Low temperature eclogite facies metamorphism in Western Tianshan, Xinjiang

摘要

According to the field occurrences and petrological study, the low temperature eclogite facies metamorphic rocks in Western Tianshan of Xinjiang can be divided into five types: (i) massive glaucophane-epidote eclogites and glaucophane-paragonite eclogites; (ii) schistose or gneissic mica eclogites; (iii) banded calcite eclogites; (iv) pillow glaucophane eclogites; (v) garnet-omphacite quartzites. Their eclogite facies metamorphism has undergone four stages of evolution: (i) pre-peak lawsonite-blueschist facies stage, T = 350—4000℃, P = 0.7—0.9 GPa; (ii) peak eclogite facies stage, T = 530 ± 20℃, P = 1.6—1.9 GPa; (iii) retrograde epidote-blueschist facies stage, T =500—530℃, P = 0.9—1.2 GPa and (iv) retrograde blueschist-greenschist facies stage, T = 450—550℃, P = 0.7—0.8 GPa. The metamorphic PT path of Western Tianshan eclogites is characterized by clockwise ITD resulting from the subduction of Tarim plate northward to Yili-Central Tianshan plate followed by fast uplift to the surface. But there were at least two stages of blueschist facies retrograde metamorphism overprinted during their uplift.