Early Paleozoic Metamorphic Evolution Of The East Kunlun Orogen

The East Kunlun Orogen（EKO）was recently considered to be an Early Paleozoic HP-UHP metamorphic belt.Many Early Paleozoic intermediate-to high-grade metabasic rocks exposed in the belt record different metamorphic conditions and ages.These meta-basic rocks are important samples for studying Early Paleozoic metamorphic evolution of the EKO.This thesis presents petrological and zircon geochronological studies of garnet-amphibolites,eclogites and biotite plagiogneiss in Langmuri,eastern section of the EKO.The garnet-amphibolites mainly consist of garnet,plagioclase,amphibole,diopside and quartz,with a small amount of biotite,chlorite,rutile,ilmenite and titanite.The porphyroblastic garnet grains include epidote,amphibole,plagioclase,rutile and quartz in their cores.Compositional profile of garnet shows a decrease in spessartine and grossular and an increase in Mg/(Mg+Fe²⁺)from the core to its rim.These compositional features suggest that the garnet crystallized in a process of prograde metamorphism.Textural relationship and mineral compositions reveal that the rock experienced three stages of metamorphism,with the metamorphic conditions of T≈610℃and P≈0.65 GPa（prograde stage）,T≈700℃and P≈1.05 GPa（peak stage）,and T≈650℃and P≈0.45GPa（retrograde stage）.The eclogites consist of garnet,omphacite,quartz and phengite,with variable amounts of symplectites（clinopyroxene+plagioclase+amphibole）formed by the decomposition of omphacite.The peak metamorphic conditions is T≈728～785℃,P≈3.36～3.49 GPa and the retrograde P-T condition is T≈600℃,P≈0.6 GPa.These metamorphic conditions form a clockwise P-T path,with a nearly isothermal decomposition process from the peak stage to the retrograde stage.This characteristics of P-T path indicates that both garnet-amphibolites and eclogites were formed in the subduction-collision tectonic process.Geochronological study reveals that the zircon U-Pb ages of garnet-amphibolites are 427.1±5.5 Ma and 492.8±5.1 Ma.The morphological characteristics of the zircons are consistent with those of typical metamorphic zircon.The mineral inclusions（garnet,amphibole and plagioclase）of the zircons are consistent with the peak mineral assemblage of the rock,which suggest that the zircons should crystallize at the peak stage of amphibolite facies.Therefore,the age of～427 Ma and～493 Ma are the peak metamorphic ages of the garnet-amphibolites.Eclogites yield a zircon U-Pb age of 438.5±2.9 Ma,which represents the age of eclogite-facies metamorphism as evidenced by the zircon REE distribution pattern.The U-Pb age of magmatic zircons in biotite plagiogneiss is～906 Ma,indicating a collisional event in the Early Neoproterozoic.Multiple ages of garnet-amphibolites and eclogites reveal that the EKO experienced Muti-stage metamorphism in the Early Paleozoic,and different rocks recorded different stages of the subduction-collision process of Proto-Tethys Ocean.The P-T path and metamorphic age presented in this paper can provide constraints for the further study of the Early Paleozoic metamorphic evolution of the EKO.