A POSSIBLE NEW HP/UHP (?) METAMORPHIC BELT IN CHINA: DISCOVERY OF ECLOGITE IN THE LASHA TERRANE, TIBET

Abstract

Eclogite was first discovered in the Lasha terrane, northeast to the Lasha City. The eclogite occurs as a 500-600 m thick belt, extending in EW for over 10 km. Most eclogites are massive, coarse-grained and fresh. Three types of eclogite have been recognized, which are rutile eclogite, quartz eclogite and phengite eclogite. The clinopyroxenes in eclogite are mainly omphacite with 16%-44% of jadeite (Jd) molecules. The end member molecules for garnets in eclogite are pyrope 16%-33%, almondite 40%-54%, and grossular 22%-31%. p-t calculation based on Grt-Omp-Phe and Grt-Omp mineral assemblages yielded 2.58 GPa and 635 °C, and 2.67 GPa and 730 °C, respectively for sample 06Y-334, and samples 06Y-345 and 06Y-336T yielded t mainly in the range of 680-780 °C and 640-740 °C. Since the phengites in these samples occur as retrogressive minerals, the peak metamorphic p for these rocks may be higher than the calculated values and enter the coesite stable field (p>2.8 GPa). Polycrystal quartz as pseudomorphs of coesite in garnet or omphacite may provide a possibility for looking for UHP metamorphic evidence. The Lasha eclogite belt most likely represents a new HP/UHP(?) metamorphic belt in China.