Original paper

Folded metamorphic reaction isograds in the Almora Nappe, Kumaun Lesser Himalaya: Field evidence and tectonic implications

Joshi, Mallickarjun; Tiwari, Amar N.

Abstract

The Barrovian sequence of metamorphic isograds has been inverted throughout the entire length of the Himalayan orogenic belt so that the higher grade rocks appear at progressively higher structural levels. However, the Precambrian psammopelitic rocks of the Almora Nappe that have been tectonically transported from the Higher Himalaya and are considered equivalent to the Himalayan Metamorphic Belt reveal four metamorphic zones progressing from the chlorite-biotite to the sillimanite-K-feldspar zone. These zones show a typical repetitive disposition. Four sets of folds, F1, F2, F3 and F4, were recognized that have affected the metamorphic rocks of Almora Nappe. The disposition of metamorphic zones in relation to the structural features indicates that the zones are involved in tight to isoclinal F2-folding. It is documented that the commonly accepted model of inverted metamorphism for the Himalayan metamorphics is not applicable to the Almora Nappe. The folded metamorphic sequence is intruded by Early Paleozoic granitoids resulting in contact metamorphism, and the well-preserved hornfelsic textures overprint the regional foliation. This suggests that the regional metamorphism is highly likely to be of Precambrian age.