Early Paleozoic Metamorphism And Anatexis In The North Wulan Gneiss Complex

The formation and evolution of orogenic belt are the manifestation of plate tectonics,and commonly accompany with multiple metamorphism,deformation and magmatism.Regional metamorphism reflects the specific tectonothermal setting in different orogenic stages,which can form various types of metamorphic rocks.LP-HT metamorphic rocks in upper plate and HP-LT metamorphic rocks usually form during early-stage oceanic subduction-accretion.Coesite-bearing or diamond-bearing UHP metamorphic rocks often form in late-stage continental subduction/collision.These high-grade metamorphism are accompanied by anatexis,which cause sufficiently decrease in crustal strength,strongly change the thermal and rheological behavior of orogenic crust,and influence deformation characteristics of rocks and geodynamic mechanism associated with orogenesis.Therefore,it is crucial to investigate metamorphism and anatexis to decipher the evolution of orogenic belt and reconstruct the regional tectonic framework.The early Paleozoic South Qilian-North Qaidam orogenic belt in northwestern China records a nearly complete history of early-stage long-lived oceanic subduction-accretion followed by late-stage continental collision.Most previous studies have focused on the low dT/dP metamorphism(HP-UHP)and anatexis in the lower plate whereas the paired high dT/dP belt in the upper plate has received little attention.Here,we investigate the timing and nature of metamorphism and anatexis in the North Wulan gneiss complex,which lies in the upper plate of the South Qilian-North Qaidam subduction zone.Integrating regional geology with the results obtained in this study,we discuss the tectonic setting of the high dT/dP metamorphism and anatexis,and the relationship between the high dT/dP belt and the HP-UHP metamorphic complex belt,which provides an important basis for understanding the evolution of the South Qilian-North Qaidam orogenic belt.On the basis of the study of field geological investigation and petrography,phase equilibria modelling is combined with zircon petrochronology to determine the P-T-t evolution of granulites in the North Wulan gneiss complex in the high dT/dP hinterland of the South Qilian-North Qaidam orogen.Granulites record a clockwise P-T path with near-peak temperatures of 800-900? at 5.5?7 kbar.Peak metamorphism was followed by high-temperature decompression.Zircon petrochronology reveals protracted zircon growth.from 474 to 446 Ma during the high-T portion of the P-T path.Broadly contemporaneous but slightly younger HP-UHP metamorphism in the foreland of the South Qilian-North Qaidam orogenic belt indicates that the region records an early Paleozoic paired metamorphic belt This early Paleozoic paired metamorphic belt provides a detailed example of dual thermal regimes in a modern-style orogenic system that can be applied to understanding the timescales and P-T conditions of high dT/dP metamorphism that accompany subduction in Phanerozoic and Precambrian orogenic belts.An integrated study of petrography,whole-rock geochemistry,geochronology,zircon trace element and Hf isotope analysis deciphers the nature and timing of partial melting in migmatitic amphibole-biotite gneiss.Zircon U-Pb geochronology reveals that the protoliths crystallized at 506-494 Ma followed by metamorphism and anatexis at ca.465 to 450 Ma.Anatexis occurred by water-fluxed melting of mafic-intermediate rocks through the breakdown of biotite and growth of peritectic amphibole.The protolith of the migmatites records Cambrian arc magmatism in an active continental margin,which was induced by northward subduction of the South Qilian ocean slab..Contemporary arc-like magmatism and high-temperature/low-pressure metamorphism in the region suggest that anatexis in the North Wulan gneiss complex likely took place in a continental arc setting,which reflects the reworking of former arc magmatic rocks in a late stage of oceanic subduction.Based on the new data presented in this study,we propose the following tectonic evolution for the early Paleozoic South Qilian-North Qaidam orogenic belt.At ca.540-440 Ma,the South Qilian ocean was opening.Subsequently,the intra-oceanic arc,seamount/oceanic plateaus and the microcontinent were accreted onto the south margin of the Qilian Block during long-lived subduction of the South Qilian oceanic plate.The northward subduction of the South Qilian ocean constructed the continental arc magmatism on the upper plate of this subduction zone,and coeval eclogite facies metamorphism occurred in the subducted oceanic slab.Slab retreat and rollback of the subducted oceanic slab induced southward migration of arc magmatism,and led to the extension and formation of an arc-back-arc system in the overriding plate.The high thermal gradient in the arc-back-arc system drove HT-LP metamorphism and partial melting in the.middle to upper crust.Simultaneously,the reworking of former arc magmatic rocks in a late stage of oceanic subduction resulted in the formation of migmatitic amphibole-biotite gneiss.At ca.440-420 Ma,continental collision between the Qilian and Qaidam Blocks closed the South Qilian ocean.Continental crust of the Qaidam Block was subducted to mantle depths and experienced UHP metamorphism.Subsequently,the exhumed high dT/dP belt and low dT/dP belt were juxtaposed during oblique collision of the two blocks and formed a paired metamorphic belt.