Composition,Metamorphic Evolution And Geochronology In Chinese Southwestern Tianshan Orogen

The Chinese Southwestern Tianshan Orogen is a Late Paleozoic collisional orogen formed by subduction of the Tarim plate beneath the Yili-Central Tianshan plate,which has finally resulted in closure of the South-Tianshan Ocean.The HP-LT(High Pressure,Low Temperature)metamorphic belt,the predominant part of the Southwestern Tianshan Orogen,is a typical orogen involving oceanic subduction followed by collision of continental blocks.In the north of the HP-LT metamorphic belt,the parallel LP-HT(Low Pressure,High Temperature)metamorphic belt is composed of sillimanite-garnet-biotite-plagioclase gneisses,two-pyroxene granulites,granitic gneisses and amphibolites.It is generally believed that the HP-LT metamorphic belt and the LP-HT metamorphic belt constitute a paired metamorphic belt in the Southwestern Tianshan.This study focuses on a variety of HP-LT metamorphic rocks from the Habutengsu Valley and the Kebuerte Valley,and LP-HT metamorphic rocks from the Muzhaerte Valley.By the comprehensive study of petrology,geochemistry,geochronology and phase equilibria modeling,this study reveal the composition and geochronology of the metamorphic belt,and try to reconstruct the geochronology framework,then discuss the tectonic evolution of the Southwestern Tianshan Orogen.The HP-LT metamorphic belt is.largely composed of metabasites and metasedimentary rocks.The former mainly includes eclogites and blueschists,and the latter mainly consists of garnet-phengite schists,muscovite schists and felsic blueschists.The metasedimentary rocks are dominant in the HP-LT metamorphic belt rocks,and the metabasites occur as rock slices or variable tectonic lenses entrapped in the metasedimentary rocks.The eclogites could be divided into four groups,including glaucophane eclogites,paragonite eclogites,epidote eclogites,glaucophane-garnet amphibolites(retrograde eclogites),whereas blueschists could be divided into three groups,including glaucophane-bearing garnet-muscovite-albite schists,garnet-muscovite-glaucophane schists and garnet-muscovite-glaucophane-quartz schists.Major mineral assemblages are Grt+Omp+Pg+Ep for the eclogites,Grt+Gln+Amp for the retrograde eclogites,and Grt+Gln+Phe+Pg+Ab+Qtz for the blueschists.Porphyroblastic garnets show perfect prograde zoning,with XMn and XFe deceasing and XMg and XCa increasing from cores to rims,which indicates a process of increasing temperature and prograde metamorphism.According to the paragenetic assemblages of inclusions within garnet and in matrix,four stages of metamorphism were recognized in the eclogites,including pre-peak lawsonite-blueschist facies metamorphic stage,peak eclogite facies stage(T=543?579?,P=1.5?1.6 GPa),retrograde epidote-blueschist facies stage(T=?450?,P<1.0 GPa),and post-peak retrograde blueschist-greenschist facies stage(T<400?,P<0.5 GPa).Based on the P-T pseudosection calculation,the peak metamorphic condition is 520?550? and 1.7?1.9 GPa for the eclogites,and 520?620? and 1.7?2.3 GPa for the blueschists,which is similar to those of the results by traditional geothermobarometer.In the LP-HT metamorphic belt,Sillimanite-garnet-biotite-plagioclase gneisses from the Muzhaerte area record four stages of metamorphism,including early prograde metamorphic stage(T=557?622 ?,P=2.5?4.5 kbar),peak amphibolite-granulite facies metamorphic stage(T=644?681?,P=4.0?5.2 kbar),near isothermal decompression retrograde stage(T=578?639?,P=2.6?4.5 kbar),and late amphibolite facies retrograde stage,which shows a clockwise P-T path.Geochemical analyses indicate that compositions of the metabasites from the Habutengsu area are mainly tholeiites with minor alkali basalts.Based on trace element and REE features,the eclogites can be subdivided into three types:N-MORB type,E-MORB type,and OIB type,while all the blueschists are OIB type.The presence of such MORB and OIB rock assemblages suggests the existence of an ocean basin in the Early Paleozoic from the study area.The geochemical features of the metabasites may also be influenced by the crystallization differentiation and different sources magma mixing,which produced their geochemical affinity of MORB and OIB.It is suggested that protolith of the metabasites were formed under the tectonic setting of mid-ocean ridge and adjacent ocean island,and the formation of the blueschists and eclogites might be related to HP-UHP metamorphism and later tectonic exhumation.40Ar/39Ar dating of muscovite yielded similar ages of 324?310 Ma from the eclogites,garnet-phengite schists,muscovite schists and felsic blueschists in the Habutengsu area.They represent the main collisional time between the Tarim plateand Yili-Central Tianshan plate,and indicate the Late Carboniferous closure of the South-Tianshan Ocean.Detrital zircons of the metasedimentary rocks from the HP-LT metamorphic belt show main age peak at 440?401 Ma,and the subordinate age peak at?699 Ma?802?770 Ma?961?926 Ma?1090 Ma and 2488?2441 Ma.It indicates that the magmatic rocks of the early of the Early Silurian and the end of the Early Devonian are the most important provenance region,and the magmatic rocks of Meso-Neoproterozoic and the early era of the Paleoproterozoic are also important provenance region.Metamorphic zircons of metasedimentary rocks in the Muzhaerte area from the LP-HT metamorphic belt report the 300?258 Ma metamorphic age.It suggests that there is a metamorphic event from the Early Permian to middle Permian,which is in agreement with the metamorphic age from the marble(270 Ma)and the diopside-garnet quartzite(270 Ma).The detrital zircons show the main age peak at 470?406 Ma,and the subordinate age peak at 1056?724 Ma.It suggests that the magmatism is quite intensely from the Neoproterozoic to the Early Devonian.Due to the effect of anatexis,various sizes of felsic leucosomes appear in the metasedimentary rocks from the LP-HT metamorphic belt.New growth zircons in the felsic leucosome show weakly oscillatory zonings and low Th/U ratios.The morphological and internal structural features of zircons suggest that their growths were associated with partial melting.The ages of anatexis from the felsic leucosome are from 276.5 Ma to 265.5 Ma,representing the formation age of the LP-HT metamorphic belt.The Early toMiddle Permian ages correspond to the widespread post-collisional magmatism events in the Southwestern Tianshan Orogen,and this study suggest that the Southwestern Tianshan Orogen have turned into post-collision orogenic evolution stage of lithospheric thinning and crustal extension in the Early Permian,with the associated anatexis.In additon,we also got a group of inheritable zircons ages of 421.8 ± 3.2 Ma in the anatectic felsic leucosome.This age consists with one of the major peak ages in the host rock sillimanite-garnet-biotite-plagioclase gneiss,and it shows that felsic leucosome could come from partial melting of the sillimanite-garnet-biotite-plagioclase gneiss,corresponding to the magma events of the Late Silurian in South Tianshan.In summary,Southwestern Tianshan Orogen in Xinjiang underwent complicated evolutionary history.During the long-term geological evolution,Tarim plate and the Yili-Central Tianshan plate experienced the process of subduction-collision-exhumation,with the multi-phase dynamic process of metamorphism,deformation and magmatism.At?470 Ma,South-Tianshan Ocean could be an open ocean basin.At?440 Ma,South-Tianshan Ocean started to subduct beneath the Yili-Central Tianshan plate,and developed a large number of continental arc magmatism along the southern margin of the Yili-Central Tianshan plate.At 401 to 320 Ma,South-Tianshan Ocean continued to die off.At 320 to 310 Ma,two plates collided in the end,and generated?300 Ma syn-collisional granites associated with the subduction.After 300 Ma,tectonic system in the Southwestern Tianshan changed from initial convergence to extension,and the magma heat of mantle source resulted in LP-HT metamorphism of the protolith of metapelites in the Muzhaerte area,with occurrences of vast anatectic felsic leucosome.In the end,Southwestern Tianshan Orogen evolution tends to terminate.