Western Qinling Orogenic Belt Research Articles

Provenance Analysis of the Cenozoic Sedimentary Successions in the Ningnan Basin, NE Tibetan Plateau: Constraints on Sedimentological and Tectonomorphological Evolution

AbstractThe evolution and reforming history of the Cenozoic basins in the northeastern Tibetan Plateau (NETP) are significant for understanding the northeastward growth of the Tibetan Plateau. The sediments in these basins archived the spatial‐temporal patterns of the surface uplift and deformation processes in response to remote effects of growth of the NETP. Here, we present an integrated provenance analysis of the Ningnan basin in the NETP to provide direct insights into the basin evolution and further constraints of the Tibetan Plateau's northeastward growth. In this study, the Cenozoic strata (the Sikouzi, Qingshuiying, Zhang'enpu and Ganhegou formations) in the Ningnan basin yield complex detrital zircon age distributions featured by predominant age populations at 200–360 Ma and 360–540 Ma with minor Precambrian ages. Detrital zircon U‐Pb ages indicate that the southwestern Ordos Block, the western Qinling orogenic belt, and the eastern Qilian orogenic belt are primary source areas. Additionally, adjacent local paleo‐uplifts (e.g., Xi‐nanhuashan, Xiangshan, Liupan mountains.) also provided minor volumes of sediment. In addition, conglomerates and paleocurrent directions of the Sikouzi–Ganhegou formations exhibit an apparent change since the late Mocene, revealing an obvious provenance shift. The quantitatively determined mixing proportions of the surrounding source areas also reveals that the early source‐to‐sink system was replaced during the deposition of the Ganhegou Formation. Combined with previous studies, we suggest that the formation of the Ningnan basin was controlled by normal faults in extension setting and experienced continued sediment accumulation during the Qingshuiying–Zhang'enpu period; since the late Miocene, the significant provenance shift reflects the obvious northeastward growth of the NETP and led to the extinction of the Ningnan basin.

Contribution of Triassic Tectonomagmatic Activity to the Mineralization of Liziyuan Orogenic Gold Deposits, West Qinling Orogenic Belt, China

The Western Qinling orogenic belt (WQOB) is one of the most important prospective gold districts in China, with widely distributed Indosinian intermediate–acidic intrusions. The Liziyuan Au deposit is a representative orogenic deposit in the northern WQOB, hosting several sections spatially associated with igneous rocks. The Au deposit is hosted by meta-sedimentary volcanic rocks of the Cambrian–Ordovician Liziyuan Group and the Tianzishan monzogranite. Two periods, including five stages of mineralization, are recognized in this area: an early metamorphic mineralization period (PI), including quartz–pyrite (Stage I) and banded quartz–polymetallic sulfide (Stage II) veins, and a later magmatic mineralization period (PII) including quartz–K-feldspar–pyrite–molybdenite veins (Stage III), quartz–polymetallic sulfide–chlorite ± calcite veinlets and stockwork (Stage IV), and late calcite–quartz veinlets (Stage V). Geochronological studies indicate a SHRIMP zircon U-Pb age of 236.1 Ma for the Tianzishan monzogranite, and our published ages of ore-bearing diorite porphyrite of the Suishizi section and granite porphyry of the Jiancaowan section being 213 and 212 Ma, respectively. Pyrites formed in association with PI and PII mineralization have well-defined Rb–Sr ages of 220 ± 7.5, 205.8 ± 8.7, and 199 ± 15 Ma, with close temporospatial coupling between mineralization and magmatism. The δ18O and δD values of fluid inclusions in Stage IV auriferous quartz veins range from −0.03‰ to +5.24‰ and −93‰ to −75‰, respectively, suggesting that mineralizing fluid was likely of magmatic origin. Three distinct ranges of δ34S values are identified in the studied sections (i.e., 7.04‰–9.12‰, −4.95‰ to −2.44‰, and 0.10‰–3.08‰), indicating a source containing multiple sulfur isotopes derived from magmatic and metamorphic fluids. The Liziyuan Au deposit is thus likely an orogenic deposit closely related to magmatism. Geochemical characteristics indicate that Tianzishan monzogranite is adakitic and was derived from thickened lower crust during Triassic orogenesis. The ore-bearing diorite porphyrite and granite porphyry formed in a post-collision extensional setting. Together with previous geological and geochemical data, our results indicate that the Liziyuan orogenic Au deposit was formed by early collisional–compressional metamorphism and late post-collision extensional magmatic fluids related to the evolution of the WQOB.

Lithospheric SH Wave Velocity Structure Beneath the Northeastern Tibetan Plateau From Love Wave Tomography

AbstractThe northeastern (NE) Tibetan plateau has been a prime site to understand the dynamic processes responsible for the rise and lateral growth of the Tibetan plateau. Here we construct a high‐resolution lithospheric scale isotropic SH wave velocity model (down to the depth of 130 km) for the NE Tibetan plateau and its adjacent regions based on the measurements of fundamental‐mode Love wave dispersions (20–100 s) with seismic data recorded by ChinArray II and China Digital Seismic Array using two‐plane wave method. Prominent slow SH wave velocity (VSH) is observed in the midlower crust of the NE Tibetan plateau, specifically in the Qilian Orogenic Belt and the Songpan‐Ganzi Terrane regions, coincident with previously indicated significantly slow SV wave velocity (VSV), probably implying the existence of partial melts in the midlower crust. This low SH wave velocity anomaly could be traced down to the uppermost mantle, indicating a weak and warm lithosphere, which we interpret as a consequence of asthenosphere upwelling after lithospheric mantle removal in the NE Tibetan plateau. The asthenosphere upwelling and associated deep‐seated thermal buoyancy could explain the occurrence of partial melting in the mid‐lower crust and account for parts of high elevations in the NE Tibetan plateau. The western Qinling orogenic belt is characterized with a high velocity anomaly in most of lithosphere, which is inconsistent with the existence of channel flow within the lithosphere along the Qinling orogenic belt from the Tibetan plateau.

3D geochemical modeling for subsurface targets of Dashui Au deposit in Western Qinling (China)

Three dimensional (3D) geochemical modeling was used to identify deposit scale exploration targets combining the geological (110 boreholes and 6 tunnels with 709 assay borehole samples) dataset and 374 litho-geochemical samples in the Dashui Au deposit (China). The Dashui Au deposit, located in the Western Qinling orogenic belt, is a magmatic-hydrothermal type large Au deposit discovered in recent years. The methodologies are summarized as follows: (1) the 3D geological bodies (ore bodies, granitic stocks/dikes, fault fractures, and alteration zones) modeling using GOCAD software. (2) 3D deposit-scale geological modeling in mining zones combination the deposit-scale plane maps, cross sections, longitudinal sections, tunnel planes, borehole datasets, and the range of 3D model is 2.3 × 0.55 × 0.6 km3. (3) The ordinary kriging and discrete smooth interpolation (DSI) interpolation methods were used to analyze the distribution trend of Au grade features combined with borehole and the primary litho-geochemical halos dataset, and the Cu/Pb ratio model is analyzed and constructed by metallogenic model/rule using primary geochemical halos data with DSI interpolation method. (4) The concentration-volume (C–V) fractal method is used to analyze the anomaly classification of grade and Cu/Pb ratio in 3D space using GeoCube software. The results show as follows: (1) the high Au grade value zones are associated with ore-bearing intrusion and the NW- and NE-trending fault structures; (2) the high Au value blocks of Cu/Pb ratio can be used to delineate ore-bearing magmatic fluids channel and mineralization zones; (3) the integrated exploration criteria are Cu/Pb ratio, the NE- and NW-trending faults, and intrusions in the Dashui deposits, the potential targets at depth are located in the southwest and southeast zones under the mining levels of No. 2, No. 170, No. 20, and No. 35 Au orebodies, and the Au resource is >172 t (the depth is <1000 m) in the study area.

Petrogenesis and tectonic implications of Early Cretaceous andesitic–dacitic rocks, western Qinling (Central China): Geochronological and geochemical constraints

40Ar/39Ar and zircon U–Pb geochronological and whole-rock geochemical analyses for the Laozanggou intermediate-acidic volcanic rocks from the western Qinling orogenic belt, Central China, constrain their petrogenesis and the nature of the Late Mesozoic lithospheric mantle. These volcanic rocks yield hornblende or whole-rock 40Ar/39Ar plateau ages of 128.3–129.7 Ma and zircon U–Pb age of 131.3 ± 1.3 Ma. They exhibit SiO2 of 56.86–66.86 wt.%, K2O of 0.99–2.46 wt.% and MgO of 1.03–4.47 wt.%, with Mg# of 42–56. They are characterized by arc-like geochemical signatures with significant enrichment in LILE and LREE and depletion in HFSE. All the samples have enriched Sr–Nd isotopic compositions with initial 87Sr/86Sr ratios ranging from 0.7112 to 0.7149 and ɛNd(t) values from −10.2 to −6.3. Such geochemical signatures suggest that these volcanic rocks were derived from enriched lithosphere-derived magma followed by the assimilation and fractional crystallization (AFC) process. The generation of the enriched lithospheric mantle is likely related to the modification of sediment-derived fluid in response to the Triassic subduction/collision event in Qinling orogenic belt. The early Cretaceous detachment of the lithospheric root provides a reasonable mechanism for understanding the petrogenesis of the Laozanggou volcanic sequence in the western Qinling orogenic belt.

Genesis of the high Sr/Y rocks in Qinling orogenic belt, central China

The genesis of high-Sr/Y rocks and their connection with specific geodynamic settings are not yet resolved. Triassic granitic intrusions with high Sr/Y ratios are widespread in the Qinling orogenic belt of China. Three intrusions of granodiorite, granite porphyry, and dacite porphyry in the western Qinling orogenic belt have zircon LA–ICP–MS U–Pb ages of 237–216 Ma. The rocks are metaluminous to weakly peraluminous and are characterized by enrichment in large-ion lithophile elements (e.g., Rb, Ba, Th, and U), depletion in high-field-strength elements (e.g., Nb, Ta, and Ti), high Sr/Y (31.8–77.5) and (La/Yb)N (13.6–72.6) ratios, very low Y (5.3–16.3 ppm) and Yb (0.35–1.53 ppm) contents, and negligible Eu anomalies. Their (87Sr/86Sr)i ratios (0.70715–0.70884), εNd(t) (−5.93 to −3.07), zircon εHf(t) (−2.34 to +0.85) values and ancient two-stage model ages (TDM2Nd = 1.26–1.47 Ga; TDM2Hf = 1.20–1.39 Ga), and characteristic of low-temperature granitoid indicate that they are partial melts of Neoproterozoic lower crust, with input of enriched-mantle-derived mafic magma. There is no evidence of formation through high-pressure magmatic processes, and a proposed partial melting model indicates that the high-Sr/Y signature of these intrusive rocks may have been inherited from source rocks (lower crustal rocks of the South Qinling terrane). Studies of regional tectonic evolution have shown that the rocks were formed in a subduction regime and/or a subduction-to-initial-collision transitional setting. A high Sr/Y ratio in the magma source may therefore have imparted the high-Sr/Y signature, rather than it being associated with a specific geodynamic setting.

Geochemistry, 40Ar/39Ar geochronology, and geodynamic implications of Early Cretaceous basalts from the western Qinling orogenic belt, China

The Qinling-Dabie orogenic belt was formed by the collision of the North and South China Cratons during the Early Mesozoic and subsequently developed into an intracontinental tectonic process during late Mesozoic. Field investigations identified the presence of late Mesozoic basalts in the Duofutun and Hongqiang areas in the western Qinling orogenic belt. The petrogenesis of these basalts provides an important constraint on the late Mesozoic geodynamics of the orogen. The representative basaltic samples yield the 40Ar/39Ar plateau age of about 112 Ma. These samples belong to the alkaline series and have SiO2 ranging from 44.98 wt.% to 48.19 wt.%, Na2O + K2O from 3.44 wt% to 5.44 wt%, and MgO from 7.25 wt.% to 12.19 wt.%. They demonstrate the right-sloping chondrite-normalized REE patterns with negligible Eu anomalies (1.00–1.10) and PM-normalized patterns enriched in light rare earth element, large ion lithophile element and high field strength element, similar to those of OIB rocks. These samples additionally show an OIB-like Sr-Nd isotopic signature with εNd(t) values ranging from +6.13 to +10.15 and initial 87Sr/86Sr ratios from 0.7028 to 0.7039, respectively. These samples are geochemically subdivided into two groups. Group 1 is characterized by low Al2O3 and high TiO2 and P2O5 contents, as well as high La/Yb ratios (>20), being the product of the high-pressure garnet fractionation from the OIB-derived magma. Group 2 shows higher Al2O3 but lower P2O5 contents and La/Yb ratios (<20) than Group 1, originating from asthenospheric mantle with input of delaminated lithospheric component. In combination with available data, it is proposed for the petrogenetic model of the Early Cretaceous thickened lithospheric delamination in response to the asthenospheric upwelling along the western Qinling orogenic belt.

Age and genesis of the Gangcha gold deposit, western Qinling orogen, China

The Gangcha gold deposit in Hezuo, Gansu, China, is a newly discovered medium‐scale (19 tons) epithermal deposit occurring within the western Qinling orogenic belt. The Daguanshan Formation, represented by metamorphosed volcanic–clastic rock suite, is the main ore‐bearing strata. Gold occurs mainly as submicroscopic inclusions in sulphides and quartz. The hydrothermal alteration is characterized by mid‐ to low‐temperature mineral assemblages such as pyrite, arsenopyrite, and fluorspar. We report results from Rb–Sr isotopic dating of pyrites in the main orebody, which yield an isochron age of 225.3 ± 3.4 to 229.9 ± 4.7 Ma suggesting that the mineral deposit formed during the middle Triassic within a collisional orogenic setting. The initial 87Sr/86Sr value of pyrite (0.710326 to 0.710349) is lower than that of the continental crust and higher than that of the mantle. The S isotope data of pyrites from the deposit show a narrow range from 0.6‰ to 1.3‰ with an average of 0.975‰. Their Pb isotopes exhibit characteristics of a crust–mantle mixed source with a μ (238U/204Pb) of 9.39 to 9.53 and a ω (232Th/204Pb) of 36.81 to 38.14. The S and Pb isotopic composition indicates that ore‐forming materials were derived from a magma sourced in the lower crust, which also incorporated mantle components.

Fabrics and geochronology of the Wushan ductile shear zone: Tectonic implications for the Shangdan suture zone in the Qinling orogen, Central China

The ductile shearing along the Shangdan suture zone during the Paleozoic time is a key to understand the collisional deformation and tectonic regime of amalgamation between the North China Block and the South China Blocks. The Wushan ductile shear zone, a branch of the Shangdan suture, records mylonitic deformation that affected granitic and felsic rocks outcropping in an over 1km wide belt in the western Qinling Orogenic belt. Shear sense indicators and kinematic vorticity number (0.79–0.99) of the mylonites reveal a dextral shear deformation. The quartz c-axis fabrics indicate activation of combined basal 〈a〉 and rhomb 〈a〉 slip, prism 〈a〉 slip and prism 〈c〉 slip. The dynamic recrystallization of quartz is accommodated by combined subgrain rotation and grain boundary migration. These characteristics suggest that the mylonites experienced ductile shear deformation under amphibolite facies conditions at temperatures of ∼500–650°C. Zircons from granitic mylonite yield a U-Pb age of 910±4.8Ma, which represents the formation age of the protolith of the mylonite. The ductile shear zone was intruded by a granitic dyke, which yields a zircon U-Pb age of 403±3.5Ma constraining the minimum age of the ductile shear deformation. Together with regional geology and available geochronological data, these structural characteristics and ages indicate that the Wushan ductile shear zone was formed by dextral shearing following the N-S shortening as a result of collision between the North China and South China blocks along the Shangdan suture.

Geochemical Characteristics and Tectonic Significance of the Acidic Volcanic Rocks from the Shetang-Boyang Area, Western Qinling Orogenic Belt, China

Acidic volcanic rocks of Shetang-Boyang area are located in the western Qinling orogenic belt, consist of rhyolite and granite porphyry. They are comparable in the chemical composition, enriched in Si, alkali, Al and a little bit of Mg, Ca and Ti. The contents of HFSE (Zr, Hf) and LILE (Rb, Th, U) are high, however, the content of Ba, Sr, Ti, P have obviously depleted and there are obvious negative Eu anomalies (Eu/Eu*=0.06-0.13). These geochemical characteristics are revealed that these volcanic rocks have an A1 type affinity. Geochemical data combined with regional studies, show that these volcanic rocks were formed in a continental extension setting and the western Qinling orogenic belt in 211Ma has been in the tectonic setting of post-collisional extension.

Geologic and geochemical insights into the formation of the Taiyangshan porphyry copper–molybdenum deposit, Western Qinling Orogenic Belt, China

Taiyangshan is a poorly studied copper–molybdenum deposit located in the Triassic Western Qinling collisional belt of northwest China. The intrusions exposed in the vicinity of the Taiyangshan deposit record episodic magmatism over 20–30millionyears. Pre-mineralization quartz diorite porphyries, which host some of the deposit, were emplaced at 226.6±6.2Ma. Syn-collisional monzonite and quartz monzonite porphyries, which also host mineralization, were emplaced at 218.0±6.1Ma and 215.0±5.8Ma, respectively. Mineralization occurred during the transition from a syn-collisional to a post-collisional setting at ca. 208Ma. A barren post-mineralization granite porphyry marked the end of post-collisional magmatism at 200.7±5.1Ma. The ore-bearing monzonite and quartz monzonite porphyries have a εHf(t) range from −2.0 to +12.5, which is much more variable than that of the slightly older quartz diorite porphyries, with TDM2 of 1.15–1.23Ga corresponding to the positive εHf(t) values and TDM1 of 0.62–0.90Ga corresponding to the negative εHf(t) values. Molybdenite in the Taiyangshan deposit with 27.70 to 38.43ppm Re suggests metal sourced from a mantle–crust mixture or from mafic and ultramafic rocks in the lower crust. The δ34S values obtained for pyrite, chalcopyrite, and molybdenite from the deposit range from +1.3‰ to +4.0‰, +0.2‰ to +1.1‰, and +5.3‰ to +5.9‰, respectively, suggesting a magmatic source for the sulfur. Calculated δ18Ofluid values for magmatic K-feldspar from porphyries (+13.3‰), hydrothermal K-feldspar from stockwork veins related to potassic alteration (+11.6‰), and hydrothermal sericite from quartz–pyrite veins (+8.6 to +10.6‰) indicate the Taiyangshan deposit formed dominantly from magmatic water. Hydrogen isotope values for hydrothermal sericite ranging from −85 to −50‰ may indicate that magma degassing progressively depleted residual liquid in deuterium during the life of the magmatic–hydrothermal system. Alternatively, δD variability may have been caused by a minor amount of mixing with meteoric waters. We propose that the ore-related magma was derived from partial melting of the ancient Mesoproterozoic to Neoproterozoic middle to lower continental crust. This crust was likely metasomatized during earlier subduction, and the crustal magmas may have been contaminated with lithospheric mantle derived magma triggered by MASH (e.g., melting, assimilation, storage, and homogenization) processes during collisional orogeny. In addition, a significant proportion of the metals and sulfur supplied from mafic magma were simultaneously incorporated into the resultant hybrid magmas.

Seismic structure of the Helan–Liupan–Ordos western margin tectonic belt in North-Central China and its geodynamic implications

We study high-resolution three-dimensional P-wave velocity (Vp) tomography and anisotropic structure of the crust and uppermost mantle under the Helan–Liupan–Ordos western margin tectonic belt in North-Central China using 13,506 high-quality P-wave arrival times from 2666 local earthquakes recorded by 87 seismic stations during 1980–2008. Our results show that prominent low-velocity (low-V) anomalies exist widely in the lower crust beneath the study region and the low-V zones extend to the uppermost mantle in some local areas, suggesting that the lower crust contains higher-temperature materials and fluids. The major fault zones, especially the large boundary faults of major tectonic units, are located at the edge portion of the low-V anomalies or transition zones between the low-V and high-V anomalies in the upper crust, whereas low-V anomalies are revealed in the lower crust under most of the faults. Most of large historical earthquakes are located in the boundary zones where P-wave velocity changes drastically in a short distance. Beneath the source zones of most of the large historical earthquakes, prominent low-V anomalies are visible in the lower crust. Significant P-wave azimuthal anisotropy is revealed in the study region, and the pattern of anisotropy in the upper crust is consistent with the surface geologic features. In the lower crust and uppermost mantle, the predominant fast velocity direction (FVD) is NNE–SSW under the Yinchuan Graben and NWW–SEE or NW–SE beneath the Corridor transitional zone, Qilian Orogenic Belt and Western Qinling Orogenic Belt, and the FVD is NE–SW under the eastern Qilian Orogenic Belt. The anisotropy in the lower crust may be caused by the lattice-preferred orientation of minerals, which may reflect the lower-crustal ductile flow with varied directions. The present results shed new light on the seismotectonics and geodynamic processes of the Qinghai–Tibetan Plateau and its northeastern margin.

Cenozoic tectonic uplift history of Western Qinling: Evidence from sedimentary and fission-track data

The western Qinling (秦岭) orogenic belt is one of the outermost ranges in the northeastern Tibetan Plateau. Its tectonic uplift history is therefore essential to insight on the evolution history of the plateau. However, the timing of deformation and uplift history is still poorly known. Fortunately, its Cenozoic orogenic history is recorded in an excellent synorogenic sedimentary sequence exposed in the Tianshui (天水) Basin, the northeastern foot of western Qinling. According to sedimentary-tectonic analysis of the Yaodian (尧店) and Lamashan (喇嘛山) sections based on the previous magnetostratigraphy studies, we speculated that two stages (occurred at 9.2–7.4 and ∼3.6 Ma) of variation in depositional facies were attributed to the uplift and deformation of the western Qinling, and the modern structure geomorphic frame of the northeastern Tibet formed after 2.6 Ma. Furthermore, four stages of active processes along the western Qinling occurred at 49–41, 34–27, 25–19 and ∼13 Ma, are deciphered from an integrated detrital apatite fission-track data of the Ganquan (甘泉), Yaodian main sections and seven small ones. The former two are represents the exhumation episodes triggered by tectonism and the others attributed to the volcanic signals.

Late Cenozoic orogenic history of Western Qinling inferred from sedimentation of Tianshui basin, northeastern margin of Tibetan Plateau

The Western Qinling orogenic belt marks the northeastern margin of the Tibetan Plateau. Its late Cenozoic orogenic history is recorded in an excellent sedimentary sequence exposed in the Tianshui sub-basin of the Longzhong basin. According to the magnetostratigraphic analysis from the Yaodian and Lamashan sections, we speculate that the late Cenozoic Tianshui basin accumulated lacustrine/floodplain deposits from ~14.8 to ~2.6 Ma. In addition, detrital apatite fission-track thermochronologic and paleocurrent data reveal that the detritus of the Tianshui basin mostly derived from the Western Qinling and that the youngest population age represents a ~14 Ma volcanic intrusion, which can be related to the lithospheric deformation and uplift of the Tibetan Plateau. Furthermore, two stages of variations in depositional facies and average accumulation rates were attributed to the pulse uplift and deformation of the Western Qinling at 9.2–7.4 and ~3.6 Ma.

Eocene to Pliocene exhumation history of the Tianshui-Huicheng region determined by Apatite fission track thermochronology: Implications for evolution of the northeastern Tibetan Plateau margin

This study presents new apatite fission-track (AFT) data from the Tianshui-Huicheng region, which is situated in the northeastern margin of the Tibetan Plateau. AFT results (including 19 detrital and 13 bedrock samples) show that deposition in the Tianshui-Huicheng basin range from early Eocene to late Miocene–Pliocene and most of the apatites derived from the Western Qinling orogenic belt, reflecting one rapid exhumation episode (at 34–29 Ma) and two phases of volcanism (at 23–18 Ma and ca. 13 Ma). The exhumation history of the Tianshui-Huicheng region is likely related to episodic tectonic activity and directly linked to the kinematics of active deformation of the Tibetan Plateau. Active processes deciphered from the detritus indicate that deformation reached the northeastern Tibetan Plateau margin as early as ca. 49 Ma, which is manifested by the initiation of the Tianshui-Huicheng basin. The 23–18 Ma event, linked to the lithosphere deformation of the Western Qinling and the Neogene basin formation along the Tianshui-Huicheng region. By modeling the bedrock samples, an 8–4 Ma event was revealed which resulted from the intense deformation of the Tianshui-Huicheng region.

Crustal thicknesses and Poisson's ratios in the eastern Tibetan Plateau and their tectonic implications

Crustal thicknesses and Poisson's ratios in the eastern Tibetan Plateau were estimated using the H‐k stacking method on teleseismic receiver functions. The data came mainly from the regional seismic networks in 2006–2008. Results show that the crust thickens from east to west, with a thickness difference of ∼30 km. A sharp change of crustal thickness from 41.5 to 52.5 km is observed across the Longmen Shan. Crustal thickness predictions based on the Airy‐type isostasy are consistent with the estimations from H‐k stacking, implying that the topography is approximately compensated. The study area can be divided into three parts according to the Poisson's ratios: (1) the northern Songpan‐Garze terrane and the western Qinling orogenic belt with low ratios (ν ≤ 0.26); (2) the southern Songpan‐Garze terrane, the Sanjiang fold system, and the Sichuan Basin with high‐to‐intermediate ratios (0.27 ≤ ν &lt; 0.30); and (3) the southwestern Yangtze platform with low to intermediate ratios (ν ≤ 0.27). Joint interpretation of the low average Vp values from deep seismic sounding profiles and the low Vp/Vs ratios from receiver function analysis favors the presence of felsic crust rather than significant volumes of partial melts in the northern Songpan‐Garze terrane and the western Qinling orogenic belt. The junction of the Longmen Shan fault zone, the Xianshuihe fault zone, and the Anninghe fault zone has an ultrahigh Poisson's ratio (ν ≥ 0.30), which could be considered as an evidence of local partial melt. We propose that the eastward movement of upper and middle crustal material in the southern Songpan‐Garze terrane is obstructed by the more rigid Sichuan Basin, resulting in strain accumulation along the Longmen Shan fault zone.

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.

Geology and geochemistry of the Dengjiashan Zn–Pb SEDEX deposit, Qinling Belt, China

Dengjiashan, with 25 Mt at 4.77% Zn, 1.27% Pb, and 14 g/t Ag, is one of the largest SEDEX (sedimentary-exhalative) Zn–Pb sulfide deposits in the western Qinling orogenic belt, China. The ore is hosted in chert stratigraphically conformable with underlying bioclastic micrite and overlying phyllite of an intensely folded and faulted Middle Devonian sedimentary sequence. The fact that the major orebodies are located between platformal limestone and overlying phyllite indicates that mineralization took place when the Dengjiashan sedimentary sub-basin subsided from an open platform to a shallow-marine basin within an extensional tectonic setting. The deposit comprises 16 sulfide orebodies, of which the #1 and #9 orebodies are the two largest and account for 95% of ore reserves. The massive sulfide lenses are mainly composed of sphalerite, pyrite, and galena, with minor chalcopyrite, tetrahedrite, cinnabar, pyrargyrite, freibergite, boulangerite, polybasite, and trace amounts of owyheeite and native gold. The gangue is dominated by quartz, calcite, barite, and ankerite, with minor amounts of sericite, chlorite, celsian, cymrite, and albite. The rare-earth element patterns of sulfide beds and host chert suggest that the mineralizing fluids were dominated by oxidized seawater that leached Eu from feldspar in basement rocks. Strontium isotopes also suggest that the hydrothermal fluids were composed of Devonian seawater that leached radiogenic Sr along the flow path. Lead isotope ratios form an array between the upper crust and orogene curves in the 207Pb/204Pb–206Pb/204Pb diagram, indicating that lead was leached from crustal rocks below the deposits. Sulfur isotopes indicate that bacterial sulfate reduction (BSR) sulfide formed in a sub-basin restricted to sulfate where it mixed with heavy hydrothermal sulfur from thermochemical sulfate reduction (TSR) of Devonian seawater sulfate.

Magmatic Gold Mineralization in the Western Qinling Orogenic Belt: Geology and Metallogenesis of the Baguamiao, Liba and Xiaogouli Gold Deposits

Abstract The superlarge Baguamiao, large Liba and Xiaogouli gold deposits represent three typical gold deposits different from the Carlin type in the western Qinling Orogenic Belt. Based on Ar‐Ar dating of quartz from ores, U‐Pb dating of single zircon from granite, tracing of H and O isotopes and studies on the mineralogy and texture of spots and bleached alteration developed in wall rocks, this paper focuses the relations between gold deposits and granite to clarify the origin of gold deposits and the metallogenesis in the tectonic evolution of the Qinling Orogenic Belt. The comprehensive studies show that the age of the granite (148.1–244 Ma) is identical with that of the gold deposits (131.91–232.56 Ma). It is suggested that the granite has close temporal, spatial and genetic relationship with the gold deposits. The granite provides a heat source, water source and considerable amount of ore‐forming material. Finally, it is concluded that the orogeny by collision, emplacement of the granite and positioning of the gold deposits represent a successive process. Both the granite and gold deposits resulted from the syn‐orogeny and post‐orogeny tectonic evolution.