Metamorphic evolution of Sumdo eclogite in Lhasa Block of the Tibetan Plateau: Phase equilibrium in NCKMnFMASHTO System

Abstract: The eclogite in the Lhasa block mainly consists of garnet, omphacite, phengite, epidote and rutile. The garnet in the eclogite has no visible zonation and a homogeneous core. The Xpy and Xgr decrease from core to rim. Most of the omphacites show weak zonation. The Jd value increases from core to rim. The growth of some narrow barroisite rods along omphacite cleavage may reflect retrograde processes. The phengite included in garnet has higher Si value than that in the matrix. The authors applied for the first time the phase equilibrium simulation method to the study of this common eclogite in the Lhasa block. P-T pseudosection of sample 13SD43 was employed in the NCKMnFMASHTO system. The maximum Xpy in the core of garnet and the maximum Si value in phengite were used to determine the peak metamorphic condition of 610℃，32Kbar, with a peak mineral assemblage of g+o+law+phn+ru. With the help of phase equilibrium and petrography research, several metamorphic evolution processes of the eclogite could be recognized. Studies show that the eclogites in the Lhasa block probably experienced UHP metamorphism and rapid exhumation to the middle crust, which might have provided new evidence for the existence of the suture zone in the Lhasa block.