拉萨地块松多榴辉岩的变质演化过程：NCKMnFMASHTO体系中的相平衡关系

摘要

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.%拉萨地块松多榴辉岩主要矿物组合为石榴子石、绿辉石、角闪石、多硅白云母、绿帘石、金红石。石榴子石环带不明显，核部成分均一，从核部到边部，镁铝榴石和钙铝榴石含量降低，可能分别记录了榴辉岩峰期及退变质过程信息。绿辉石显示微弱的成分环带，硬玉含量从核部到边部略有升高，部分绿辉石边部发育韭闪石退变质边，反映了在减压过程中外来流体进入体系的过程。多硅白云母具有高的Si含量（3.5~3.6），其中石榴石包体中的多硅白云母相对基质中的白云母有更高的Si值。本文利用Thermocalc变质相平衡模拟软件，结合详细岩相学观察，在NCKMnFMASHTO体系下，模拟松多含多硅白云母榴辉岩的变质演化过程。其中，榴辉岩峰期矿物组合为g+o+law+phn+ru，石榴石核部最大镁铝榴石值和石榴子石包体中多硅白云母最大Si值确定的榴辉岩峰期温压条件约为620℃，32×105 Pa，榴辉岩经历了近等温降压的退变质过程。相平衡模拟结果表明拉萨地块松多榴辉岩经历了超高压变质作用过程，并经历了相对快速的折返过程到中部地壳层次。