Abstract

• The Mogok-Kyatpyin area forming a mountainous region with two distinct valleys of Mogok valley and Kyatpyin valley is well known for its gem minerals. A variety of marbles, calc-silicate rocks, gneisses, migmatites and locally granulites are well exposed with the emplacement of various types of granitoid rocks. The high-grade assemblages of marble samples are characterized by calcite + dolomite + diopside + spinel + graphite with forsterite and clinohumite in some samples. Based on the mineral assemblages and textural relationship, temperature and fluid (T-XCO2) diagrams indicate temperature and XCO2 conditions of > 800 °C and > 0.5 at an isobaric pressure of 0.8 GPa for the clinohumite-bearing and forsterite-bearing marble samples for the peak stage. Re-equilibrium texture of clinohumite + tremolite + dolomite indicates a decrease in temperature and XCO2 condition, and yields <540 °C and <0.02 at an isobaric pressure of 0.4 GPa. The present study combining with other high-grade areas along the Mogok metamorphic belt suggest widespread distribution of upper amphibolite to granulite facies metamorphism in the middle segment including Mogok-Kyatpyin area. During peak metamorphic stages, the fluid conditions had higher XCO2 value. In contrast, decreasing XCO2 values with decreasing temperature and pressure might indicate that infiltration of H2O-rich fluid occurred during retrograde metamorphism.