Metamorphic Petrology of Clinopyroxene Amphibolite from the Xigaze Ophiolite, Southern Tibet: P-T Constraints and Phase Equilibrium Modeling

Abstract

The clinopyroxene amphibolite from the Bailang terrane is located in the central section of the Yarlung Zangbo suture zone (YZSZ), southern Tibet. The study of it is expected to provide important clues for the subduction of the Neo-Tethyan Ocean below the Asian Plate and thus for better understanding of the development of the India-Asia collision zone. Based on integrated textural, mineral compositional, metamorphic reaction history and geothermobarometric studies of the clinopyroxene amphibolite within a serpentinite melange, four overprinted metamorphic stages are established. They are the first metamorphic record of M1 stage indicated by a relict assemblage of plagioclase+clinopyroxene+amphibole, an early M2 stage characterized by an assemblage of medium-grained clinopyroxene+amphibole+plagioclase+quartz as well as rutile inclusion in titanite, which is formed during burial process, an isobaric cooling M3 stage which is characterized by an assemblage of clinopyroxene+amphibole+plagioclase+titanite, and a decomposing retrograde stage M4, which is represented by the amphibolite+plagioclase symplectite+titanite+ rutile+quartz. By applying the THERMOCALC (versions 6.2 and 6.3) technique in the NCFMASHTO system, the P-T conditions estimated from M1 to M4 stages are ca. 8.6 kbar/880 °C, 10.8-13.4 kbar/800-840 °C, 12.7-13.2 kbar/650-660 °C and <11.2 kbar/640 °C, respectively. The mineral assemblages and their P-T conditions define a counterclockwise P-T path for the clinopyroxene amphibolite of the Xigaze ophiolite, suggesting that the rocks underwent a cooling process during burial from magmatic protolith, and a decompressing stage after the pressure peak metamorphic conditions, which implies that the Bailang terrane of the Xigaze ophiolite may have experienced subduction/collision-related tectonic processes. The peak metamorphism reaches to the transitional P-T conditions among amphibolite facies, granulite facies and eclogite facies with a burial depth of 30–40 km. After exhumation of the ophiolitic unit to the shallow crustal levels, the clinopyroxene amphibolite exposes to a high fO2 condition on the basis of the stable epidotebearing assemblage in the T-MO2 diagrams. A late subgreenschist facies overprinting subsequently occurs, the relevant mineral assemblage is prehnite+albite+chlorite+epidote+quartz.