Neoproterozoic(750–711 Ma)Tectonics of the South Qinling Belt,Central China:New Insights from Geochemical,Zircon U-Pb Geochronological,and Sr-Nd Isotopic Data from the Niushan Complex

The Xiejiaba and Fuqiangbei plutons form part of the newly identified Neoproterozoic Niushan complex,which is located in the southern South Qinling belt(SQB).The plutons are compositionally similar,were emplaced at 750–711Ma,and provide insights into Neoproterozoic tectonism within the South Qinling belt.The Xiejiaba pluton contains diorite,quartz diorite,granodiorite,and granite phases,all of which are sub-alkaline and have variable major element compositions with negative correlations between SiO_(2)and MgO,TFe_(2)O_(3),Al_(2)O_(3),CaO,TiO_(2)and P_(2)O_(5).These rocks are enriched in light rare earth elements(LREEs)and large ion lithophile elements(LILEs)and have negative Nb,Ta,P and Ti anomalies,all of which are indicative of arc-type magmatism.The Fuqiangbei pluton contains granitoids that are compositionally similar to the rocks in the Xiejiaba pluton.Samples from these plutons have similarε_(Nd)(t)values(1.24–5.99)but very variable(^(87)Sr/^(86)Sr)i values(0.7010–0.7054).Combining these data with the geochemical data for these rocks suggests that the magmas that formed the Niushan complex were derived from the crust–mantle boundary.This,combined with the results of previous research,suggests that the transition from low pressure-low temperature to low pressure-high temperature conditions within a subduction zone caused melting during a period of late subduction and backarc extension.This constrain the timing of subduction within the South Qinling belt and the northern Yangtze Block(YB)to 750–711 Ma,with this Neoproterozoic subduction associated with an ocean to the north overprinting an existing continental rift-type tectonic setting within the northern margin of the Yangtze Block and the South Qinling belt.

Petrological and Geochemical Constraints on the Origin of Strongly Peraluminous Granitoids from the Triassic Guangtoushan Pluton in South Qinling

As an important part of the early Mesozoic granites in the South Qinling tectonic belt(SQTB),the Guangtoushan pluton provides a material basis for research on the composition of magma sources and the effects of peritectic assemblage entrainment(PAE)on the changes in the granite composition.As shown by the results of LA-ICP-MS zircon U-Pb dating,the Guangtoushan pluton was emplaced during the Late Triassic(214-212 Ma)and was formed in the post-collision stage between the SQTB and the Yangtze plate.The collected samples had high SiO_(2)content and low Cr and Ni contents,indicating that the magmas did not undergo significant crust-mantle mixing during their evolution.The Guangtoushan granitoids were distributed along the trend line of magmatic fractional crystallization in the F-An-Or diagram.This result,combined with the relatively homogeneous Sr-Nd isotopic composition,implies that the Guangtoushan pluton underwent slight assimilation and contamination.As can be inferred from the comparison between the compositions of the Guangtoushan granitoids and various fluid-absent experimental melts,the magma sources of the Guangtoushan granitoids contain a variety of materials,such as graywackes,pyroclastic graywackes,and pelites and are not derived from lower crustal mafic rocks.The correlation between the maficity and the major and trace elements further indicates that the strongly peraluminous granitoids from the Guangtoushan pluton was formed by the partial melting of biotite-bearing crustal rocks and its magmatic evolution was accompanied by the entrainment of clinopyroxenes and accessory minerals.

U-Pb Zircon and Re-Os Molybdenite Geochronology of the W-Mo Mineralized Region of South Qinling, China, and their Tectonic Implications

A W-Mo mineralized region is located along the northern margin of the South Qinling tectonic belt of China. WMo mineralization occurs mainly in Cambrian–Ordovician clastic and carbonate rocks, and the ore bodies are structurally controlled by NW–SE-and NNE–SSW-striking faults. Evidence for magmatism in the area is widespread and is dominated by intermediate–felsic intrusives or apophyses, such as the Dongjiangkou, Yanzhiba, Lanbandeng, and Sihaiping granitic bodies. Quartz-vein-type mineralization and fault-controlled skarn-type mineralization dominate the ore systems, with additional enrichment in residual deposits. At present, there are few or insufficient studies on(1) the age of mineralization,(2) the relationship between intermediate–felsic granite and W-Mo mineralization,(3) the source of ore-forming materials, and(4) the metallogenic and tectonic setting of the mineralized area. In this paper, we present geochronology results for numerous intrusive granitic bodies in the South Qinling tectonic belt. U-Pb zircon geochronology of the Lanbandeng monzogranite and Wangjiaping biotite monzogranite yields ages of 222.7 ± 2.3 and 201.9 ± 1.8 Ma, respectively. In contrast to the Late Triassic age of the Lanbandeng monzogranite, the age of the newly discovered Wangjiaping biotite monzogranite places it at the Triassic–Jurassic boundary. Re-Os molybdenite geochronology on the Qipangou W-Mo deposit yielded a model age of 199.7 ± 3.9 Ma, indicating the deposit formed in the early Yanshanian period of the Early Jurassic. Granitoid intrusions in the mineralized area are characterized by composite granite bodies that crystallized at ca. 240–190 Ma. While there were multiple stages of intrusion, most occurred at 210–220 Ma, with waning magmatic activity at 200–190 Ma. The Re-Os age of molybdenite in the region is ca. 200–190 Ma, which may represent a newly discovered period of W-Mo metallogenesis that occurred during the final stages of magmatism. The heat associated with this magmatism drove ore formation and might have provided additional ore-forming components for metallogenesis(represented by the Wangjiaping biotite monzogranite). Ore materials in the mineralized area were derived from mixed crustal and mantle sources. Enrichment of the region occurred during intracontinental orogenesis in the late Indosinian–Yanshanian, subsequent to the main Indosinian collision. At this time, the tectonic environment was dominated by extension and strike-slip motion.

Genesis of the Taishanmiao Au Deposit in the Ningshan-Zhenan Ore Field,South Qinling Orogen,China:Evidence from Geochemical,Geochronological and S-Pb-H-O Isotopical Study

The Taishanmiao Au deposit is in the western part of the Ningshan-Zhenan ore field,in the South Qinling orogen.Based on geological and geochemical features,we propose that the Taishanmiao Au deposit is a magmatichydrothermal type of deposit.All samples have high SiO_(2),K_(2)O+Na_(2)O contents and differentiation index values,low CaO,MgO,P_(2)O_(5),and TiO_(2)contents,are enriched in high field-strength elements,and depleted in large ion lithophile element.The stable isotopeδ^(34)S values of pyrite vary from 6.8%-7.8%,and the H-O isotopic compositions of quartz from quartzpyrite veins indicate the ore-forming fluid is a mixture of a small amount of magmatic-hydrothermal solution and groundwater.Lead isotope ratios of pyrite and silicalite can show that the ore-forming materials were derived from a mixed source containing mantle and crustal materials.At the same time,the LA-ICP-MS U-Pb dating of monzogranite is 198.4±4.2 Ma.Combined with the regional geological background,the intracontinental extension in the late collisional orogeny and large-scale lithospheric thinning associated with mantle uplift may lead to large-scale mineralisation in the region.

U-Pb Zircon Ages and Geochemistry of the Metasedimentary Rocks from the Foping Area in the South Qinling Belt:Evidence for Early Devonian Amalgamation between North China and South China Blocks

The provenance and tectonic implications of Late Paleozoic sedimentary rocks in the South Qinling Belt(SQB) provide important clues for understanding the timing and mechanism of the collision between North China Block(NCB) and South China Block(SCB).Here we report new LAICP-MS zircon U-Pb ages and geochemical composition of metasedimentary rocks from the Foping area in the SQB.The results indicate that the depositional age of the Wenquan Group can be limited to the Early Devonian by the youngest U-Pb age of 398 Ma,whereas those of the quartz schist from the Changjiaoba Group could be constrained after the Carboniferous by the youngest206Pb/238U peak age of 306 Ma.Therefore,much of the previously-assumed “Neoarchaean or Paleoproterozoic” strata,including the Wenquan and Changjiaoba groups,were actually deposited in the Late Paleozoic.Based on analysis and comparison of the detrital zircon ages of the Devonian tectonic units in the SQB,we found that most prominent peak ages of detrital zircons from metasedimentary rocks in the Foping area shared similar characteristics with others.They mainly derived from the North Qinling Belt and South Qinling Belt-Yangtze Block,with a minor source probably from the North China Block,implying that the Shangdan Ocean between NCB and SCB had closed in the Early Devonian.Combined with regional geological background and geochemical data,we inferred that these Devonian strata formed in the foreland basin after the amalgamation of the North China Block and South China Block.

Mesozoic uplift of the Dabashan and Micangshan-Hannan Dome in the South Qinling orogenic belt

The topographic evolution of continental orogens is important for understanding continental orogenic processes,geodynamic mechanisms,and climatic and environmental changes.The Qinling Orogen is a major orogenic belt in China,and its uplift history can provide insights into the tectonic configuration and geodynamics of China and East Asia.Previous studies have shown that the Dabashan and Micangshan-Hannan Dome(MHD)in the South Qinling orogenic belt were uplifted during the Mesozoic.However,the magnitude of the uplift remains unclear.In this study,using sedimentary records in the northern Sichuan Basin and lithospheric flexural modeling,we estimated the magnitude of Mesozoic uplift of the Dabashan and MHD,along with the effective elastic thickness(Te)of the Sichuan Basin.The Dabashan and MHD were uplifted by approximately 1220 and 880 m during the Middle Jurassic and Early Cretaceous,respectively.Therefore,we propose that the present-day elevation of the Dabashan and MHD is primarily the result of Mesozoic uplift.The differences in the duration and amount of uplift between different tectonic units indicate that the uplift processes and driving mechanisms in the South Qinling orogenic belt were different in the Mesozoic and Cenozoic.Mesozoic uplift was the result of convergence of the North China and South China blocks advanced from east to west,whereas Cenozoic uplift was driven by ongoing indentation of the Indian Plate into Eurasia from southwest to northeast.The lithospheric strength of the northern Sichuan Basin was weakened from the Middle Jurassic to Early Cretaceous,and Tedecreased from 73 to 57 km.This may have been caused by the flexure-related bending stresses in the lithosphere that developed due to the large topographic loading.

Rock Assemblages and Formation Ages of the Baishuijiang Group in the Southwest Qinling Orogenic Belt, Northwest China

The Baishuijiang Group, located in the southwest Qinling orogenic belt, is divided into three belts according to the characteristic of the matrix and rock blocks based on the large scale geological mapping. The north belt and south belt are composed of abyssal mudstone and siltstone, and limestone, chert and basic and ultrabasic rock blocks. The middle belt consists of a few limestone blocks and turbidites, which were formed in the trench environment. At present, in the Baishuijiang Group, many fossils were found in matrix and rock blocks, the fossils contain the Cambrian small shell fossils(Xiao, 1992;Tao et al., 1992), Silurian chitinozoas, scolecodonts and spores, and Ordovician graptolites, and middle Devonian Coral and conodonts in limestone and chert blocks(Wang et al., 2011a), and Permian radiolarians in the matrix(Wang et al., 2007). The volcanic rock blocks have undergone different degree of metamorphism. Their geochemical characteristics indicate that the rocks are similar to oceanic island arc and seamount(Wang et al., 2009), and SHRIMP U-Pb dating yielded ages from Neoproterozoic to early Paleozoic(Yan et al., 2007;Wang et al., 2009, 2011b). Therefore, comprehensive analysis of regional data, the Baishuijiang group is an accretionary complex which was consisted of matrix and blocks, and was finally formed during Permian-Triassic.

Drainage evolution in intermontane basins at the Qinling-Daba Mountains

River capture is of great significance to landform evolution and hominine migration.In the Qinling-Daba Mountains,there is a viewpoint that Jialing River captured Hanjiang River,but this is still controversial.In this paper,we discuss the drainage evolution processes in intermountain basins at the Qinling-Daba Mountains based on a combination of detrital zircon UPb geochronology and geomorphic indexes.We suggest that the Hanjiang River gradually captured the Jialing River from east to west,accompanied by the evolution of the ancient Yangtze River.In terms of geomorphic evidences,wide valleys did not match with discharge,and a series of wind gaps developed in the Shiquan-Ankang basin.In addition,the valley shapes and width-toheight ratios(Vf)indicate two possible rapid incisions.The hypsometric integrals(HI)reflect that the landform gradually changes from the old stage to the youth stage from west to east.Theχvalues show that the drainage divide is moving to the side of the Yuehe River,and the Yuehe River is gradually shrinking.According to the sedimentary records,the zircon U-Pb age distributions indicate the provenance change.The high-altitude terraces show three age peaks(200–250,400–505,and 700–900 Ma),with the dominant Indosinian age peak(200–250 Ma),while the modern fluvial sediments only show a single peak of Jinning(700–900 Ma).These data show that there are two major river captures:(1)The ancient Hanjiang River cut through the regional compression ridge,and then captured the Hanzhong Basin river system(a part of the ancient Jialing river system)from east to west,and(2)The southern tributary captured the trunk with the uplift of the divide in the Shiquan-Ankang Basin,forming the modern drainage pattern in the upper Hanjiang River.The activities of the regional strike-slip fault,and the associated compression uplift played a key role in the river captures,the drainage evolution,and related landforms in the Shiquan-Ankang basin.In addition,it is shown that the evolution of the upper tributary basins lagged behind the response of the trunk channel to the tectonic activities and river captures.The interconnected wide valleys caused by river capture may have provided convenient geomorphological conditions for human migration into the Qinling-Daba Mountains along those river valleys.