P-T CONDITION OF METAMORPHISM IN THE GNEISSES OF ASKOT KLIPPE, KUMAUN, HIMALAYA, INDIA

Abstract

Metamorphics comprising the Askot Klippe occur as an apparently folded doubly plunging synform with its axis trending roughly NW-SE and plunging north-westerly. In the central parts of the klippe, cordierite bearing gneisses are exposed in an asymmetric overturned anticline with its northern (steep)and the southern (gentle) limbs both dipping due north. Though the northern flank of the klippe dips due south and the southern flank dips due north. The reaction isograd map for the area shows garnet-biotite zone, kyanite-biotite zone and sillimanite-K-feldspar zone in the central parts of the klippe. There is a strong F₂ deformation that has folded the isograds suggesting that the F₂ deformation is post regional metamorphism. These isograds have also been affected by the later F₃ folding.Basal Ramgarh Group and the overlying Almora Group are the two tectono-metamorphic units of Askot Klippe. Pelitic metamorphics belonging to the Almora Group occupy the central parts of the area while the periphery comprises highly mylonitized granite gneisses of the Ramgarh Group. Interestingly the Askot Klippe is also separated from the underlying sedimentary rocks by a basal shear zone similar to the Almora Nappe. The basal shear zone of Askot Klippe developed as a consequence of southward thrusting of the rocks. The pelitic assemblages of Almora Group are characterized by the presence of muscovite, chlorite, biotite, garnet, cordierite, K-feldspar and sillimanite. Basic assemblages are characterized by the presence of plagioclase and hornblende. The temperature and pressure exceeded 780^oC and 6.2 kbar respectively and the rocks reached amphibolite-granulite transition.