Evolution of metamorphic fluid recorded in granulite facies metacarbonate rocks from the middle segment of the Mogok metamorphic belt in central Myanmar

The Mogok metamorphic belt of Paleogene age, which records subduction- and collision-related events between the Indian and Eurasian plates, lies along the western margin of the Shan plateau in central Myanmar and continues northwards to the eastern Himalayan syntaxis. Reaction textures of clinohumite- and scapolite-bearing assemblages in Mogok granulite facies metacarbonate rocks provide insights into the drastic change in fluid composition during exhumation of the collision zone. Characteristic high-grade assemblages of marble and calc-silicate rock are clinohumite + forsterite + spinel + phlogopite + pargasite/edenite + calcite + dolomite and scapolite + diopside + anorthite + quartz + calcite, respectively. Calculated petrogenetic grids in CaO-MgO-Al2O3-SiO2-H2O-CO2 and subsets of this system were employed to deduce the pressure-temperature-fluid evolution of the clinohumite- and scapolite-bearing assemblages. These assemblages suggest higher temperature (> 780–810 °C) and XCO2 [= CO2/(CO2 + H2O) > 0.17–0.60] values in the metamorphic fluid for the peak granulite facies stage, assuming a pressure of 0.8 GPa. Calcite grains commonly show exsolution textures with dolomite particles, and their reintegrated compositions yield temperatures of 720–880 °C. Retrograde reactions are mainly characterized by a reaction zone consisting of a dolomite layer and a symplectitic aggregate of tremolite and dolomite grown between clinohumite and calcite in marble, and a replacement texture of scapolite by clinozoisite in calc-silicate rock. These textures indicate that the retrograde reactions developed under lower temperature (< 620 °C) and XCO2 (< 0.08–0.16) conditions, assuming a pressure of 0.5 GPa. The metacarbonate rocks share metamorphic temperatures similar to the Mogok paragneiss at the peak granulite facies stage. The XCO2 values of the metacarbonate rock at peak metamorphic stage are, however, distinctly higher than those previously deduced from carbonate mineral-free paragneiss. Primary clinohumite, phlogopite, and pargasite/edenite in marble have F-rich compositions, and scapolite in calc-silicate rock contains Cl, suggesting a contrast in the halogen compositions of the metamorphic fluids between these two lithologies. The metamorphic fluid compositions were probably buffered within each lithology, and the effective migration of metamorphic fluid, which would have extensively changed the fluid compositions, did not occur during the prograde granulite facies stage throughout the Mogok metamorphic belt. The lower XCO2 conditions of the Mogok metacarbonate rocks during the retrograde stage distinctly contrast with higher XCO2 conditions recorded in metacarbonate rocks from other metamorphic belts of granulite facies. The characteristic low XCO2 conditions were probably due to far-ranging infiltration of H2O-dominant fluid throughout the middle segment of the Mogok metamorphic belt under low-amphibolite facies conditions during the exhumation and hydration stage.
ファイル公開：2019-09-01