Multiple garnet growth in garnet–kyanite–staurolite gneiss, Pangong metamorphic complex, Ladakh Himalaya: New constraints on tectonic setting

Abstract

Garnet–kyanite–staurolite gneiss in the Pangong complex, Ladakh Himalaya, contains porphyroblastic euhedral garnets, blades of kyanite and resorbed staurolite surrounded by a fine-grained muscovite-biotite matrix associated with a leucogranite layer. Sillimanite is absent. The gneiss contains two generations of garnet in cores and rims that represent two stages of metamorphism. Garnet cores are extremely rich in Mn (XSps=0.35–0.38) and poor in Fe (XAlm=0.40–0.45), whereas rims are relatively Mn-poor (XSps=0.07–0.08), and rich in Fe (XAlm=0.75–0.77). We suggest that garnet cores formed during prograde metamorphism in a subduction zone followed by abrupt exhumation, during early collision of the Ladakh arc and Karakoram block. The subsequent India–Asia continental collision subducted the metamorphic rocks to a mid-crustal level, where the garnet rims overgrew the Mn-rich cores at ca. 680°C and ca. 8.5kbar. PT calculations were estimated from phase diagrams calculated using a calculated bulk chemical composition in the Mn–NCKFMASHT system for the garnet–kyanite–staurolite-bearing assemblage. Muscovites from the metamorphic rocks and associated leucogranites have consistent K–Ar ages (ca. 10Ma), closely related to activation of the Karakoram fault in the Pangong metamorphic complex. These ages indicate the contemporaneity of the exhumation of the metamorphic rocks and the cooling of the leucogranites.