The mineralogy and ages of the Yarlung Zangbo ophiolitic mélange zone in the Namjagbarwa area,Xizang

The Yarlung Zangbo ophiolitic mélange zone is aligned as an arc along the Yarlung Zangbo Grand Canyon in the Namjagbarwa area,Xizang. The lithologies there consist mainly of quartizite,mica quartz schist,green schist,amphibolite,metagabbro and blastodiabase. The major rock-forming minerals are composed of:(1) amphibole including Mg-rich magnesio-amphibole,magnesio-cummingtonite,pargasite and edenite;(2) clinopyroxene including Mg-rich species corresponding to diopside in composition;(3) olivine including representative forsterite,and (4) plagioclase dominated by andesine of metamorphic origin in amphibolites. The temperatures and pressures for the metamorphism processes estimated from the geothermometer and geobarometer are:t=500 -650℃ and p=0.75-0.8 Gp,corresponding to those for the amphibolite facies dated back to ca. 47-43 Ma. The later low-temperature and high-pressure metamorphism resulted in the formation of the high pressure-type lepidomorphite dated back to ca. 27-20 Ma,relative to the intense strike-slipping of the Aniqiao fault and Mainling-Lunang fault at the boundary of the Yarlung Zangbo ophiolitic mélange zone. The rapid uplifting and erosion of the crust in the study area permitted the formation of lepidomorphite of medium-to low-pressure types.