Mianlue Suture Research Articles

Propagation tectonics and multiple accretionary processes of the Qinling Orogen

The Qinling Orogen was built through collision between the North China and South China Blocks. Previous detailed geological, geochemical and geochronological investigations revealed that the mountain range can be divided into four tectonic units with distinct tectono-lithostratigraphy, which are, from north to south, the southern sector of the North China Block, North Qinling Belt, South Qinling Belt and northern sector of the South China Block, separated by the Kuanping, Shangdan and Mianlue sutures. According to the petrology, geochemistry and geochronology of ophiolitic mélanges and related magmatic rocks, as well as the features of sedimentary units, we think that the North China Block, the North Qinling Belt and the South China Block were originally independent continental units while the South Qinling Belt had been the northern part of the South China Block. These units experienced three episodes of accretionary tectonic processes and amalgamation from south to north.The Neoproterozoic accretion took place along the Luonan-Luanchuan Fault and Kuanping ophiolitic mélange belt as a result of southward subduction and subsequent collision between the North Qinling and North China Blocks during ca. 1.0–0.8Ga related to the formation of the supercontinent of Rodinia. The Paleozoic accretion occurred along the Shangdan suture resulted from northward subduction of oceanic lithosphere in the Early Paleozoic and subsequent continental subduction in the Late Paleozoic. Late Triassic accretion took place along the Mianlue suture between the South Qinling and South China Blocks due to northward subduction of the Mianlue oceanic lithosphere during the Permian–Early Triassic and subsequent collision in the Late Triassic. After the Late Triassic collision along the Mianlue suture the whole Qinling Mountain range entered the phase of intense intracontinental deformation.

Episodic Triassic magmatism in the western South Qinling Orogen, central China, and its implications

The Qinling Orogen is located between the North China and South China cratons. The Mianlue Suture is the key boundary between the orogen and the South China Craton marking where the South China Craton subducted northward beneath the North China Craton during the late Triassic. Another older suture, the Shangdan Suture, divides the orogen into the North Qinling and South Qinling terranes. Our new geochronological data on eight granite plutons in the South Qinling Terrane indicate that the major phase of the Zhongchuan Granite is 219.5 ± 2.1 Ma, Luchuba is 217.1 ± 1.0 Ma, Baijiazhuang is 215.9 ± 3.0 Ma, Jiaochangba is 213.9 ± 1.7 Ma, Lvjing is 214.5 ± 2.1 Ma, Xiba is 215.6 ± 1.8 Ma, Wenquan is 217.3 ± 2.1 Ma and Fengxian is 240.9 ± 1.0 Ma. Compiled results reveal widespread magmatism began in the northwestern part of the South Qinling Terrane at ~240–250 Ma and episodically progressed to the southeast in the North Qinling Terrane. Magmatism was then widespread throughout the South Qinling Terrane during the Late Triassic. A new division of Triassic intrusions has been proposed across the Qinling Orogen based on critical geochemical and geochronological differences. First, the western part of the South Qinling Terrane, (western South Qinling suite), is significantly enriched in large ion lithophile elements, including Cs, Rb, U, Th and K, whereas the eastern part of the South Qinling Terrane, (eastern South Qinling suite), has moderate enrichment in these elements. Second, there is an obvious transition from calcium to potassium enrichment in the Late Triassic western‐South‐Qinling suite, whereas the Late Triassic eastern South Qinling suite appears to follow a curved trajectory starting from high Na through K to high Ca. Third, the Ba and Sr concentrations of the western South Qinling suite have marked troughs, whereas the eastern South Qinling suites have relative enrichment of the two elements. The source of the Triassic magma becomes increasingly juvenile towards the east‐southeast in the Qinling Orogen. Geochemical and isotopic data indicate that there are differences in the granites across the Chengxian–Huixian–Fengxian Fault within the western South Qinling Orogen, which highlight the tectonic significance of this northeast‐trending fault that is now recognized as a significant boundary within the orogen. The identification of such a large‐scale structure should attract some exploration attention, since this fundamental lineation is likely to be associated with some large mineral systems. Copyright © 2014 John Wiley & Sons, Ltd.

Geology, geochemistry and genesis of the Huachanggou gold deposit, western Qinling Orogen, central China

The large Huachanggou gold deposit, on the southern margin of the Mian‐Lue Suture in western Qinling Orogen (central China), is hosted in the Devonian Sanhekou Group composed of spilite, limestone and phyllite. The mineralization is considered to have occurred in three stages: (1) Early quartz–pyrite veins, (2) Middle quartz–sulphides veins, and (3) Late carbonate–quartz veinlets, with gold being introduced mainly in the second stage. Quartz formed in the two earlier stages contains four types of fluid inclusions, namely (1) Pure CO2, (2) CO2–H2O, (3) Solid‐bearing and (4) NaCl–H2O, whereas the late‐stage minerals contain only the NaCl–H2O inclusions. The inclusions in quartz formed in the early‐, middle‐ and late stages have yielded total homogenization temperatures of 272−385 °C, 232−395 °C and 118−228 °C, respectively, with salinities no higher than 12 wt.% NaCl equiv. For the middle‐stage, trapping pressures estimated from the CO2–H2O inclusions for the altered spilite‐type (123−326 MPa) and quartz vein‐type (132−342 MPa) orebodies correspond to mineralization depths of 12 km and 13 km, respectively. We suggest that fluid boiling and mixing may have caused rapid precipitation of metallic sulphides and native gold. Through boiling and influx of meteoric water, the ore‐forming fluid system has evolved from CO2‐rich to CO2‐poor and from metamorphic to meteoric, as indicated by the decreasing δ18Owatervalues from the early‐ to late stages. Integrating the evidence obtained from regional geology, ore deposit geology, fluid inclusion and C–H–O isotope geochemistry, we conclude that the Huachanggou gold deposit is best ascribed to be an orogenic‐type system formed in the tectonic setting of the north‐vergent oceanic plate subduction along the Mian‐Lue Suture during the Late Triassic. Copyright © 2014 John Wiley & Sons, Ltd.

Origin of carbonatites in the South Qinling orogen: Implications for crustal recycling and timing of collision between the South and North China Blocks

Most studies of compositional heterogeneities in the mantle, related to recycling of crustal sediments or delaminated subcontinental lithosphere, come from oceanic setting basalts. In this work, we present direct geochronological and geochemical evidence for the incorporation of recycled crustal materials in collision-related carbonatites of the South Qinling orogenic belt (SQ), which merges with the Lesser Qinling orogen (LQ) to separate the South and North China Blocks. The SQ carbonatites occur mainly as stock associated with syenites. The data presented here show that zircon from the syenites yields an age of 766±25Ma, which differs significantly from the age of primary monazite from the carbonatites (233.6±1.7Ma). The syenites contain lower initial 87Sr/86Sr and higher εNd values. This indicates that the carbonatites do not have genetically related with the silicate rocks, and were directly derived from a primary carbonate magma generated in the mantle. The carbonatites show a Sr–Nd isotopic signature similar to that of the chondritic uniform reservoir (CHUR), and parallel Sm–Nd model ages (TCHUR) of 190–300Ma. However, the rocks have extremely variable Pb isotopic values straddling between the HIMU and EM1 mantle end-members. Most carbon and oxygen isotopic compositions of the SQ carbonatites plot outside the field for primary igneous carbonates. Their δ13C shows higher value than a ‘normal’ mantle, which implies an incorporation of recycled inorganic carbon. The carbonatites were emplaced close to the Mianlue suture, and followed the closure of the Mianlue ocean and Triassic collision of the South and North China Blocks. However, direct melting of the subducted Mianlue oceanic crust characterized by high εNd and low (EM1-like) 206Pb/204Pb values cannot explain the CHUR-like Nd signature and the Pb isotopic trend toward HIMU in the SQ carbonatites. We conclude that their parental magma was derived from a source incorporating the Mianlue oceanic crust mixed with an asthenospheric (or deeper) material characterized by high Pb and low Nd isotopic values. This material represents a deep-seated Proterozoic carbonate component recycled via mantle convection or localized upwelling. Notably, this model cannot explain the isotopic compositions of the Late Triassic (209–221Ma) carbonatites in the LQ, characterized by a mantle-derived δ13C, but EM1-like Sr–Nd–Pb isotopic compositions. This signature is best explained in terms of delamination of the lower continental crust thickened during the collision of the South and North China Blocks, and partial incorporation of the delaminated material into the LQ mantle source. Modeling of the measured Sr–Nd–Pb isotopic variations suggests that the source of the LQ carbonatites could be produced by mixing of 80–85% of mantle material and 15–20% of delaminated lower continental crust. The emplacement of the SQ and LQ carbonatites marked a gradual transition from a compressional tectonic regime, brought about by the collision of the South and North China Blocks to intra-orogenic extension in the waning stages of the Triassic orogeny.

Compositional polarity of Triassic granitoids in the Qinling Orogen, China: Implication for termination of the northernmost paleo-Tethys

The Qinling Orogen was formed from the closure of the northernmost paleo-Tethys sea and the tectonic suturing of the Yangtze and North China Cratons. The timing of this collision and the tectonic framework are debated. The widely developed Triassic granitoids in the western Qinling Orogen offer a key to understand the tectonic evolution of this region. Here we compile the geological, geochemical and geochronological data of the Triassic granitoids from the Qingling Orogen and conclude that the granitoids north of the Mian-Lue Suture were emplaced in an active continental margin related to the northward subduction of the Mian-Lue oceanic plate during 248−200Ma. The granitoids can be classified into I- and S-types, with the former constituting the major variety. Northward from the Mian-Lue Fault, an S-type granite belt appears, followed by and locally overlapping with an I-type granite belt. The I-type granite belt can be subdivided into four sub-belts from south to north, with increasing contents of K2O, K2O+Na2O, SiO2, Th and U, and the ratios of K2O/Na2O, Rb/Sr and (87Sr/86Sr)i, but decreasing contents of Na2O, Al2O3, Mg#, and εHf(t), except for the northernmost belt which occurs in the Huaxiong Block of the North China Craton. This geochemical polarity of I-type granitoids and the zoned distribution of S- and I-types granitoids cannot be explained through continental collision orogeny (including syn- to post-collision), but can be well interpreted by considering the progressive subduction of the northernmost branch of the paleo-Tethys, as represented by the Mian-Lue Ocean. Thus, the termination of the northernmost paleo-Tethys and the onset of the continental collision between the Yangtze and the North China plates are considered to have occurred at about 200Ma, during the transition from Triassic to Jurassic. Our model is also supported by the available data from other studies and provides a revised framework for the timing and tectonics of assembly of the Yangtze and the North China Cratons.

Heterogeneous sources of the Triassic granitoid plutons in the southern Qinling orogen: An E‐W tectonic division in central China

The Qinling orogen is an important orogenic belt formed by the collision between the North and South China blocks along the Mianlue suture during the Triassic. The orogen is customarily divided into the western and eastern Qinling terranes. However, the boundary has long been a matter of debate. There are many Triassic granitoid plutons in the orogen, especially in South Qinling, along the southern part of the Shangdan suture. Systematic analysis of U‐Pb ages and Hf isotopes in zircons frxom six granitoid plutons, including Guangtoushan, Gaoqiaopu, Xiba, Laocheng, Dongjiangkou, and Zhashui from west to east, allows us to trace their formation ages and source regions. All plutons yield ages that vary only from 218 to 211 Ma. However, zircon Hf‐isotope data for these Triassic plutons cluster in two distinct groups. Granitoids from the western segment (i.e., Guangtoushan, Gaoqiaopu, and Xiba) of the South Qinling belt have negative εHf(t) values (−20.9 to −5.2) and relatively old two‐stage Hf model ages (1.58 to 2.57 Ga). In contrast, those from the eastern segment (i.e., Laocheng, Dongjiangkou, and Zhashui) show higher εHf(t) values (−5.4 to +6.8) and younger two‐stage Hf model ages (0.82 to 1.60 Ga). Integrating these analyses with published Sr‐Nd‐Pb‐Hf isotopic data, we suggest that the division between the western and eastern South Qinling segments is located near the Taibai‐Chenggu line, between the Baoji‐Chengdu railway and 108°E longitude. The integrated data suggest that the western South Qinling segment separated from the North China block during the Paleoproterozoic and early Mesoproterozoic and then switched into continental convergence with the eastern South Qinling segment (northern margin of the Yangtze block) during the late Mesoproterozoic to early Neoproterozoic; finally, the two segments amalgamated during the late Neoproterozoic.

Geochemical characteristics of rare earth elements and their implications for the Huachanggou gold deposit in Shaanxi Province, China

The Huachanggou gold deposit is located in the south part of the Mian-Lue suture zone in the Qinling orogenic belt. Rare earth element (REE) concentrations determined by ICP-MS are shown to characterize the ore samples and their wall rocks in three ore zones in order to reveal the origin of ore-forming materials and fluid. In AuI, REE chondrite normalized patterns of ore are similar to those of ore-controlling spilite; the ore-forming materials originated from deep magma, and magmatic activity offered main hydrothermal source for gold mineralization. The REE characteristics of AuII and AuIII are similar, and most of the ore samples are similar with the wall rocks. The ore-forming fluids of AuII and AuIII were metamorphic hydrothermal fluids which had extracted ore-forming materials part from the wall rocks, and part from the spilite in AuI.

Mesozoic basin development and its indication of collisional orogeny in the Dabie orogen

The Dabie orogen underwent deep continental subduction, rapid exhumation, and the huge amount of erosion during the Mesozoic. Its tectonic evolution, especially how its evolution was recorded by sedimentary basins at the flanks of the Dabie orogen is one of the most important issues of the world’s attention. These years, newly studies of basin sedimentology, combined with structural geology, have shown a fundamental progress. The overall distribution of different basin types in the orogen indicates that shortening and thrusting at the margins of the orogen from the Late Triassic to the Early Cretaceous controlled the foreland basins, and extension, doming and rifting were initiated in the core of the orogen from the Jurassic to the Early Cretaceous and were expanded to the whole orogen after the Late Cretaceous. Therefore, The Dabie orogen records gradual transition from overall shortening and thrusting to dominantly extension and rift basin formation expanded from its core to its margins, although these shortening and extension overlapped in time from the Jurassic through Early Cretaceous at crustal levels. The unroofing ages of the ultra-high pressure (UHP) metamorphic rocks in the Dabie orogen change from Early Jurassic to Late Jurassic westward. The depth of exhumation increases eastwards. The sediment sources for the Hefei basin are mostly composed of the deeply exhumed, axial Dabie metamorphic complex, and the sediment sources for the Middle Yangtze basin are mostly from cover strata in the southern orogen and related strata with subjacent (i.e. subsequently overthrusted) Mianlue suture belt. Geodynamic analysis represents that continental collision between the North China Block and the South China Block along the Shangdan and Mianlue sutures, subsequently northwestward progradation of the Jiangnan fold and thrust belt, and the underthrusting of the North China Block along the Northern Boundary Fault of Qinling Range led to crustal thickening, gravitational spreading and balanced rebound of the resultant thick crustal welt, and multi-episodic exhumation of the HP/UHP metamorphic rocks. The future studies by the methods of tracing the Dabie orogeny through deposition in the marginal basins should focus on eastward extension of the Mianlue suture, thrust and overlap of the Dabie HP/UHP metamorphic block on different lithotectonic zones and basins along the northern South China Block, the structural framework of the source area of the basins in the syn-depositional stage, the basin lateral extension, huge amount of erosion and sediment transportation from the Dabie blanket and basement rocks, and recovery of subducted and removed structural units within the Dabie orogen, etc.

Middle Triassic arc magmatism along the northeastern margin of the Tibet: U–Pb and Lu–Hf zircon characterization of the Gangcha complex in the West Qinling terrane, central China

The tectonic setting of Mesozoic magmatic complexes in the northeastern margin of the Tibet plateau is disputed, and hence gives rise to uncertainty concerning the tectonic evolution of the northeastern Tibet Plateau and the timing of the closure of the Palaeo-Tethys ocean. The Gangcha complex is typical of these complexes, consisting of andesite, dacite, gabbro, gabbroic diorite, granodiorite, quartz diorite, and diorite with typical chemical traits of continental margin arc rocks. Andesite, gabbroic diorite, and mineralization-associated potassic-altered diorite yield weighted mean 206 Pb/ 238 U ages of 242.1 ± 1.2 Ma, 243.8 ± 1.0 Ma and 234.0 ± 0.6 Ma respectively. Zircon ϵ Hf (t) for magmatic grains ranges from −3.5 to +5.7, interpreted to demonstrate that the Gangcha complex contains crustally contaminated mantle magmas. Inherited zircons in the complex yield similar U–Pb ages (777–310 Ma) to the A’nyemaqen composite ophiolite assemblage with ϵ Hf (t) of −17.4 to +11.6. This suggests that components of this older ophiolite melted and contributed to the Gangcha complex magmas. Hence the Gangcha complex is considered to have formed as a continental margin arc in northeastern Tibet by northward subduction during consumption of the Palaeo-Tethys ocean. Regionally, it corresponds to the arc magmatism along the eastern and western Kunlun sutures to the west and the Mianlue suture to the east. Supplementary material: Geochemical data, and zircon U–Pb and Lu–Hf data for the Gangcha complex are available at www.geolsoc.org.uk/SUP18521 .

Pb–Sr–Nd isotope constraints on the fluid source of the Dahu Au–Mo deposit in Qinling Orogen, central China, and implication for Triassic tectonic setting

The Dahu Au–Mo deposit is a structure-controlled lode system occurring in the northern Xiaoqinling terrane, Huaxiong Block, Qinling Orogen. This paper reports a new Sr–Nd–Pb isotope dataset obtained for ore sulfides and the hostrocks within the Taihua Supergroup, in an attempt to constrain the source of the ore-forming fluids from a new dimension. 16 sulfide samples yield ISr ratios of 0.70470–0.71312, with an average of 0.70854; εNd(t) values between −13.5 and −18.1, with average of −15.1; and (206Pb/204Pb)i, (207Pb/204Pb)i and (208Pb/204Pb)i ratios of 17.033–17.285, 15.358–15.438, and 37.307–37.582, with averages of 17.162, 15.405, and 37.440, respectively. 5 gneiss samples from the Taihua Supergroup yield ISr ratios of 0.70947–0.73201, averaging 0.72294; εNd(t) values of −20.0 to −31.1, averaging −25.1; and (206Pb/204Pb)i, (207Pb/204Pb)i, (208Pb/204Pb)i ratios of 17.127–18.392, 15.416–15.604 and 37.498–37.814, with averages of 17.547, 15.470 and 37.616, respectively. These data show that the ore sulfides have less radiogenic Sr–Nd–Pb isotope systematics than the hostrocks, and suggest that the ore-forming fluids, which interacted with the wallrocks to form ores, must be sourced from a depleted mantle or a depleted, subducted oceanic slab. In combination with the spatial scenario and geochemical signatures of the Triassic magmatites and mineral systems, we argue that in the Triassic the Mianlue Ocean was not completely closed, and that the northward oceanic plate subduction still survived along the Mian-Lue suture, which caused the Late Triassic magmatism and associated mineralization in Qinling Orogen, including the Au–Mo Dahu deposit.

Thermochronological constraints on two-stage extrusion of HP/UHP terranes in the Dabie–Sulu orogen, east-central China

Along the Qinling–Dabie–Sulu orogenic belt in China crops out the world's largest terrane composed of ultrahigh-pressure (UHP) metamorphic rocks. Differences in the timing and mechanisms of oceanic and continental subductions are assumed to be responsible for different ages of high-pressure (HP) and UHP slices in different parts of the belt. The western part of the Dabie orogen (western Dabie terrane) holds a key to understanding of the transition from oceanic to continental subduction. This paper reports geochronological results to test a two-stage tectonic model for the exhumation of HP/UHP rocks in western Dabie. This model involves two different stages and types of extrusion for exhumation of the HP/UHP rocks in east-central China. Mica Ar/Ar ages, ranging from 241 to 231 Ma, indicate a general middle Triassic cooling probably driven by early upward extrusion during the collision between the North and South China Blocks. Late Triassic–Early Jurassic cooling was associated with later eastward extrusion, ranging from 200 to 184 Ma. The second event is recorded also in mica in the region that was not affected by later deformation and magmatism. The lateral movement along lithosphere-scale faults resulted in the eastward extrusion of the HP–UHP metamorphic terrane, which was followed, in the Late Triassic–Early Jurassic time, by a major compressive event. These two extrusion events are correlative with the two stages of Triassic exhumation of the western Dabie HP–UHP rocks, respectively. Wintin the framework of the Qinling–Dabie–Sulu orogenic belt, it is suggested for western Dabie that the subduction/exhumation of blueschist-facies unit is related to the Mianlue suture, whereas the subduction/exhumation of HP/UHP eclogite-facies units is related to the Shangdan suture.

Zircon U-Pb ages and geochemistry of the Wenquan Mo-bearing granitioids in West Qinling, China: Constraints on the geodynamic setting for the newly discovered Wenquan Mo deposit

East Qinling is the largest porphyry molybdenum province in the world; these Mo deposits have been well documented. In West Qinling, however, few Mo deposits have been discovered although granitic rocks are widespread. Recently, the Wenquan porphyry Mo deposit has been discovered in Gansu province, which provides an insight into Mo mineralization in West Qinling. In this paper we report Pb isotope compositions for K-feldspar and sulfides, S isotope ratios for sulfides, the results obtained from petrochemical study and from in situ LA-ICP-MS zircon U-Pb dating and Hf isotopes. The granitoids are enriched in LILE and LREE, with REE and trace element patterns similar to continental crust, suggesting a crustal origin. The Mg # (40.05 to 56.34) and Cr and Ni contents are high, indicating a source of refractory mafic lower crust. The ε Hf( t) values of zircon grains from porphyritic monzogranite range from − 2.9 to 0.6, and from granitic porphyry vary from − 3.3 to 1.9. The zircons have T DM2 of 1014 to 1196 Ma for the porphyritic monzogranite and 954 to 1224 Ma for the granitic porphyry, implying that these granitoids were likely derived from partial melting of a Late Mesoproterozoic juvenile lower crust. The Pb isotope compositions of the granitoids are similar to granites in South China, showing that the magma was sourced from the middle–lower crust in the southern Qinling tectonic unit. The Pb isotopic contrast between the Mo-bearing granitoids and ores shows that the Pb in the ore-forming solution was derived from fractionation of a Triassic magmatic system. δ 34S values of sulfides are between 5.02 and 5.66‰, similar to those associated with magmatic-hydrothermal systems. LA-ICP-MS zircon U-Pb dating yields crystallization ages of 216.2 ± 1.7 and 217.2 ± 2.0 Ma for the granitoids, consistent with a previously reported molybdenite Re-Os isochron age of 214.4 ± 7.1 Ma. This suggests that the Mo mineralization is related to the late Triassic magmatism in the West Qinling orogenic belt. In view of these geochemical results and known regional geology, we propose that both granitoid emplacement and Mo mineralization in the Wenquan deposit resulted from the Triassic collision between the South Qinling and the South China Block, along the Mianlue suture. Since Triassic granitoid plutons commonly occur along the Qinling orogenic belt, the Triassic Wenquan Mo-bearing granitoids highlight the importance of the Triassic tectono-magmatic belt for Mo exploration. In order to apply this metallogenic model to the whole Qinling orogen, further study is needed to compare the Wenquan deposit with other deposits.

Slab Breakoff Model for the Triassic Post-Collisional Adakitic Granitoids in the Qinling Orogen, Central China: Zircon U-Pb Ages, Geochemistry, and Sr-Nd-Pb Isotopic Constraints

Triassic granites in the Qinling orogen of central China provide insights into regional tectonomagmatic events and tectonic evolution. Zircon LA-ICP-MS U-Pb dating shows that the Yangba, Xichahe, Wulong, and Guangtoushan plutons have magmatic crystallization ages of 209 ± 2 Ma, 212 ± 2 Ma, and 208 ± 2 Ma, respectively, slightly younger than the time of collision between the Yangtze and North China cratons (242 Ma). Major- and trace-element geochemistry indicate that the granitoids have adakitic signatures, suggesting that they were generated by partial melting of a thickened lower crust. Geochemistry and petrology of mafic microgranular enclaves in some of the plutons indicate that they were derived from partial melting of the subcontinental lithospheric mantle (SCLM) and are genetically related to the host adakitic granites. We proposed that slab-breakoff explains the spatial, age and geochemistry characters of Triassic magmatism in the Qinling orogen. In summary, Triassic collision between the Yangtze and North China cratons caused crustal thickening in the Qinling-Dabie orogen to >50 km, with breakoff of the subducting oceanic slab occuring at shallow depth. This caused: (1) upwelling of the asthenosphere along the Mianlue suture, with initial melting at the base of the lower crust, producing mafic melt; (2) mantle-derived melt underplating the thickened lower crust so that temperatures at the boundary between it and the mantle were high enough to trigger dehydration melting of garnet-bearing rock at a pressure of >1.5 GPa, resulting in the generation of adakitic magma with high Sr, Ba, and Sr/Y ratios, no negative Eu anomalies, and leaving eclogite as a residue. (3) Moreover, given that lithospheric mantle-derived magmas intruded the adakitic magma chamber, interaction between pristine adakitic magmas and mantle-derived magmas result in formation of the mafic enclaves, providing the adakitic magma with a mantle/crustal component, e.g. high Mg# and Cr and Ni contents. We conclude that partial melting of lithospheric mantle played an important role in generation of the Triassic adakitic granites in the Qinling orogen, reflecting post-collisional slab-breakoff.

Geochemistry of Ophiolites from the Mian-Lue Suture Zone: Implications for the Tectonic Evolution of the Qinling Orogen, Central China

The Mian-Lue suture zone along the southern margin of the Qinling orogenic belt contains abundant ophiolite fragments that record collision between the North and South China terranes. The ophiolites are well exposed in the areas of Lueyang, Pipasi, and Derni. In Lueyang, ultramafic rocks comprise serpentinized harzburgite and dunite exhibiting LREE depletion and marked positive Eu anomalies, gabbroic rocks enriched in Sr and Ba and positive Eu anomalies, diabase dikes enriched in LREE with slightly negative Eu anomalies, and tholeiite metabasalts with MORB-type characteristics of LREE depletion, low Nb (3.9-8.4 ppm), TiO2 (0.92-1.86 wt%,), Ti/V (18.14-23.76), Th/Ta (0.04-1.86), Th/Y (0.01-0.02), and Ta/Yb (0.01-0.13). Ophiolites from Pipasi comprise tholeiitic basalt with average low (La/Yb)N(0.65-0.97), Ti/V (26.98), Th/Ta (1.06), Th/Y(0.01), and Ta/Yb(0.05). Ophiolites from Derni are represented by basalt, gabbro, and metaperidotites. The basalts are LREE depleted and lack prominent Eu anomalies, indicating a depleted asthenosphere source. Isotopic ages of 350-245 Ma imply the existence of a Paleozoic ocean basin from Devonian to Permian time in central China. The MORB-type ophiolites in the Mian-Lue suture zone are regarded as the fragments of Paleo-Tethys oceanic lithosphere that separated the Qinling and South China terranes during middle Paleozoic to early Mesozoic.

U–Pb zircon age, geochemical and isotopic characteristics of carbonatite and syenite complexes from the Shaxiongdong, China

The Qinling is an important orogenic belt, which formed by the joining of the North China and South China blocks. The Shaxiongdong carbonatite–syenite complexes were emplaced at the southern margin of South Qinling and border the South China block. LA-ICPMS (Laser Ablation Inductively Coupled Plasma Mass Spectrometry) zircon U–Pb geochronology constrains the syenite emplacement age to be 441.8 ± 2.2 Ma, which is significantly earlier than the collision of the South China block and South Qinling along the Mianlue suture (200–240 Ma) near where the complexes reside. Trace-element abundances and C–O–Sr–Nd–Pb isotopes for the carbonatites and calcite separates indicate an igneous origin. They and associated syenites show overlaps of initial 87Sr/ 86Sr ratios (0.7029–0.7033, 0.7030–0.7032) and ε Nd (2.8 to 4, 2.5 to 6 for syenites and carbonatites, respectively), which implies that the carbonatites may derive from a carbonated alkali silicate melt. However, the rocks are composed dominantly of calcite. They have markedly lower Th, Nb, Zr and P contents compared to average calciocarbonatites worldwide. Calcites from the carbonatites are also characterized by low REE contents and relatively flat REE patterns. This indicates that the carbonatites are calcite-rich cumulates, which were produced from a residual liquid derived from an intensively fractioned carbonatite magma. In addition, the syenites are characterized by negative Pb and no Nb anomalies. They show lower Sr isotopes and higher ε Nd than syn/post-orogen related granites emplaced in the Qinling. This indicates that the complexes formed in a rifting environment. It is noted that Sr and Nd isotopic compositions from the carbonatites and syenites are close to HIMU mantle source values. Variations in 207Pb/ 206Pb (0.785–0.842) and 208Pb/ 206Pb (1.954–2.110) ratios from the calcites best fit a model involving mixing HIMU and EM1 components. Therefore, plume activity may play an important role in the complex generation and tectonic evolution of the South China block. Geological support for this deduction is the presence of numbers of Silurian dyke swarms. We hypothesize that the upwelling plume metasomatizing the continental lithosphere resulted in the South Qinling separating from South China block along the Mianlue suture during the early Paleozoic period.

Post-collisional plutonism with adakitic signatures: The Triassic Yangba granodiorite (Bikou terrane, northern Yangtze Block)

The post-collisional Yangba granodiorite intruded into the Bikou metasedimentary-volcanic group, southern Mianlue Suture, central China. The host granodiorites contain many mafic microgranular enclaves which have acicular apatite, phenocrysts of host granodiorites, implying that the enclaves have been incorporated as magma globules into host granodioritic magma and undergone rapid cooling. The variation trends of major and trace elements between enclaves and host rocks suggest a mixing and mingling process with respect to their petrogenesis. The mafic microgranular enclaves are characterized by shoshonite with SiO2≤63%, σ (4.54–6.18)〉3.3, high K2O content (4.22%–6.04%), K2O/Na2O〉1; in the K2O-SiO2 diagram, all the samples plot in the shoshonite field, which are enriched in LILE and LREE, with obvious Nb, Ta negative anomalies, indicating a subducting fluid-metasomatised mantle source. Zircon LA-ICP-MS dating of the granodiorites yielded an age of 215.4±8.3 Ma, indicating they were formed during the late-orogenic or post-collisional stage (≤242±21 Ma) of the South Qinling Mountain Belt. The host granodiorites have many close compositional similarities to high-silica adakites from supra-subduction zone setting, but tend to have a higher concentration of K2O (3.22%–3.84%) and Mg#. Chondrite-normalized rare-earth element patterns are characterized by high ratios of (La/Yb)N, the extreme HREE deple-tion and a lack of significant Eu anomalies. In conjunction with the high abundances of Ba and Sr as well as the low abundances of Y and HREE, these patterns suggest a feldspar-poor, garnet ± amphibole-rich fractionation mineral assemblage. High Mg# values demonstrate that the host granodiorites were contaminated by enclave magma. On a whole, integrated geological and geochemical studies suggested the Yangba granodiorites and their mafic micro-granular enclaves resulted from mixing of enriched mantle-derived shoshonitic magma and thickened lower crust-derived felsic magma. In combination with previous studies we consider that the Yangba granodiorites were likely to represent underplating activities and delamination of the lower crust during the late orogenic stage in western Qinling Orogenic Belt.

Zircon LA-ICP MS U-Pb Dating of the Longkang Andesitic Ignimbrites from Jiuzhaigou: Evidence of the Mianlue Suture Westward Extension

Genesis of metasedimentary-volcanics outcroped in Kangxian-Pipasi-Nanping tectonic zone is closely related with tectonic evolution of the Mianlue Ocean. In the west end of Kangxian-Pipasi-Nanping tectonic zone, there are hundred meters of andesitic ignimbrites and tuffites. Zircon U-Pb dating on these volcanics has great significance to the tectonic implication of the volcanic rocks. Zircons from an ignimbrite sample (JZG-L) have been collected from Yazhagou in the east end of the Tazang-Longkang Section for LA-ICP MS U-Pb dating. The zircons from Longkang andesitic ignimbrites are magmatic genesis with oscillatory zoning and high U ((35–750) ppm), Th ((311–717) ppm) contents with high Th/U (0.44–1.30) ratios. The measured 206Pb/238U ratios are in good analytical precision, yielding a weighted mean age of (246 ± 3) Ma (MSWD=2.6, n=12, 2σ). Some detrital zircons have also been observed, they have maximal concordia age of (2 481 ± 33) Ma, which indicates the existence of Precambrian material in the Jiuzhaigou area. Combined with regional geology and the volcanic rocks in the studied area, the (246 ± 3) Ma zircon U-Pb age suggests a Late Permian orogenic volcanism and provides important geochronology evidence for the Mianlue suture westward extension.

Geochemistry of the Mian-Lue ophiolites in the Qinling Mountains, central China: Constraints on the evolution of the Qinling orogenic belt and collision of the North and South China Cratons

The Anzishan ophiolite, a typical ophiolitic block of early Carboniferous age in the Mian-Lue suture zone of the Qinling Mountains, central China, consists of amphibolites/metabasalts, gabbros and gabbroic cumulates. All of these rocks, as well as those in the Hunshuiguan-Zhuangke (HZ) block, have compositions similar to normal MORB and back-arc basin basalts (BABB) with high ε Nd( t ) values, indicating that they were derived from a depleted mantle source. The Mian-Lue suture zone also contains blocks of other lithologies, e.g., rift volcanic rocks in the Heigouxia block and arc volcanic rocks in the Sanchazi block. Although they are in fault contact with each other, the presence of these different blocks in the Mian-Lue suture zone may represent a complete Wilson cycle, from initial rifting to open ocean basin to final subduction and continent-continent collision, during the late Paleozoic-early Triassic. In this region, the North and South China Cratons were separated by Paleo-Tethys at least until the early Carboniferous, and final amalgamation of both cratons along the Qinling orogenic belt took place in the Triassic.

Post-collisional adakitic biotite plagiogranites from Guangtoushan pluton (Mianxian, central China): Petrogenesis and tectonic implication

The post-collisional Guangtoushan pluton intruded into the Mianlue suture, Central China. Geochemically, the Guangtoushan biotite plagiogranites show many close compositional similarities to high-silica adakites from the supra-subduction zone setting, but tend to have a higher concentration of K2O (3.22%–3.84%). Chondrite-normalized rare earth element patterns are characterized by high ratios of (La/Yb)N, concave-upward shapes of the heavy rare earth element (HREE), and a lack of significant Eu anomalies. In conjunction with high abundances of Ba and Sr, as well as low abundances of Y and HREE, these patterns suggest a feldspar-poor, garnet±amohibole-rich fractionation mineral assemblage. Coupled with previous studies, we suggest that the Guangtoushan biotite plagiogranites were likely to be caused by subducting slab break-off during the late orogenic stage in the West Qinling orogenic belt.

SHRIMP Zircon U‐Pb Dating of Gabbro and Granite from the Huashan Ophiolite, Qinling Orogenic Belt, China: Neoproterozoic Suture on the Northern Margin of the Yangtze Craton

Abstract: The recently identified Huashan ophiolitic mélange was considered as the eastern part of the Mianlüe suture in the Qinling orogenic belt. SHRIMP zircon U‐Pb geochronology on gabbro from the Huashan ophiolite and granite intruding basic volcanic rocks indicates crystallization ages of 947 ± 14 Ma and 876 ± 17 Ma respectively. These ages do not support a recently proposed Hercynian Huashan Ocean, but rather favor that a Neoproterozoic suture assemblage (ophiolite) is incorporated into the younger (Phanerozoic) Qinling orogenic belt.