Petrology and U–Pb zircon dating of coesite-bearing metapelite from the Kebuerte Valley, western Tianshan, China

Abstract

This paper deals with the petrology and U–Pb dating of coesite-bearing garnet–phengite schist from the Kebuerte Valley, Chinese western Tianshan. It mainly consists of porphyroblastic garnet, phengite, quartz and chlorite with minor amounts of paragonite, albite, zoisite and chloritoid. The well preserved coesite inclusions (∼100μm) in garnet are encircled by a narrow rim of quartz. They were identified by optical microscopy and confirmed by Raman spectroscopy. Using the computer program THERMOCALC, the peak metamorphic conditions of 29kbar and 565°C were obtained via garnet isopleth geothermobarometry. The predicted UHP peak mineral assemblage comprises garnet+jadeite+lawsonite+carpholite+coesite+phengite. The metapelite records prograde quartz–eclogite-facies metamorphism, UHP coesite–eclogite-facies peak metamorphism, and a late greenschist-facies overprint. Phase equilibrium modeling predicts that garnet mainly grew in the mineral assemblages garnet+jadeite+lawsonite+chloritoid+glaucophane+quartz+phengite and garnet+jadeite+lawsonite+carpholite+glaucophane+quartz+phengite. SHRIMP U–Pb zircon dating of the coesite-bearing metapelite yielded the peak metamorphic age 320.4±3.7Ma. For the first time, age data of coesite-bearing UHP metapelite from the Chinese western Tianshan are presented in this paper. They are in accord with published ages obtained from eclogite from other localities in the Chinese western Tianshan and the Kyrgyz South Tianshan and therefore prove a widespread occurrence of UHP metamorphism.