Polyphase tectonothermal history recorded in granulitized gneisses from the north Qaidam HP/UHP metamorphic terrane,western China:Evidence from zircon U-Pb geochronology

摘要

High-grade gneisses of the north Qaidam high-pressure/ultrahigh-pressure metamorphic terrane enclose minor eclogites and ultramafic rocks.In combination with pet-rological data,sensitive high-resolution ion microprobe U-Pb geochronology on the granulitized gneisses in the Luliangshan and Xitieshan,western north Qaidam Mountains,reveals a polyphase tectonothermal history including Early Neoproterozoic and Early Paleozoic events.The rocks investigated are two granulite-facies paragneisses and one orthogneiss that both surround garnet peridotite in the Luliangshan,and two paragneisses that enclose eclogite in the Xitieshan.The inherited zircon cores from paragneiss and orthogneiss yield ages between ca.1000 Ma and ca.2500 Ma,representing Mesoproterozoic to Archaean source material for these gneisses.A ca.900 Ma age obtained from one orthogneiss and one paragneiss is interpreted as the age of simultaneous Early Neoproterozoic magmatism and metamorphism.This Early Neoproterozoic tectonothermal event is similar in age to the Jinning orogeny,which is commonly recognized in the metamorphic basement of the south China block,and suggests that the north Qaidam Mountains have an affinity to the south China block.Early Paleozoic metamorphism is recorded in all gneiss samples.In conjunction with cathodoluminescence imagery and mineral inclusions,U-Pb dating of zircons reveals two Early Paleozoic age groups:ca.450 Ma represents the age of high-pressure granulite metamorphism (Grt + Rt inclusions in zircon;kyanite porphyro-blasts),whereas ca.425 Ma reflects the time of medium-pressure,granulite-facies metamorphism (PI + Sil inclusions in zircon) and associated anatexis during decompression.The ages obtained suggest that the granulite-facies metamorphism lasted for ~25 m.y.and was related to the Early Paleozoic continental collision between the Qilian and Qaidam blocks,and to subsequent thermal relaxation and exhumation.