Metamorphic Evolution of Rare Garnet-Al-Silicates (Kyanite-Sillimanite-Andalusite)-Staurolite ± graphite bearing Anatectic Metapelites from Eastern Himalayan Syntaxis, NE India

The present study integrates petrochemical analyses and phase equilibria modeling of rare Garnet-Al-Silicates (Kyanite-Sillimanite-Andalusite) - Staurolite ± graphite bearing anatectic metapelites to comprehend its tectono-metamorphic evolution from the Eastern Himalayan Syntaxis (EHS), NE India. Extensive field studies reveal the occurrence of high-grade metapelitic rocks as enclaves in the Trans-Himalayan Lohit group of rocks (migmatitic gneisses), Dibang Valley, Arunachal Pradesh, India. Quartz-garnet-kyanite-plagioclase represents the peak metamorphic assemblage, in which garnets (Fe-rich) occur as porphyroblast and kyanite exhibit long crystals embedded in polygonal quartz and feldspar. The presence of peritectic kyanite and overgrowth of muscovite corona over kyanite (back reaction of melt with Al-silicates) indicates melting in the metapelitic rocks. Additionally, the presence of hydrous phases (e.g., muscovite and biotite), in the matrix points towards a wet melting condition. Furthermore, the post-peak metamorphic history of the rocks can be characterized by the presence of sillimanite and andalusite formed during exhumation. Thermobarometric studies estimate the peak metamorphic conditions for the metapelite at T =650±25°C, P~8 Kbar. The P-T pseudosection analysis predicts the peak metamorphic condition for the metapelites at T~ 650°C, P=7-8 Kbar; with clockwise PT paths. This suggests a wet melting condition at T~ 680°C (Solidus) and P~8 Kbar for the high-grade metapelites possibly related to the India-Asia collision from the Eastern Himalayan Syntaxis of Arunachal Himalaya.