Discovery of eclogites in Jinsha River suture zone, Gonjo County, eastern Tibet and its restriction on Paleo-Tethyan evolution

摘要

As the important component of the eastern Tethys tectonic region,the Jinsha River-Ailao Mountain suture zone is often considered to be an ophiolitic mé1ange belt.However,the P-T-t path and chronological framework of the metamorphic evolution in the collisional orogenetic process of this zone are still poorly understood owing to the lack of metamorphism research of symbolic high-pressure rocks.During a regional geological survey on a scale of 1:50000 in Gonjo County,Tibet Autonomous Region involved in this paper,(retrograde)eclogites lenses of different scales were found in Jinsha River suture zone,eastern Tibet for the first time.The(retrograde)eclogites can be divided into garnet-albite-chlorite-actinolite schists and eclogites according to retrograde degrees.The mainly mineral components of eclogites include garnet(45%-50%),clinopyroxene(about 25%),and hornblende(5%-10%)primarily,and biotite,quartz,rutile,and muscovite secondarily.According to the data of electron probe micro analysis(EPMA),clinopyroxenes feature high content of Na2O(5.6%-6%)and corresponding jadeite(Jd)molecules of 37%-44%,and they fall within the omphacite region in Quad-Jd-Ae diagram.The temperature and pressure of the metamorphism at peak are P≈2.2-2.34 GPa and T≈622-688℃respectively as measured with geobarometry Grt-Omp-Phe and geothermometer Grt-Omp.This will provide a new reference for the understanding of Paleo-Tethyan evolution.In this paper,two samples of eclogites were chosen for LA-ICP-MS zircon U-Pb dating and their 206pb/238U ages obtained are 240±3 Ma and 244±1 Ma respectively.Furthermore,the zircons feature extremely low Th/U ratio(<0.01),extremely low content ofNb,Ta,and HREE,and invisible negative Eu anomaly.Therefore,the genesis of the zircons shall be eclogites-facies metamorphism,indicating that the 240-245 Ma determined in this paper shall be the age of eclogites-facies metamorphism and may represent the westward subduction-collision epoch of Paleo-Tethys Ocean located between Zhongza Block and Qamdo Block.