The Metamorphism Of Two Metamorphic Belts In Southern Altyn And Their Tectonic Relationship

The P-T-t paths of high-pressure/ultra high-pressure(HP-UHP)metamorphic rocks can reflect the complex process in the subduction and exhumation of oceanic/continental crust,which is essential to understanding formation and evolution history of orogenic belts.The protolith property and formation age of metamorphic rocks also provide crucial constraints on the tectonic setting prior to orogeny.The Altyn orogenic belt locates the northeastern margin of the Tibet Pleateau,and is the convergence zone of the main tectonic units in western China,among the Tarim block,the Kunlun belt,the Qaidam block,and the Dunhuang orogen.In recent years,with the unceasingly thorough studied in South Altyn(SA)metamorphism and related researches,multiple HP-UHP terranes have been identified in four regions of Jianggalesayi,Yinggelisayi,Danshuiquan and Munabulake,which constituted the SA HP-UHP metamorphic belt.This dissertation finds a fresh terrane with variable types of HP-UHP rocks named Younusisayi in SA,and firstly reports the Silurian middle-pressure/high-pressure(MP-HP)metamorphic belt in the South Margin of Middle Altyn(SMMB).Comprehensive studies of metamorphism,geochemistry and geochronology have constrained the metamorphic evolution history and protolith attributes of both the HP rocks in new finding Younusisayi area in SA and MP-HP rocks in SMMB.Combined with the previous research,the formation mechanism and tectonic genetic relationship of these two metamorphic rock belts are discussed.The main innovative cognitions and achievements of this dissertation are as follows:1.A new HP-UHP terrane named Younusisayi(originally classified as "Altyn Group")was discovered in the SA,where develops metamorphic rocks of phengite eclogite,garnet pyroxenite,garnet/kyanite-granitic gneiss,phengite granitic gneiss,etc.The determination of metamorphic evolution process,peak and retrometamorphic ages of these rocks,and construction of complete P-T-t paths,further extended the spatial distribution of HP-UHP metamorphic rocks in SA.The discovery of omphacite inclusions in garnet in the garnet pyroxenite,and the > 24 kba peak pressure recovered from garnet/kyanite granitic gneiss,indicate they both experienced eclogite facies metamorphism.And garnet zonings in the phengite eclogite and granitic gneiss may record the peak metamorphic conditions(maximum peak pressure)near or above the stishovite domain,which provides a new target area for the study of ultra-deep subduction of the SA continental crust.2.MP-HP rocks including phengite pelite gneiss,HP mafic granulite,garnet/sillimanite pelite gneiss,and LP-MP rocks including biotite-plagioclase gneiss,biotite amphibole gneiss,amphibolite have been discovered in the Munabulake,Jianggalesayi,Younusisayi,Yaolesayi area in SMMB(originally designated as "Bashikurgan Group of the Changcheng Period").These rocks generally recorded three stages of metamorphic age from Silurian to Devonian(～430 Ma,～400 Ma,～375 Ma).The confirmed metamorphic evolution and P-T-t paths of these MP-HP rocks revealed they experienced HP granulite-amphibole eclogite facies metamorphism with low geothermal gradient(peak d T/d P < 9.2～11.8 ℃/km,Alpine type).These different types of metamorphic rocks constituted another MP-HP metamorphic belt along the SMMB,which is different from the HP-UHP metamorphic belt in SA.3.The phengite pelite gneiss,HP mafic granulite and marble in the silurian MP-HP metamorphic belt in the SMMB show a similar geological occurrence of interbedding distribution,indicating that the protolith assemblage should be volcanic-sedimentary formation.Geochemical studies show that mafic metamorphic rocks may mainly formed by magmatism with the attribute of island arc,and the metasedimentary rock are derived from the felsic source of continental island arc.The protolith magmatic crystallization ages of the meta-mafic rocks are 455.9±2.6 Ma and 462.4±1.4 Ma.The minimum age of detrital zircons in the biotite-plagioclase gneiss is 471 Ma.These ages suggest that the formation age of the "Bashikurgan Group of the Changcheng Period" in the Munabulake,Jianggalesayi,Younusisayi,Yaolesayi area in SMMA should be471-456 Ma,rather than mesoproterozoic.Combined with the regional geological background,it is suggested that the SMMB may be a back-arc basin with the attributes of rifting trough or rift valley in the extensional background during middle to late Ordovician.4.Combining metamorphic evolution,metamorphic age,protolith property and formation age of SA HP-UHP metamorphic belt and SMMB MP-HP metamorphic belt,and regional early Paleozoic ophiolite,magmatic activity and partial melting studies,this dissertation proposed that the SMMB Silurian MP-HP metamorphic belt may be producted by the closure of the back-arc basin which opened(in the SMMB,middle to late Ordovician)during the exhumation of deep-ultra deep subducted SA continental plate,and reconstructed the tectonic evolution framework of southern Altyn from Neoproterozoic to Paleozoic.5.In combination with regional geological correlation,and considering the consistent age of peak and retrograde metamorphism of the SMMB MP-HP metamorphic belt and North Qaidam HP-UHP metamorphic belt(continental crust),we suggest that the SMMB MP-HP metamorphic belt may be the western extension of the North Qaidam HP-UHP metamorphic belt.This understanding is more reasonable than considering the SA HP-UHP metamorphic belt as the western extension of North Qaidam HP-UHP metamorphic belt.