Pressure-temperature Evolution of the Metapelites in the Motuo Area,the Eastern Himalayan Syntaxis

The Motuo area is located in the east of the Eastern Himalayan Syntaxis. There outcrops a sequence of high‐grade metamorphic rocks, such as metapelites. Petrology and mineralogy data suggest that these rocks have experienced three stages of metamorphism. The prograde metamorphic mineral assemblages (M₁) are mineral inclusions (biotite + plagioclase + quartz ± sillimanite ± Fe‐Ti oxides) preserved in garnet porphyroblasts, and the peak metamorphic assemblages (M₂) are represented by garnet with the lowest X_Sps values and the lowest X_Fe# ratios and the matrix minerals (plagioclase + quartz ± K‐feldspar + biotite + muscovite + kyanite ± sillimanite), whereas the retrograde assemblages (M₃) are composed of biotite + plagioclase + quartz symplectites rimming the garnet porphyroblasts. Thermobarometric computation shows that the metamorphic conditions are 562–714°C at 7.3–7.4 kbar for the M₁ stage, 661–800°C at 9.4–11.6 kbar for the M₂ stage, and 579–713°C at 5.5–6.6 kbar for the M₃ stage. These rocks are deciphered to have undergone metamorphism characterized by clockwise P‐T paths involving nearly isothermal decompression (ITD) segments, which is inferred to be related to the collision of the India and Eurasia plates.