METAMORPHIC EVOLUTION OF ECLOGITE FROM SUMDO [1]LAHSA BLOCK OF TIBETAN PLATEAU:PHASE EQUILIBRIA IN NCKMNFMASHTO SYSTEM

Lhasa block lies between Bangongnvjiang suture and Yalvzangbu suture. Sumdo eclogite is in the south of Lhasa block, exposing in the geological section of Quartzite, ultrabasic rock, serpentinite and schist. The eclogites in the Lahsa block has a similar mineral assemblage involving garnet, omphacite, phengite andepidote with rutile. This paper mainly reports the research on petrography and the phase equilibrium calculation via Thermocalc, making a discussing on metamorphic evolution of Sumdo eclogites and providing new evidence to prove whether a suture exists between north Lhasa and south Lhasa. The garnet in eclogite has no obvious zonation and a homogeneous core. The Xpy and Xgr decrease from core to rim. Most of the omphacite shows weak zonation. The Jd value increase from core to rim. The growth of some narrow barroisite rods along omphacite, cleavage may reflect retrograde process. The phengite included in garnet has higher Si value than that in the matrix. The metamorphic evolution of Sumdo eclogite can be divided into three stages：(Ⅰ) pre-peak prograde stage; (Ⅱ) peak metamorphism stage; (Ⅲ) post-peak decompression stage. Peak mineral composition is garnet, omphacite, lawsonite and phengite. Sumdo eclogite had experienced ultra-high pressure metamorphism. The peak metamorphic condition is 620℃,32kbar. Sumdo eclogite is low temperature and ultra-high pressure eclogite, which has similar P-T path with glaucophane eclogite. This may be new evidence for the suture existing in Lhasa block.