Phase Equilibrium Modeling of Retrograded Eclogite at the Kekesu Valley, Eastern Segment of SW Tianshan Orogen and Tectonic Implications

Abstract

In this study, we report newly found strongly retrograded eclogite blocks in the Kekesu Valley, eastern segment of southwestern Tianshan, northwestern China. Based on detailed petrographical studies and mineral chemistry analyses, we constrain P-T evolution of two representative samples via phase equilibrium modeling method using THERMOCALC software. The peak stage is evidenced by the primary inclusions of omphacite+glaucophane+lawsonite (inferred)+chlorite+white mica in garnet. Using grossular and pyrope isopleth thermobarometry in garnet, the P peak conditions for this stage were constrained to be ~23 kbar, 480–500 °C and the T peak conditions were constrained to be 19–20 kbar, 500–520 °C. Subsequent exhumation led to the transformation of lawsonite to epidote. Using Si-inphengite barometry, the P was constrained to be 16–20 kbar at T of 500–520 °C for this stage. Further retrograde stage was evidenced by the mineral assemblages of glaucophane+garnet+epidote+paragonite+ chlorite+white mica+quartz. Later stage retrogression is evidenced by glaucophane+epidote+chlorite+ quartz+white mica+albite+actinolite with P of 5–9 kbar when T<490 °C, indicating the eclogite may have been exhumed to the middle crust level and overprinted by blueschist-greenschist facies metamorphism. The results show that eclogite at the eastern segment of SW Tianshan shares similar P-T path to the HP rocks from the western segment, implying a similar P-T evolution for these rocks.

Keywords

eclogite / P-T path / phase equilibria / southwestern Tianshan / Kekesu Valley

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