Neoproterozoic-Early Paleozoic Metamorphism In The Qinling Orogenic Belt And Its Geologic Implication

The formation and evolution of accretionary orogen is a prominent feature of convergent plate margins,which is also one of the key subjects of geodynamics.More and more studies have revealed that it may be a prolonged and complex process in the formation and evolution of an accretionary orogen,which may include multistage compression and extension.Regional metamorphism corresponds to characteristic tectonothermal environments in different orogenic stages,which results in different types of metamorphic rocks.In this regard,metamorphic pressure（P）-temperature（T）paths serve as reliable indicators of burial,heating,exhumation,and cooling for metamorphic rocks,and thus provide important constraints on the tectonic evolution of oceanic subduction,slab rollback,continental collision and/or final extensional collapse.The Qinling orogen records multiple orogeny since Neoproterozoic to Palaeozoic,and preserves different types of high-grade metamorphic rocks,which is an ideal place to study the orogeny and plate dynamics of convergent plate margins.In this study,we carried out a comprehensive study of petrology,mineralogy,zircon U-Pb-Hf-O isotope and trace element compositions,as well as conventional geothermobarometry and phase equilibrium modeling for Neoproterozoic and Palaeozoic high-grade metamorphic rocks from the Qinling orogenic belt.The result not only quantify the P-T conditions,the timing and tectonic setting for the amphibolite-facies metamorphism in the South Qinling orogen,but also shed light on the timing of peak,retrograde stage and exhumation duration of the HP-UHP terrane in the North Qinling orogen.The main achievements are listed as follows:1.We conducted an integrated study of petrography,zircon U-Pb age and Hf-O isotopic compositions for two garnet amphibolites and one gneiss from the Douling complex.Phase equilibrium modeling and conventional geothermobarometry calculation define peak P-T conditions of 9-10 kbar and 670-690℃,and shape a clockwise P-T path followed by retrograde isothermal decompression.Zircon grains from the gneiss mostly reveal core-rim structures.The cores display oscillatory zoning,high Th/U ratios and trace element contents,HREE enriched patterns,high ¹⁷⁶Lu/¹⁷⁷Hf ratios andδ¹⁸O values,consistent with a magmatic origin.They yielded a weighted mean²⁰⁶Pb/²³⁸U age of 868±9 Ma,which is taken as the protolith age.The cores have strongly positiveε_Hf（t）values of 7.0-13.5 and young two-stage Hf model ages of ca.1310-890 Ma,indicating that the protolith was dominantly sourced from prompt reworking of Meso-Neoproterozoic juvenile crust.On the other hand,the zircon overgrowth rims in the gneiss exhibit no zoning patterns,low Th/U ratios and trace element contents,flat HREE patterns and pronounced negative Eu anomalies,variable¹⁷⁶Lu/¹⁷⁷Hf ratios and comparableε_Hf（t）values to the magmatic cores,indicating their coeval formation with garnet and plagioclase under amphibolite-facies metamorphism and might be generated by dissolution-reprecipitation.Zircon crystals from the two amphibolites performed weak zoning patterns,low Th/U ratios and trace element contents,high formation temperatures,flat to weakly enriched HREE patterns,insignificantly and/or clear negative Eu anomalies,low ¹⁷⁶Lu/¹⁷⁷Hf ratios andε_Hf（t）values,suggesting that they may form near the peak stage of the amphibolite-facies metamorphism.Metamorphic zircons in the three samples yielded homogeneously highδ¹⁸O values（10.81±0.17‰,11.45±0.40‰,10.92±0.08‰）,indicating the participation of external fluid originated from surrounding metasedimentary rocks.These metamorphic zircon grains together yielded a weighted mean age of 817±4 Ma（MSWD=0.63）,which is taken as the best estimated age for the upper amphibolite-facies metamorphism in the Douling complex.Considering the widespread contemporaneous magmatism in the northern Yangtze Craton,we propose that the Douling complex recorded a tectonic regime switch from compression to extension at ca.830-820 Ma,in response to outboard oceanic subduction on the periphery of the Yangtze Craton and then tectonic extension and/or collapse of the thickened lithosphere.2.We performed an integrated petrologic,zircon U-Pb isotopic and trace element study of six plagioclase-bearing felsic dikes from the North Qinling HP-UHP metamorphic rocks.The felsic dikes show no to strong deformation and are composed mainly of quartz,K-feldspar,plagioclase,and amphibole,with minor biotite and muscovite.Zircon crystals from these dikes have euhedral shape and exhibit weak zoning,planar zoning,or oscillatory zoning.They possess very low Th/U ratios,and steep to relatively flat heavy rare earth element（HREE）enriched patterns with variable Eu anomalies.The zircon formation temperatures calculated by the Ti-in-zircon thermometer vary between ca.600 and 660℃.These features suggest that the zircon crystals precipitated from the felsic dikes under relatively low pressure and high temperature conditions.These felsic dikes yielded identical zircon U-Pb ages of 488±6to 494±10 Ma with a weighted mean of 490±2 Ma（MSWD=0.39）.This age is indistinguishable from the peak HP-UHP metamorphic age of 492±4 Ma within analytical error in the North Qinling（NQ）unit.The resultant exhumation rate is averaged at about 3.2 cm/yr and the minimum is about 0.8 cm/yr from the peak UHP metamorphic to the middle crust depth.Our results demonstrate that the North Qinling terrane is a typical small,young,and fast HP-UHP terrane.Zircon petrochronology of felsic dikes has the potential to provide substantial constraints on the exhumation processes of HP-UHP terranes involving in complex retrogression.Different types of regional metamorphism will be formed at different evolution stages of accretionary orogen,accompanied with magmatism of different properties.An in-depth study of the properties and temporal-spatial dirtribution patterns of different types of metamorphic rocks exposed in the orogenic belt can clarify the history of the formation and evolution of different regions,and further reveal the complex evolution process of the orogenic belt.The Neoproterozoic-early Paleozoic high-grade metamorphic rocks in the Qinling orogenic belt recorded the multi-stage accretionary orogeny associated with the assemble and break-up of the Rodinia supercontinent and the subduction of the Proto-Tethys Ocean.