Shangdan Suture Research Articles

Late Mesozoic ductile deformation and exhumation along the Shangdan suture zone in the Qinling Orogenic Belt, China

The Qinling Orogenic Belt (QOB) is a multi‐stage orogenic belt recording subduction and collision processes. The ductile shearing deformation with different properties and ages is developed on the boundary of the different litho‐tectonic units, which can be used to decipher the tectonic evolution of the QOB. Previous studies mainly concentrated on the pre‐Late Mesozoic shearing deformation, while the Late Mesozoic shearing deformation received less attention. It restricts the understanding of the Late Mesozoic tectonic evolution of the QOB. Hence, we conducted detailed field structural analysis and laboratory analysis of the Shagou shear zone (SGSZ) in the middle part of the Shangdan suture zone (SDSZ). The SGSZ, developed in the southern margin of the Baliping pluton, is an S–SSW‐dipping sinistral strike‐slip ductile shear zone with a top‐to‐the‐ESE shear sense. The mylonite protoliths are voluminous Late Triassic granites (200–215 Ma) and a small amount of Neoproterozoic granites (759 Ma) and Early Cretaceous granites (145 Ma). Microstructures and geothermobarometer results suggest that the ductile shearing deformation occurred under high greenschist–low amphibolite‐facies conditions (400–650°C) at middle crustal depths (13–21 km). Zircon U–Pb and amphibole‐biotite 40Ar/39Ar dating results constrain the timing of shearing deformation to 135–145 Ma and the rapid exhumation of the SGSZ to 115–135 Ma. Combining with the regional data, we consider that the sinistral strike‐slip shearing deformation in the SGSZ controls and accommodates the south‐eastward extrusion of the East SQB under the intracontinental compression. Subsequently, the intracontinental extension led to the rapid exhumation of the SGSZ.

Structural styles and chronological sequence of the Mianyuzui ductile strike-slip fault along the Shangdan suture Zone: Implications for the Paleozoic tectonic evolution of the Qinling Orogenic Belt, China

The Shangdan suture zone (SDSZ) subdivides the Qinling Orogenic Belt into North Qinling and South Qinling belts, marking the main tectonic boundary between the North China and South China Blocks. The Mianyuzi ductile strike-slip fault (MYZF) within the SDSZ separates the North China Block from the South China Block and contains valuable collisional information. This paper integrates the structural and geochronological data of the MYZF to investigate the Paleozoic tectonic evolution of the Qinling Orogenic Belt and the collision between the North China and South China Blocks. Two phases of deformation (D1-D2a-b) are identified along the southern boundary of the SDSZ. The metamorphic temperature of mylonites is ∼ 600–700 ℃ for the early stage and ∼ 300–500 ℃ for the late stage of shearing deformation. U-Pb zircon analysis of the mylonites and diabase dike in the ductile shear reveals that zircons have major younger protolith populations at 480–460 Ma, 450–430 Ma, and 405–425 Ma, which suggests that rocks were probably derived from the North Qinling belt. The metamorphism and deformation cooling age from the MYZF have been constrained to be 339–328 Ma and 259 ± 1.6 Ma using U-Pb and 40Ar/39Ar dating. Combined with previously published data, we concluded that the subduction of the Shangdan Ocean was active until the Early Silurian and subsequent collisions occurred during the Late Silurian. Meanwhile, the shearing might have been related to the intraplate extension-exhumation during the Carboniferous-Late Permian in the Qinling Orogenic Belt.

Structural Geology and Chronology of Shear Zones along the Shangdan Suture in Qinling Orogenic Belt, China: Implications for Late Mesozoic Intra-Continental Deformation of East Asia

Structural Geology and Chronology of Shear Zones along the Shangdan Suture in Qinling Orogenic Belt, China: Implications for Late Mesozoic Intra-Continental Deformation of East Asia

Late Palaeozoic tectonic transformation of the North Qinling Orogenic Belt: Insight from the geochronology, geochemistry and Sr‐Nd‐Hf isotopes of Carboniferous magmatic rocks

The North Qinling Orogenic Belt (NQB) records the pivotal geological information for understanding the Palaeozoic evolution of the Qinling Orogenic Belt (QOB). Previous studies mainly focused on the subduction–collision process along the Shangdan suture zone before the Early Devonian. However, as the significant interim period between the Early Palaeozoic and Mesozoic orogeny, the Late Palaeozoic tectonic evolution of the NQB remains poorly understood. Fortunately, the Carboniferous magmatic rocks discovered this time provide substantial geological evidence for revealing the Late Palaeozoic tectonic history of the Qinling Orogenic Belt. This paper will provide a detailed analysis of zircon U–Pb ages, geochemical characteristics and Sr‐Nd‐Hf isotopic compositions of Carboniferous rocks. The Carboniferous magmatic rocks are categorized as highly differentiated S‐type granite and monzodiorite, formed at 350 ± 2.4 Ma and 353 ± 5.2 Ma, respectively. The granite shows weak peraluminous (A/CNK = 1.02–1.11) and shoshonite nature. Based on the isotopic composition and trace element characteristics, we propose that the Qinling Group paragneiss is the primary source of the Carboniferous granite. Monzodiorite is characterized by enriched LREEs and LILEs (Ba, K and Pb), depleted HFSEs (Th, Nb, Ta and Ti) and enriched Sr‐Nd isotopic composition. Monzodiorite magma source region consists of continental crustal material and lithospheric metasomatized mantle. Carboniferous magmatic rocks are the product of an extensional tectonic setting, which indicates the NQB tectonic regimes transition from compression to extension during the Carboniferous.

Middle to Late Triassic felsic and mafic magmatism in the eastern West Qinling Orogen, Central China: A record of the Paleo-Tethyan oceanic crust advance to retreat

The tectonic setting of Indosinian magmatism in West Qinling Orogen and the tectonic evolution of the Paleo-Tethys remain hotly debated. Here we present SHRIMP zircon UPb dating, major-trace elemental and Sr-Nd-Pb-Hf isotopic data for four granitic and gabbroic plutons across the eastern West Qinling Orogen, from north to south including Guanshan, Fengxian, Mishuling and Miba. Our new data indicate that the Fengxian granitoids, emplaced at 238 Ma in a continental arc setting, show adakitic affinity and were formed by partial melting of subducted sediments. The southern Guanshan granitoids, emplaced at 235–229 Ma in a continental arc setting, also show adakitic affinity but were generated by partial melting of thickened (>50 km) lower crust. The northern Guanshan granites and gabbros were emplaced at 217–216 Ma in a back-arc setting, with the former generated by partial melting of normal (40–50 km) lower crust and the latter derived from asthenospheric mantle. The Mishuling granitic pluton contains abundant mafic microgranular enclaves, both emplaced at 214 Ma in a back-arc setting with the host granitoids formed by partial melting of normal (40–50 km) lower crust and the enclave magmas derived from asthenospheric-lithospheric mantle. The Miba granitoids, emplaced at 216 Ma in a continental arc setting, show adakitic affinity and were generated by partial melting of thickened (>50 km) lower crust. We suggest that the Paleo-Tethyan oceanic crust had subducted near the Shangdan suture by 238 Ma and successively advanced northward, approaching southeastern Qilian Orogen at 235–229 Ma. Then slab rollback initiated and gradually migrated southward. The slab subduction did not cease before 214 Ma.

Monazite and titanite U[sbnd]Pb ages of granitoids from the North Qinling Terrane: Implications for the subduction of the Shangdan Ocean and accretionary processes

The early Paleozoic granitoids of the North Qinling Terrane have been widely viewed as the products of an Andean-type margin related to the long-lasting subduction of the Proto-Tethys (Shangdan) ocean. However, the spatial and temporal distributions of the arc granitoids coupled with subduction-accretion processes are still a subject of debate. Here, we report LA-ICP-MS UPb monazite and titanite ages and trace element concentrations for granites and a paragneiss from the North Qinling Terrane. The results, together with data in the literatures, are used to reconstruct the magmatic evolution related to the subduction of the Shangdan ocean during the early Paleozoic. As revealed by UPb geochronology, subduction-related magmatism in the southern belt of the North Qinling magmatic arc (NQMA) was most active between 423 and 404 Ma, and therefore younger and shorter than previously speculated. The ages are in agreement with a metamorphic monazite UPb age (415 Ma) obtained from the paragneiss, suggesting that crustal anatexis of the NQMA was contemporaneous with the magmatism. Overall, magmatism in the NQMA moved progressively from north to south. This southward propagation is consistent with roll-back of the Shangdan oceanic slab and transition from an oceanic arc into a back-arc basin setting. The timing of magmatism and migmatization as well as high-temperature granulite metamorphism in the NQMA signifies that the Shangdan ocean retreated to the North Qinling Unit before c. 423 Ma. Monazites from NQMA generally show HREE depletion and negative Eu anomalies, and some display the REE tetrad effect. These geochemical features indicated that the NQMA granites were highly fractionated. In addition, the abrupt changes in REE compositions displayed by titanites probably record a mixing event with a mafic magma along the Shangdan suture zone.

Early Mesozoic orogenic gold mineralization in the North Qinling Terrane: Insights from rutile U-Pb, mica and K-feldspar 40Ar/39Ar, and H-O-S-Pb isotopes of the Yangxie gold deposit

The North Qinling Terrane is located between China’s second largest, Early Cretaceous gold province (southern North China Craton) to the northeast and the third largest, Triassic gold province (western South Qinling Terrane) to the southwest. However, only several small- to medium-sized Au deposits and occurrences have been found so far in the North Qinling Terrane, and their genesis and possible links with the gold mineralization in the above two large gold provinces remain unclear. In this study, we present a comprehensive geochronology, stable and radiogenic isotope geochemistry study of the Yangxie gold deposit in the North Qinling Terrane to provide new insights into gold genesis in the North Qinling Terrane and its genetic relations to the neighboring gold provinces. The Yangxie gold deposit consists of quartz-sulfides veins that are hosted by NW- and NE-striking brittle faults cutting amphibolite- to granulite-facies metamorphic rocks of the Paleoproterozoic to Neoproterozoic Qinling Group. Gold mineralization is represented by four paragenetic stages consisting of milky quartz-K-feldspar (I), light grey quartz-pyrite (II), light grey quartz-polymetallic sulfides (III), and quartz-calcite (IV). Gold was deposited mainly in stages II and III, forming abundant native gold grains that occur as inclusions in pyrite, microfracture infillings in sulfide minerals, or interstitial growths to other ore and gangue minerals. Ore-related alteration assemblages comprise mainly quartz, sericite, muscovite, K-feldspar, and calcite. Hydrothermal rutile is widely recognized in gold ores and texturally intergrown with gold-bearing pyrite and other alteration minerals, indicating synchronous precipitation of rutile and gold. Laser ablation ICP-MS U-Pb dating on two rutile samples yield reproducible dates of 214.6 ± 5.5 Ma (MSWD = 0.82, 2σ) and 210.1 ± 6.9 Ma (MSWD = 1.3, 2σ), which are interpreted as the time of gold deposition. 40Ar/39Ar dating of sericite, muscovite, and K-feldspar from the alteration assemblages yield more scattered and younger dates ranging from 199.8 ± 1.1 to 119.3 ± 0.6 Ma, which are considered as the cooling ages of the deposit related to later tectonothermal events. Oxygen and hydrogen isotopes of ore-related quartz and extracted fluid inclusion waters are 10.4 ∼ 15.0 and −87.7 ∼ -67.2 per mil, respectively. Pyrite and galena have variable δ34S values ranging from −9.4 to 0.5 per mil. These sulfides have lead isotopic compositions of 17.956–18.067 for 206Pb/204Pb, 15.565–15.621 for 207Pb/204Pb, and 38.045–38.301 for 208Pb/204Pb. The isotope data collectively suggest that the ore-forming fluids and metals were derived from the devolatilization of the early Paleozoic sedimentary rocks distributed in the North Qinling Terrane. The results presented here, combined with independent studies, show that the Yangxie deposit was a product of the Late Triassic orogenic gold mineralization event that formed the western South Qinling gold province. Recognition of the Late Triassic Yangxie gold deposit suggests that faults within and around the metamorphosed Paleozoic sedimentary rocks and subsidiary faults affiliated to the Shangdan Suture in the North Qinling Terrane could be favorable targets for further Triassic orogenic gold exploration.

Zircon U–Pb dating and geochemistry of intrusive rocks in the Shangdan suture zone of the Qinling Orogenic Belt: petrogenesis and tectonic implications

We studied the zircon U–Pb ages, whole-rock geochemistry and Sr–Nd isotopic compositions of a Paleozoic gabbro–diabase–granite association in the Shangdan suture zone to constrain the petrogenesis of these intrusive rocks and identify their tectonic implications. These rocks have distinctive crystallization ages: the Guchenggou gabbro yielded zircon U–Pb ages of 424–423 Ma, the average age of the Yueritan diabase was 411 Ma and the Danfeng granites yielded zircon U–Pb ages of 414–411 Ma. The Guchenggou gabbro was formed by partial melting of a phlogopite-bearing garnet lherzolite modified by subduction fluids in a continental arc setting. The Yueritan diabase was generated by mixing a phlogopite-bearing garnet lherzolite modified by subduction melts with continental crustal rocks in a post-collision setting. The Danfeng granites were weakly peraluminous I-type granites formed by mixing juvenile crust with metagreywackes at high temperatures and pressures in a tectonic environment transitioning from compression to extension. We suggest that the subduction of the Shangdan Ocean induced the formation of the gabbro. The diabases and granites are related to relamination of the continental crust in response to the collision of the North Qinling Belt with the South Qinling Belt, suggesting that closure of the Shangdan Ocean may have occurred at c. 410 Ma.

Mesozoic intracontinental ductile shearing along the Paleozoic Shangdan suture in the Qinling Orogen: Constraints from deformation fabrics and geochronology

AbstractThe Qinling Orogen, located between the North and South China Blocks, records subduction-collisional orogeny along the Paleozoic Shangdan and Triassic Mianlue sutures, and Mesozoic intracontinental orogenesis, which all played an important role in building the present tectonic framework and topography. The strike-slip Shagou ductile shear zone that overprints the Paleozoic Shangdan suture between the North and South Qinling Belts is crucial for understanding the Mesozoic intracontinental deformation in the Qinling Orogen. The microstructures, asymmetrical fabrics, and kinematic vorticities (0.81–0.95) suggest sinistral simple shear. The quartz c-axis patterns from felsic mylonites exhibit Y-maximum fabrics, indicating the activity of prism <a> slip, while those from amphibole-rich mylonites display both Y-maximum and Z-maximum fabrics showing the combined activity of prism <a> and basal <a> slip systems. In the mafic mylonites, the plagioclase fabrics were induced by combined (010)[100] and (001)[100] slip systems, while the amphibole fabrics were probably related to anisotropic growth or passive rotation of rigid clasts. Equilibrium P–T conditions of 4.28–6.12 kbar and 646–727 °C estimated from geothermobarometry suggest that the main deformation occurred under amphibolite facies conditions at middle–lower crustal depths. Zircon U-Pb ages constrain their protolith rocks to have crystallized at 816 ± 25 Ma and 726–718 Ma, while the intense mylonitization and sinistral shearing occurred at ca. 200–187 Ma. A U–Pb zircon age of 132 ± 3 Ma from a granitic dike cutting the foliation and an amphibole 40Ar/39Ar plateau age of 119.0 ± 0.9 Ma from mylonites together suggest that the Shagou shear zone evolved through decompression and exhumation stages in the time period of 132–119 Ma.

Exhumation of plutons controlled by boundary faults: Insights from the kinematics, microfabric, and geochronology of the Taibai shear zone, Qinling Orogen, China

Abstract The Taibai granitic plutons lie between the Taibai ductile shear zone to the north and the Shangdan suture to the south. The deformation mechanism of the ductile shearing is crucial to understanding the exhumation processes of the multiple plutons that formed after the Late Mesozoic period. Geological investigations, microstructures, and kinematic vorticity calculations indicate that the Taibai shear zone deformed in response to pure shear-dominated (54%–65%) transpression and top-to-NW shear sense as a result of NE–SW oblique contractional tectonics. The quartz crystallographic preferred orientations of the prism <a> slip system, the grain boundary migration, and sub-grain rotation dynamic recrystallization of quartz—combined with the plagioclase–hornblende thermometer—constrain the main deformation temperatures to a range of 400–650 °C, which suggests amphibolite to greenschist facies conditions. In addition, it is extremely likely that the mylonites experienced late-stage, lower temperature deformation as demonstrated by the sporadic bulging recrystallization, the quartz basal <a> slip system, and the two-feldspar geothermometer calculation. The samples collected from the weakly deformed mylonitic granite pluton and the undeformed quartz-feldspathic dike that intruded into the mylonites yield zircon U–Pb ages of 129 ± 1 Ma and 115 ± 1 Ma, respectively. This information, with the lower intercept ages of ca. 120 Ma obtained from the mylonite samples, suggests that the ductile shearing probably occurred from ca. 129 Ma to 115 Ma. Combined with the regional geological data, these findings suggest that the Taibai shear zone and the Shangdan suture accommodated the oblique upward extrusion of the Taibai plutons during Early Cretaceous time.

Zircon U–Pb geochronology and geochemistry of the Lajimiao mafic complex in the Shangdan Suture Zone, Qinling orogen: Petrogenesis and tectonic implications

The Shangdan Ocean represents the eastern branch of the Proto-Tethys Ocean, which is important to decipher the evolution of the Central China Orogenic Belt. However, its evolution is still in debated. In this contribution, we carried out an integrated study of zircon UPb ages, whole-rock geochemistry, as well as zircon HfO isotope compositions for the Lajimiao mafic complex in the Shangdan Suture Zone in the North Qinling orogen. The Lajimiao mafic complex is composed of coarse-grained gabbros to norite-gabbros (Group 1) in the inner and fine-grained diorites (Group 2) in the outer part. The Group 1 samples are comprised predominantly of salite to augite (Mg# = 75–80), magnesi-hornblende (Mg# = 65–67) and Ca-rich plagioclase (An = 45–67); the Group 2 samples are comprised predominantly of magnesi-hornblende (Mg# = 60–66) and plagioclase (An = 28–53). New SIMS and LA-ICP-MS UPb dating of zircon grains from a gabbro and a diorite sample yielded similar weighted mean ages of 412 ± 4 Ma and 414 ± 2 Ma, indicating that they were formed simultaneously. The Group 1 samples have relatively low SiO2 (49.56–50.78 wt%), high MgO (6.67–7.70 wt%) and Fe2O3T (7.46–9.61 wt%) contents, while the Group 2 samples have relatively high SiO2 (57.74–59.53 wt%), low MgO (3.28–3.42 wt%) and Fe2O3T (5.76–7.01 wt%) contents. They both belong to calc-alkalic series and enriched in large ion lithophile elements (LILEs), light rare earth element (LREEs) and depleted in high field strength elements (HFSEs), which are typical of arc-related rocks. The Group 2 diorites exhibit relative enrichment in light rare earth element (LREE) ((La/Yb)N = 10.9–13.1) with slightly negative Eu anomalies (Eu/Eu⁎ = 0.82–0.93), compared to the Group 1 samples ((La/Yb)N = 5.2–9.6; Eu/Eu⁎ = 0.91–1.43). Both of them have enriched initial Sr (0.7049–0.7054), and chondritic to slightly depleted Nd (ƐNd(t) = −0.09 ~ +1.50), but depleted whole-rock and zircon Hf isotopic compositions (ƐHf(t) = 7.81–8.10 and 8.2–9.1). The HfNd isotopic systems are decoupled with positive △ƐHf values (5.22 to 5.92) (△ƐHf = ƐHf(t) – (1.59 × ƐNd(t) + 1.28)). The zircon grains display higher δ18O values of 6.3 to 6.5‰ than those of the normal mantle zircon. Whole-rock Pb isotopic compositions of the studied samples are of 15.610–15.624 for 207Pb/204Pb, 38.027–38.178 for 208Pb/204Pb, and 18.100–18.269 for 206Pb/204Pb. These geochemical features indicate that the Lajimiao mafic complex was derived from relatively depleted mantle that had been metasomatized by 2–5% subducted zircon-barren oceanic sediments. Combining with regional geological, geochronological, mineralogical, and geochemical data, the Lajimiao mafic complex was suggested to be generated by partial melting of a phlogopite-bearing mantle source at relatively low pressure in an arc setting. We suggested that the generation of the Lajimiao mafic complex was induced by the subduction of Shangdan Ocean during the early Devonian and thus the eastern branch of Proto-Tethys Ocean might have not been closed until that time.

Genesis of the Maanqiao gold deposit in the western Qinling orogen, central China: Constraints from fluid inclusions and in-situ sulfur isotopes

The western Qinling orogen (WQO) is one of the most important prospective gold provinces in China. The Maanqiao gold deposit, located on the southern margin of the Shangdan suture, is a representative gold deposit in the WQO. The Maanqiao deposit is hosted by the metasedimentary rocks of the Upper Devonian Tongyusi Formation. The EW-trending brittle-ductile shear zone controls the orebodies; they occur as disseminated, and auriferous quartz–sulfide vein. The ore-related hydrothermal alteration comprises silicification, sulfidation, sericitization, chloritization, and carbonatization. Native gold is visible and mainly associated with pyrite and pyrrhotite. Mineralization can be classified into the following three stages: bedding-parallel barren quartz–pyrite–(pyrrhotite) (early-stage), auriferous quartz–polymetallic (middle-stage), and carbonate–(quartz)–sulfide (late-stage).Detailed fluid inclusion (FI) studies revealed three types of inclusions in quartz and calcite: aqueous (W-type), CO2–H2O (C-type), and pure carbonic (PC-type) FIs. The primary FIs in the early-stage quartz are C- and PC-type, in the middle-stage quartz are mainly W- and C-type, and in the late-stage calcite are only W-type. During gold mineralization, the total FI homogeneous temperatures evolved from 189–375 °C (mostly 260–300 °C) to 132–295 °C (mostly 180–240 °C) to 123–231 °C (mostly 130–150 °C), and the salinities varied among 2.2–9.1 wt.% NaCl equiv. (mostly 5–8 wt.%) to 0.2–9.0 wt.% NaCl equiv. (mostly 3–6 wt.%) to 0.3–3.6 wt.% NaCl equiv. (mostly 2–4 wt.%). The ore-forming fluid was characterized as an H2O–NaCl−CO2−CH4–(N2) system with medium-low temperature and low salinity. The fluid immiscibility and fluid-rock interaction may be responsible for the precipitation of the sulfides and gold at the Maanqiao gold deposit. Three types of pyrite corresponding to the three mineralization stages, as well as pyrrhotite and arsenopyrite in the middle stage, are micro-analyzed for in-situ sulfur isotopic composition by LA-ICP-MS. Py1 yield near-zero δ34S values of −2.5‰ to 3.0‰, which are somewhat lower than that of the granite hosted pyrites (Py-g, 4.8‰ to 6.6‰). The result suggests a mixed sulfur source from magmatic-hydrothermal fluids and the metamorphism of diagenetic pyrite. Pyrite + pyrrhotite + arsenopyrite assemblages in the middle-stage have relatively higher δ34S values (6.6‰ to 12.3‰) and are mainly developed due to the metamorphism of the ore-host and underlying Devonian sedimentary sequences. The low δ34S values of the late-stage fracture-filled Py3 (−21.9‰ to −17.0‰) resulted from an increasing oxygen fugacity, which was caused by the inflow of oxidized meteoric waters.Based on our studies, the Maanqiao gold deposit is considered to be an orogenic type and closely related to the Indosinian Qinling orogeny.

Structural style and geochronology of ductile shear zones in the western north Qinling orogenic belt, Central China: Implications for Paleozoic orogeny in the Central China orogeny

The Qinling Orogen, which was formed by long-lived subduction and collision between the North and South China blocks, can be divided into the South Qinling orogenic belt (SQOB) and the North Qinling orogenic belt (NQOB) by the Shangdan suture zone. Orogen-scale shear zones developed on the boundaries within the NQOB can be used to enhance our understanding of collisional orogeny that occurred during the Paleozoic in the Qinling Orogen. Previous studies have identified several ductile shear zones on boundaries between different lithotectonic units in the eastern NQOB. However, ductile shear zones related to orogenic processes in the western NQOB have attracted relatively little attention. Herein, we present structural and chronological data for the ductile shear zones in the Tianshui region, western NQOB. Three sub-parallel ductile shear zones (DSZ-1, DSZ-2, and DSZ-3) are recognised in the southern Qinling Complex or as boundaries between the Qinling Complex and adjacent lithotectonic units in the Huamiao area, Tianshui City. Microstructural and fabric analyses indicate that these shear zones have kinematics of transpressive sinistral strike-slip. Zircon U-Pb and 40Ar-39Ar dating data suggest that these ductile shear zones experienced a prolonged period of deformation from 420 to 370 Ma during exhumation, which was related to oblique convergence between the SQOB and the NQOB. Combining with the synchronous Xinyang-Yuanlong dextral shear zone in the northern Qinling Complex, we suggest that the transpressive sinistral shearing and dextral transpressive ductile shearing on both sides of the Qinling Complex subjected it to orogen-parallel oblique-slip extrusion during the middle Paleozoic. Our results, together with previously published data in the eastern NQOB, indicate the occurrence of orogen-scale strike-slip shearing along the whole NQOB during the Paleozoic orogeny.

Thermotectonic evolution of the Paleozoic granites along the Shangdan suture zone (central China): Crustal growth and differentiation by magma underplating in an orogenic belt

Abstract The nature of source rocks and the pressure-temperature-hydration (P-T-H2O) condition are the two main factors that control the geochemical properties of granites. Therefore, the evolution of P-T-H2O conditions can be used to deduce the tectonic setting of granites. In this paper, we report on three Paleozoic granite plutons along the Shangdan suture that revealed increasing melting temperature and decreasing pressure from 437 to 403 Ma, suggesting a crustal thinning process. The Tieyupu granodiorites (437 ± 4 Ma) display Na-rich adakite affinity, i.e., SiO2 = 69.1–70.1 wt%, Na2O/K2O = 1.9–2.26, positive zircon εHf(t) values (+4.29 to +12.04), and high Sr/Y (137–160) and Y/Yb (9.89–10.25) ratios, implying a garnet-rich residue in their source. In combination with moderate zircon saturation temperatures (814–822 °C), we infer that the Tieyupu granodiorites were formed by melting of Neoproterozoic metabasites under high-pressure (>1.5 GPa) and moderate-temperature (HP-MT) conditions. The Liangchahe granodiorites (415 ± 8 Ma) also display Na-rich adakite affinity, i.e., higher Na2O/K2O (2.16–3.11) and lower Sr/Y (77–88) ratios, and higher zircon saturation temperatures (854–874 °C), and they are interpreted to have been derived from melting of metabasites under moderate-pressure (>1.0 GPa) and high-temperature (MP-HT) conditions. Their variable zircon εHf(t) values (−14.97 to +9.80) and the existence of zircon xenocrysts suggest that the primitive adakitic melts were assimilated by evolved crustal components. The Yaogou monzogranites (403 ± 4 Ma) have the highest K2O/Na2O (0.81–1.00) ratios and total rare earth element (ΣREE; 105–191 ppm) contents, lowest Sr/Y (14–43) ratios, positive zircon εHf(t) values (+6.79 to +12.22), and highest zircon saturation temperatures (891–973 °C), showing they were formed by high-temperature melting of intermediate rocks under low-pressure conditions (<1.0 GPa). The evolution of P-T conditions revealed by these three granites suggests that crustal growth and differentiation were related to gradual extensional and melting of mafic protoliths in the orogenic belt.

Paleomagnetic Constraints of the Lower Triassic Strata in South Qinling Belt: Evidence for a Discrete Terrane Between the North and South China Blocks

AbstractThe paleoposition of the South Qinling Belt is a crucial key to understanding the collisional process between the North and South China blocks. In order to identify the paleogeography of the South Qinling Belt during the Early Triassic and its correlation to the North and the South China blocks, combined paleomagnetic studies have been conducted on the Lower Triassic sedimentary rocks in the South Qinling Belt. After systematic thermal demagnetization, a high‐temperature characteristic remanence was isolated from 133 specimen embracing 22 sites with a mean direction ofD= 330.5°,I= 27.7° (α95= 2.5°). The positive fold and reversal tests suggest that the characteristic remanence was acquired during deposition. This characteristic remanence corresponds to a mean paleopole atφp= 7.2°E,λp= 62.5°N (A95= 2.4°), indicating a paleolatitude of ~23.6°N for the South Qinling Belt. These new data are in concordance with the coeval paleomagnetic pole of the North China Block but are distinctly different from that of the South China Block with ~10.0° paleolatitude and ~73.6°declination differences. Together with the regional geology, our new data suggest that the South Qinling Belt had amalgamated with the North China Block along the Shangdan Suture by the Early Triassic times; however, it was still separated from the South China Block by the Mianlue Ocean up to the Early Triassic. The final collision between the North and South China blocks resulted from a ~73.6° clockwise rotation of the South China Block relative to the South Qinling Belt along the Mianlue Suture during the Late Triassic.

Early‐Paleozoic mafic intrusion in North Qinling (Central China): Implication for the initiation back‐arc system along the Shangdan suture zone

The relationship between the Shangdan and Erlangping suture zone (ESZ) and the initiation stretch time of the Early Paleozoic back‐arc system in the Qinling Orogen still remains unclear. This paper reports the chronological and geochemical data of the Miaogou mafic intrusion along the southern margin of the Shangdan suture. The pyroxenite in the mafic intrusion has a zircon U–Pb age of 485 ± 5 Ma (Mean Standard Weighted Deviation (MSWD) = 0.08, n = 18). It has low SiO2 (45.41–46.02 wt%), K2O (0.42–0.44 wt%), and TiO2 (0.73–0.83 wt%) contents, high MgO (13.9–15.13 wt%) contents with high Mg# (74–77). In combination with its high clinopyroxene portion, the pyroxenite represented the accumulates from the mafic magma. Some sieve rims of the clinopyroxene indicate that the primitive pyroxenite was metasomatized by subduction‐related fluids. However, their consistent trace element and rare earth element (REE) patterns suggest that the metasomatism process has insignificant effects on their geochemical features. Zircons from the pyroxenite have positive εHf(t) (+7.18 to +11.75) values, reveal a depleted source region. In combination with its high V (322–1,093 ppm) contents, low Ti/V (10–15) ratios and smooth REE patterns, we propose that the pyroxenite was derived from metasomatized depleted mantle wedge in a back‐arc basin. In considering the duration limit (<475 Ma) of the back‐arc basin along the ESZ, we argued that the ~485 Ma Miaogou pyroxenite represented the initiation back‐arc basin in the Shangdan suture, which might represent the initial rifting of the North Qinling microcontinent induced by the northward subduction of the Shangdan Ocean.

Fabrics, geothermometry, and geochronology of the Songshugou ophiolite: Insights into the tectonic evolution of the Shangdan suture, Qinling orogen, China

Abstract The Songshugou ophiolite, located in the northern Qinling belt, consists mainly of metamorphosed mafic and ultramafic rocks recording details of deformation and metamorphism that occurred during subduction, accretion, and collision along the Shangdan suture in the Qinling orogenic belt. Electron backscatter diffraction measurements revealed that the harzburgites are dominated by olivine C-type crystal preferred orientations (CPOs), which were possibly induced by high pressure during slab subduction. Olivine A-type CPOs were also observed in some harzburgites, representing the remnants of the original fabric in oceanic mantle rocks formed in the spreading center of the Shangdan ocean. Coarse-grained dunites are characterized by B-type CPOs, which may have been caused by melt-rock reactions and/or high water contents in a suprasubduction-zone setting during exhumation. Fine-grained dunites are also dominated by B-type CPOs, suggesting that grain-size reduction related to mylonitization did not result in fabric variations. Combined with the mineral assemblages, application of geothermometry suggested that the Songshugou ophiolite has experienced metamorphism and deformation under amphibolite-facies conditions. Zircons from garnet-bearing amphibole schist are characterized by flat heavy rare earth element (HREE) patterns and low Th/U ratios and yielded a mass spectrometry U-Pb age of 500.5 ± 8.8 Ma, representing the peak metamorphic age of the metamafic rocks. Other zircons displayed relative HREE enrichment and a clearly negative Eu anomaly and gave an age of 492.5 ± 3.0 Ma, constraining the time of the exhumation of the ophiolite. Integrated with all the available regional geology, our new fabric, geochemical, and geochronological data suggest that the tectonic evolution of the Songshugou ophiolite can be proximately constrained as subduction at ca. 500 Ma and exhumation at ca. 492 Ma.

Subduction and collision processes between the North China and South China blocks constrained by the geochronology and geochemistry of the Wuguan Complex in the Qinling orogen, China

ABSTRACT The Qinling orogen is a key area for understanding the processes of subduction and collision between the South China Block (SCB) and North China Block (NCB). The Wuguan Complex, distributed along the southern margin of the Shangdan suture zone, can provide important constraints on the age of collision between NCB and SCB and the tectonic evolution of the Qinling orogen in Late Paleozoic. Detrital zircons from meta-sedimentary rocks of the Wuguan Complex in the Danfeng-Shangnan area have an age spectrum with two main peaks at ~448 Ma and ~819 Ma, and two subordinate peaks at ~938 and ~1440 Ma, respectively, and are interpreted to have been derived from the North Qinling terrane (NQT). The petrographic and geochemical characteristics of the meta-sedimentary rocks indicate that they were deposited in a fore-arc basin along the southern margin of the NQT. The youngest detrital zircons yield a weighted mean age of 378 ± 3 Ma, indicating that the fore-arc deposition was continuing at least to this time, which implies that the Paleo-Qinling Ocean between the NCB and SCB was not finally closed until at least the late Devonian. In combination with regional data, we propose that sedimentary rocks of the Wuguan Complex might once have been a sequence of late Ordovician to late Devonian strata with intercalated mafic rocks, which has been dismembered by the later tectonic activity. It was metamorphosed during northward subduction of the Paleo-Qinling Ocean at ca. 320 Ma, and slowly cooled through ca. 350°C at ca. 247 Ma (muscovite 40Ar/39Ar age). It has recorded the detailed processes of subduction and collision between the NCB and SCB.

Thickening and partial melting of the Northern Qinling Orogen, China: insights from zircon U–Pb geochronology and Hf isotopic composition of migmatites

The widespread migmatites in the Precambrian basement of the North Qinling Belt are products of partial melting of the Earth's crust and record the tectono-thermal response to Late Paleozoic collision along the Shangdan suture. The morphology and trace element geochemistry of zircons from the migmatites suggest a metamorphic origin related to partial melting. The zircons from leucosome, melanosome and banded migmatites yield U–Pb weighted mean ages of 408 ± 2, 403 ± 2 and 400 ± 2 Ma, respectively, constraining the timing of migmatization. The granitic dyke intruded into the migmatites gives a zircon U–Pb weighted mean age of 396 ± 2 Ma, constraining the minimum time of migmatization. Zircons from the melanosome and banded migmatite yield upper intercept ages of 1088–1061 Ma, representing the protolith ages of the basement of the North Qinling Belt. The zircon rims of the melanosome and banded migmatite predominantly display negative and positive ε Hf ( t ) values, respectively, suggesting the variable input of mantle material. Together with the regional geology and previous geochronological data, we propose that the thickening and partial melting in the North Qinling Belt was induced by the collision along the Shangdan suture between the North and South China Blocks from c. 408 to 400 Ma. Supplementary material: LA-ICP-MS U–Pb data, Lu–Hf isotopic compositions and trace element geochemistry data of the zircons and Ti-in-zircon thermometry are available at https://doi.org/10.6084/m9.figshare.c.4521389

Origin of Chromitites in the Songshugou Peridotite Massif, Qinling Orogen (Central China): Mineralogical and Geochemical Evidence

The Songshugou peridotite massif is located in the north of Shangdan suture zone, North Qinling orogenic belt of Central China. The massif is mainly composed of fine-grained mylonitic dunites, coarse-grained dunites, fine- and coarse-grained harzburgites, and minor clinopyroxenites. The coarsegrained dunites as well as parts of the harzburgites host small-scale chromitites. Chromite grains from various textural types of chromitites and dunites pervasively contain primary and secondary silicate inclusions. Primary inclusions are dominated by monophase olivine, with minor clinopyroxene and a few multiphase mineral assemblages consisting of olivine and clinopyroxene. Secondary inclusions, mainly Cr-chlorite and tremolite, show irregular crystal shapes. Besides, Cr2O3 contents (0.08 wt.%–0.71 wt.%) of primary olivine inclusions are remarkably higher than those of interstitial olivine (<0.1 wt.%). Chromites in the Songshugou peridotite massif are high-Cr type, with Cr# and Mg# values ranging of 67.5–87.6, and 23.4–41.2, respectively. The Cr-chlorite, formed by reactions between olivine and chromite in the presence of fluid under middle temperature, indicates the Songshugou peridotite massif has undergone alteration/metamorphism process during emplacement. Chromite grains are modified by these processes, resulting in the various degrees of enrichment of Fe2O3, Cr2O3, Zn, Co and Mn, depletion of MgO, Al2O3, Ga, Ti and Ni. Due to low silicate/chromite ratios in the massive ores, chromites from them are slightly influenced by alteration/metamorphism and thus preserve the pristine magmatic compositions. The parental magma calculated based on them has 11.17 wt.%–13.57 wt.% A12O3 and 0.15 wt.%–0.27 wt.% TiO2, which is similar to the parental melts of high-Cr chromitites from elsewhere and comparable with those of boninites. Combined with informations from previous studies, major and trace elements geochemistry of chromite, as well as the nature of the parental magma, it can be revealed that the Songshugou chromitities formed in a supra-subduction zone environment.