement in our study area. The electrical structure characteristics of the South Qinling Belt and North Qinling Belt are obviously different, the South Qinling Belt is relatively low resistivity, while the North Qinling Belt is relatively high resistivity. This may indicate that the Southern Qinling Belt is affected by the later strike slip extrusion and the Northern Qinling Belt is a relatively stable block. The lithospheric mantle of middle Qinling Orogenic Belt is characterized by overall high resistivity, which may indicate that the lateral extrusion effect of the Tibetan Plateau material is negligible in this region.

Predicting the Areas of Suitable Distribution for Zelkova serrata in China under Climate Change

Predicting the geographic distribution of a species together with its response to climate change is of great significance for biodiversity conservation and ecosystem sustainable development. Zelkova serrata is an excellent shelterbelt tree species that is used for soil and water conservation due to the fact of its well-developed root system, strong soil fixation, and wind resistance. However, the wild germplasm resources of Z. serrata have been increasingly depleted due to the fact of its weak ability to regenerate naturally and the unprecedented damage humans have caused to the natural habitats. The present work using Maxent aimed to model the current potential distribution of this species as well as in the future, assess how various environmental factors affect species distribution, and identify the shifts in the distribution of this species in various climate change scenarios. Our findings show habitat in provinces in the southern Qinling and Huai river basins have high environmental suitability. Temperature seasonality, annual precipitation, annual mean temperature, and warmest quarter precipitation were the most important factors affecting its distribution. Under a climate change scenario, the appropriate habitat range showed northeastward expansion geographically. The results in the present work can lay the foundation for the cultivation and conservation of Z. serrata.

Geology and in-situ sulfur and lead isotope analyses of the Jinlongshan Carlin-type gold deposit in the Southern Qinling Orogen, China: Implications for metal sources and ore genesis

The Jinlongshan gold deposit, Shaanxi Province, central China, is a large deposit (>150 t Au) in the Southern Qinling Orogen. Gold mineralization is hosted in Upper Devonian to Lower Carboniferous carbonaceous clastic-carbonate sequences. Four stages of pyrite and paragenetic sulfides have been identified based on morphology and paragenesis. Laser-ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) and laser ablation multi-collector inductively coupled plasma mass spectrometry (LA–MC–ICP–MS) analyses were conducted to determine the chemical and S–Pb isotopic composition of the sulfides. Pre-ore stage pyrites (Py1 and Py2) contain relatively high concentrations of Mn, Co, Ni, Cu, Zn, Sb, and Pb exhibiting a wide range of δ34S values, ranging from –29.9‰ to +48.4‰. These two pyrites were formed by bacterial and thermochemical sulfate reduction of Upper Devonian marine sulfate, respectively. Main-ore stage pyrite (Py3) and arsenopyrite (Apy) contain high concentrations of Au, As, Cu, Sb, Hg, and Tl with restricted ranges of δ34S values from 9.6‰ to 15.3‰, which is interpreted as resulting from the underlying low-grade metamorphic Paleozoic sedimentary rocks by the decomposition of pyrite and/or inorganic reduction of sulfate. Late- or post-ore stage pyrite (Py4) is characterized by low concentrations of Au, Ag, Sn, Hg, and Bi and exhibited variable δ34S values, ranging from –4.4‰ to +27.7‰, that may reflect the mixing of metamorphic fluids with meteoric water. Similarly, the Pb isotope composition of ore-stage sulfides resembles those of underlying low-grade metamorphic Paleozoic sedimentary rocks whereas differs from Precambrian metamorphic basement rocks and granitoids around the basin, indicating that the metals of the Jinlongshan gold deposit were sourced from the underlying Paleozoic sedimentary rocks.Detailed fieldwork revealed that the deposit has mainly undergone four deformation events. The ore bodies developed in NE-trending dextral oblique-slip reverse faults of D3 that crosscut WNW-trending shear zones of D2. The timing and corresponding stress regimes of the D3 event suggest that it formed in a local extensional environment of Yanshanian intracontinental orogeny of Qinling orogenic belt. Gold metallogenesis of the Jinlongshan gold deposit occurred during the transition of tectonic setting from compression to extension, which reactivated the shear zones and migrated deep ore-bearing fluids to NE-trending brittle faults. The geological and geochemical features of the Jinlongshan gold deposit are more analogous to the Carlin-type deposits than other orogenic gold deposits worldwide; therefore, we propose it is a typical Carlin-type gold deposit in South Qinling Orogen.

Geology, fluid inclusions, and H–O–S–Pb isotopes of the Chigou porphyry Cu deposit in Southern Qinling, central China: Implication for ore genesis

The Chigou porphyry Cu deposit is located in eastern segment of the Southern Qinling metallogenic belt, on the southern margin of the Central China Orogenic Belt. Copper mineralization is associated with quartz diorite, quartz diorite porphyry and granite porphyry, and occurs as veins, veinlets, disseminations, or veinlet-disseminations within host porphyries. Ore-forming process could be divided into three stages: stage I quartz + K-feldspar + pyrite ± chalcopyrite, stage II quartz + chlorite + chalcopyrite ± molybdenite and stage III quartz + calcite + pyrite. Four types of fluid inclusions (FIs) were distinguished in quartz-bearing veins, including vapor-rich two-phase (V-type), liquid-rich two-phase (L-type), hypersaline (H-type), and CO2-rich (C-type) inclusions. The FIs were homogenized at temperatures of 330–473 °C for stage I, 174–345 °C for stage II, and 128–272 °C for stage III, with corresponding salinities of 3.8–53.6 wt% NaCl equiv., 1.4–15.1 wt% NaCl equiv., and 0.4–7.7 wt% NaCl equiv., respectively. Hydrogen and oxygen isotope data indicate that the ore-forming fluids have a dominant magma signature and were diluted by meteoric water during the late stage. Chalcopyrite and pyrite from different stages record a narrow range of δ34S values from −0.2 to −2‰, consistent with the sulfur isotopic values of typical magmatic-hydrothermal system. Chalcopyrite has similar 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios to those of porphyry intrusions. All stable and radiogenic isotope results reveal that the ore-forming components (i.e. metal and sulfur) were sourced from a magmatic reservoir with some crustal materials. Laser Raman micro spectroscopy data and fluid inclusion analytical results suggest that the Chigou deposit was formed by an initially high- to intermediate-temperature, high-oxygen fugacity (fO2) and CO2-rich fluid system at an estimated depth of 0.7–2.1 km. Decreases in temperature and fO2 as well as fluid boiling were critical factors controlling sulfide precipitation at Chigou.

Mantle plume: the dynamic setting of the origin of Early Paleozoic mafic dykes in Ziyang, Shaanxi Province, Southern Qinling Block, China

The mafic dykes (dolerites) during the Early Paleozoic are widely spread in Langao-Ziyang, southern Qiling Block, and the investigation on these dykes are very important. Previous studies have mainly focused on the Silurian mafic dykes; however, research on the Earlier Paleozoic mafic dykes is relatively weak at present. Therefore, the overall understanding of the mantle source and genetic dynamic setting during the Early Paleozoic in this area is lacking. To study the accurate age and origin of the Early Paleozoic mafic dykes in Ziyang, southern Shaanxi Province, the mafic dykes from dabacunand Qinmingzhai were selected and the petrology, zircon U–Pb chronology, geochemistry, and Sr–Nd–Hf isotopes were studied. Analysis indicates that the mafic dykes studied are mainly composed of dolerite, and they are the products of the Early Ordovician (475.8–480.7 Ma). Furthermore, the dolerites belong to alkaline rock series, and they are characterized by enrichment in LREE, Rb, Ba, Sr, Nb, (87Sr/86Sr)i = 0.7020–0.7050, eNd(t) = 3.0–4.0), eHf (t) = 4.5–12.1,176Hf/177Hf = 0.282681–0.282844. This suggests that the mafic dyke were derived from the partial melting of a depleted lithospheric mantle, and the genetic process is mainly controlled by the mantle plume based on the discussion of the genetic model. Furthermore, the genetic process experienced the separation and crystallization of olivine and clinopyroxene at the same time, with little crustal contamination.

Paleozoic peralkaline A-type magmatism of the Tongbai Orogen, Central China: Petrogenesis and tectonic implications

The precise timing, petrogenesis, and geodynamic significance of the Paleozoic magmatic rocks in the southern Qinling–Tongbai Orogenic Belt (QTOB), Central China, are poorly constrained. We performed an integrated study of petrology, zircon UPb ages, geochemistry, and Hf–O–Nd isotopic compositions of the Huangyangshan pluton from the southern QTOB to constrain its genesis and tectonic implications. A granite sample from the pluton yields a weighted mean zircon 206Pb/238U age of 445 ± 6 Ma, representing the age of the pluton. The Huangyangshan pluton is dominated by alkaline granite and quartz syenite, has a peralkaline signature (A/NK = 0.82–0.96), and belongs to the shoshonite and high-K calc-alkaline series. Both the alkaline granite and quartz syenite have geochemical signatures similar to A-type granite, with high SiO2 and total alkali (Na2O + K2O) contents, high Fe-numbers and TiO2/MgO and Ga/Al ratios, and high field-strength elements (HFSEs), along with low MgO, CaO, and P2O5 contents. These samples are enriched in light rare earth elements (LREEs) and have flat heavy rare earth element (HREE) patterns and strongly negative Eu anomalies. They are enriched in most large-ion lithophile elements (LILEs) and HFSEs, with significant negative Eu, Ba, Sr, and Ti anomalies in primitive-mantle-normalized trace-element diagrams. The samples have relatively high Zr contents (175–776 ppm), yielding minimum magmatic temperatures of 768 °C–891 °C. The Huangyangshan pluton has initial zircon εHf(t) values of +6.6 to +10.0 and whole-rock εNd(t) values of +2.86 to +3.62. The analyzed zircons yield a mean δ18O value of 5.14‰ ± 0.26‰, indicating negligible incorporation of hydrothermally altered crustal materials. The geochemical signatures of the Huangyangshan pluton indicate that the magmas were derived from prompt partial melting of juvenile crust rocks. Combined with previous results, we suggest that the Paleozoic Huangyangshan peralkaline A-type granites resulted from the rifting of a passive continental margin in the northern South China Block, followed by the opening of the Paleotethyan Ocean.

Cosmogenic 26Al/10Be Burial Dating of the Uplift Rate of Southern Qinling Mountains since the Middle Pleistocene

Cosmogenic ²⁶Al/¹⁰Be Burial Dating of the Uplift Rate of Southern Qinling Mountains since the Middle Pleistocene

Re‐Os Isotopic Dating of a W‐Be Polymetallic Deposit in the Southern Qinling Region, China

Re‐Os Isotopic Dating of a W‐Be Polymetallic Deposit in the Southern Qinling Region, China

Spatiotemporal variations of forest phenology in the Qinling Mountains and its response to a critical temperature of 10°C

Based on the fifth version MOD09A1 product of NASA LP DAAC (Land Processes Distributed Active Center) from 2001 to 2016, an enhanced vegetation index (EVI) time series dataset in the Qinling Mountains was reconstructed; it was used to extract the plant phenological parameters of the region by employing the maximal slopes of the alteration method and threshold method. The results show that the Whittaker filter method was better than other methods for the reconstruction of shrubland, farmland, and forest in the Qinling Mountains. Based on the reconstructed EVI, the characteristics of vegetation coverage, the start of growth season (SOG), the end of growth season (EOG), and the length of growth season (LOG) in the Qinling Mountains were all analyzed. There was an increasing trend of vegetation coverage in most regions (about 89.93% of the monitored areas) over the Qinling Mountains in the past decades, and the average phenological distribution pattern in the Qinling Mountains was closely related to the local hydrothermal conditions. From the high altitude mountainous areas to the farming areas, the SOG was gradually postponed, the EOG was gradually earlier, and the LOG gradually shortened. Furthermore, the time series variation of SOG, EOG, and LOG from 2001 to 2016 in the Qinling Mountains was also studied. The variation showed that the SOG shifted earlier, which was more prominent in high-altitude areas, while, for some southern and northern foothills altitude below 500 m and a few areas of the eastern Funiu Mountains, the SOG was delayed. The EOG shifted later, which was more apparent in the northern Qinling foothills and mid- to low-altitude areas of the eastern ranges, and the LOG was extended. Finally, the studies of correlation analysis between the plant phenology and a temperature threshold of 10°C showed that global warming was the major factor affecting the phytophenology of the Qinling Mountains, and the effects were concentrated mainly in the 1000- to 2000-m zones of the southern and northern Qinling Mountains. Moreover, the northern Qinling foothills showed a greater response to accumulated temperature than the southern foothills, and the low-altitude area of the eastern Funiu Mountains exhibited the lowest correlation with the accumulated temperature.

Provenances and tectonic implications of Paleozoic siliciclastic rocks from the Baishuijiang Group of the southern Qinling belt, central China

The Paleozoic Baishuijiang Group is exposed in the southern Qinling belt and consists of turbidite sediments. The provenances of the Paleozoic sedimentary rocks is constrained by the integration of major and trace elements and detrital zircon U–Pb dating, which can help to understand the connection between the provenance and the Paleozoic tectonic evolution of the Qinling orogenic belt. The sandstones and mudstones have intermediate SiO2/Al2O3 and K2O/Na2O ratios, and high Fe2O3+MgO contents. In comparison with average upper continental crust, they show strong negative Nb–Ta and Sr anomalies, slight depletion of Zr–Hf and Th, but moderate enrichment of Sc, Ni and Cr. These rocks show LREE enrichment, and pronounced negative Eu anomalies in chondrite-normalized REE patterns, similar to post-Archean shales. The weathering trend of the sandstones and mudstones suggests andesitic and granodioritic provenances. These sediments are geochemically similar to continental island arc sediments, and therefore were probably deposited at an active continental margin. Detrital zircon U–Pb dating of five sandstones from the Baishuijiang Group yielded ages ranging from 407 to 3000Ma, with six peaks at ca. 425, 710, 780, 930, 1800, and 2500Ma. The youngest age peak at 425Ma and two zircon grains with a weighted average age of 414Ma indicate that the maximum depositional age of the Baishuijiang Group is late Paleozoic. Our new age data thus suggest that these turbidite sediments were mainly derived from the northern Qinling belt and the northern margin of the Yangtze Block. We propose that the Paleozoic sediments were deposited in an ocean basin between the northern Qinling belt and the Yangtze Block. Combined with regional geological evidences, our results indicate that there was a paleo-ocean in the Qinling orogenic belt since Neoproterozoic era. The oceanic crust was continually subducted northward, resulting in multiple accretion events from the early Paleozoic to early Triassic, which ultimately led to the amalgamation of the north China and Yangtze plates.

Neoproterozoic subduction-accretionary tectonics of the South Qinling Belt, China

The Neoproterozoic magmatism in the South Qinling Belt and northern margin of the South China Block is important key to understanding the tectonic evolutionary history of the South China Block, and to reconstruct the tectonics of the supercontinent Rodinia. The Xiaomoling magmatic complex in the South Qinling Belt consists mainly of basalts in the Mogouxia area, gabbros and diorites in the Chishuigou, Lengshuigou and Qianyouhe areas. The basalts in the Mogouxia are characterized by depletion of LREEs and LILEs, and non- differentiation of HFSEs, suggesting their deviation from a slightly depleted MORB mantle source. Taken together with geochemical and geological data, the basalt in the Mogouxia area is interpreted to have derived from a subduction-related back-arc basin setting. The gabbros from both Chishuigou and Lengshuigou sections display clear LREE, Th, and LILEs enrichment, and depletion of Nb, Ta and Ti, and highly differentiation in HFSEs, which are similar to that of a typical Island Arc Basalts. The diorites from both Chishuigou and Qianyouhe areas display strong LREE enrichment, depletion of Nb, Ta, P and Ti, enrichment of LILEs, and flat distribution pattern of HFSEs in the primitive mantle-normalized trace element pattern. According to their geochemistry, both gabbros and diorites from different localities of the Xiaomoling magmatic complex were mostly likely formed in an island-arc tectonic setting. The zircons concentrated from the gabbro and diorite yield LA-ICPMS U-Pb ages of 753±4Ma and 843±5Ma, respectively, representing their formation ages. Integrated all the available geological, geochemical and geochronological data, we propose that the Xiaomoling magmatic complex represents the northernmost of the accretionary belt to the north of the South China Block, while the Hannan-Huangling Neoproterozoic intrusions mark the southernmost of the accretionary belt. Therefore, the most of southern Qinling represents a wide subduction and accretionary tectonic belt in Neoproterozoic times. Our new geochronology and previously published reliable ages estimate a long-lived subduction during ca. 850–700Ma at the present South Qinling area, to the north of the South China Block.

Early–Middle Jurassic evolution of the northern Yangtze foreland basin: a record of uplift following Triassic continent–continent collision to form the Qinling–Dabieshan orogenic belt

The northern Yangtze foreland basin system was formed during the Mesozoic continental collision between the North and South China plates along the Mianlue suture. In response to the later phase of intra-continental thrust deformation, an extensive E–W-trending molasse basin with river, deltaic, and lake deposits was produced in front of the southern Qinling–Dabieshan foreland fold-and-thrust belt during the Early–Middle Jurassic (201–163 Ma). The basin originated during the Early Jurassic (201–174 Ma) and substantially subsided during the Middle Jurassic (174–163 Ma). A gravelly alluvial fan depositional system developed in the lower part of the Baitianba Formation (Lower Jurassic) and progressively evolved into a meandering river fluvial plain and lake systems to the south. The alluvial fan conglomerates responded to the initial uplift of the southern Qinling–Dabieshan foreland fold-and-thrust belt after the oblique collision between the Yangtze and North China plates during the Late Triassic. The Qianfoya Formation (lower Middle Jurassic) mainly developed from shore-shallow lacustrine depositional systems. The Shaximiao Formation (upper Middle Jurassic) predominantly consists of thick-bedded braided river delta successions that serve as the main body of the basin-filling sequences. The upward-coarsening succession of the Shaximiao Formation was controlled by intense thrusting in the southern Qinling–Dabieshan fold-and-thrust belt. Palaeogeographic reconstructions indicated an extensive E–W foredeep depozone along the fold-and-thrust belt during the Middle Jurassic (174–163 Ma) that was nearly 150 km wide. The depozone extended westward to the Longmenshan and further east to the northern middle Yangtze plate. The northern Yangtze foreland basin was almost completely buried or modified by the subsequent differential thrusting of Dabashan and its eastern regions (Late Jurassic to Cenozoic).

Late Mesozoic development of the southern Qinling–Dabieshan foreland fold-thrust belt, Central China, and its role in continent–continent collision

The southern Qinling–Dabieshan foreland fold-thrust belt is a key region in which to investigate the continental–continental collision between North China and South China. We provide structural evidence of a diachronous dextral strike-slip thrusting deformation from the Middle Triassic in the east to the late Late Triassic in the west, which indicates a northwestward oblique subduction and suturing of the South China plate under the North China–Qinling–Dabieshan plate. We also identify a later phase of intracontinental deformation that included an orthogonal intracontinental collision and south-vergent thrusting during the Early and Middle Jurassic, indentation of South China into the Qinling–Dabieshan Orogen and arc-shaped extrusions of the southern Qinling–Dabieshan foreland fold-thrust belt from the Late Jurassic to the Cretaceous. Our results reveal that the long-term intracontinental collision rotated clockwise in a northwest–southeast-vergent direction in the Late Triassic to northeast–east–southwest–west-vergent in the Late Cretaceous. This collision may have been driven by a north-vergent subduction of the Meso-Tethys Ocean in the south, west-vergent subduction of the Izanagi plate in the east, and south-vergent compression of the Eurasia plate in the north.

鄂西地区三叠纪嘉陵江组二段与三段局部平行不整合识别及沉积古地理研究

研究区位于上扬子地块的中北缘，南秦岭造山带的南侧，西侧为米仓山断裂带与大巴山弧形断裂带，是北东向构造区与近东西向构造区的叠合位置，在构造分区上具有重要的意义，构造运动在本区具有敏感的响应。前人在对本区的研究中认为晚二叠世–早三叠世的地层中均为整合接触。但在本次调查过程中，通过沉积地层、构造资料的研究对比分析，识别出了嘉陵江组二段与三段(T1-2j2/T1-2j3)之间的局部平行不整合接触关系，代表区域内又一次差异升降运动。同时将印支运动在本区活动的时限由中晚三叠世向前推至早–中三叠世。 The study area is located on the northern edge of the Yangtze Block and the south side of the Southern Qinling orogenic belt, and on the west side of this area there are Micang mountain fault belt area and Dabashan arc fault belt area. This area is an overlaying zone between NE and EW trending structure, and it shows great influences on the structure partition, with sensitive response to tectonic movement in this area. The early study believes that the formations from the Late Permian to the Early Triassic are conformities. However, this study, from the analysis of sediment formations and structure, identifies the accord unconformity between the middle and the upper Jialingjiang formation (T1-2j2/T1-2j3), which stands for another differential subsidence. The latest effect time of Indosinian movement in this area is early-middle Triassic.