Phase equilibrium modeling for metamorphic evolution of garnet-bearing mica-quartz schist in Sumdo UHP metamorphic belt, Lhasa Block

Abstract: The garnet-bearing mica-quartz schist of Sumdo UHP belt in Lhasa block occurs as country rocks of eclogite, and is mainly composed of garnet, muscovite, albite, chlorite, quartz and minor rutile and sphene. Garnet displays an obvious compositional zonation where Xprp increases from the core to the mantle and then decreases in the rim, whereas Xsps decreases gradually from the core to mantle, with the trend of declining following rising in the rim, indicating that garnet composition profiles from core to mantle have preserved the prograde growth zoning and were partially reset during retrogression. The model system MnNCKFMASHO was chosen to calculate P-T and P-M(H2O) pseudosections of the garnet-bearing mica-quartz schist. Garnet isopleth thermobarometry involved plotting compositional isopleths of garnet as contours on a P-T pseudosection, with the combination of contours of saturated H2O content, thus obtaining estimated peak P-T conditions of 27 kbar, 523/580 ℃ and peak mineral assemblages of g-jd-cr-law (+phn +q/coe+H2O). The compositional profile of garnet from the core to the mantle and contouring of the H2O content saturated indicate that prograde metamorphic evolution represents a cold subduction stage with heating with the increasing pressure, and the rocks experienced blueschist-facies to eclogite-facies metamorphism during this stage. P-M(H2O) pseudosections and isopleth of saturated H2O content could be used to assess evolution of mineral assemblages in terms of changes in water content during decompression, which shows that garnet-bearing mica quartz schist experienced an early isothermal decompression process and was then followed by a cooling with decompression evolution during the late stage. Amphibolite-facies to epidote-amphibolite-facies metamorphism occurred during early stage and was followed by greenschist-facies metamorphism. The isothermal decompression of garnet-bearing mica-quartz schist probably represents a fast tectonic exhumation. Albite was likely to replace early jadeite at this stage. A comparison with the P-T path and contact relationship in the field of garnet-bearing mica-quartz schist and eclogite shows evidently that garnet-bearing mica-quartz schist and the eclogite it hosted experienced similar subduction and exhumation processes.