Metamorphic conditions and CHIME monazite ages of Late Eocene to Late Oligocene high-temperature Mogok metamorphic rocks in central Myanmar

Abstract

The high temperature (T)/pressure (P) regional Mogok metamorphic belt is situated in central Myanmar, and is mainly composed of pelitic gneisses, amphibolites, marbles, and calc-silicate rocks. The garnet–biotite–plagioclase–sillimanite–quartz assemblage and its partial system suggest equilibrium P/T conditions of 0.6–1.0GPa/780–850°C for the peak metamorphic stage, and 0.3–0.5GPa/600–680°C for the exhumation and hydration stage. Monazite grains show complex compositional zoning consisting of three segments—I, II, and III. Taking into consideration the monazite zoning and relative misfit curves, the calculated chemical Th–U–total Pb isochron method (CHIME) monazite age data (284 spot analyses) indicated four age components: 49.3±2.6–49.9±7.9, 37.8±1.0–38.1±1.7, 28.0±0.8–28.8±1.6, and 23.7±1.3Ma (2σ level). The ages of the Late Eocene and Late Oligocene epochs were interpreted as the peak metamorphic stage of upper-amphibolite and/or granulite facies and the postdated hydration stage, respectively.