New Petrographic Data on the Metapelites of Almora Crystallines in Kumaun Himalaya

Detailed structural and metamorphic study has revealed four tectonic episodes in the Almora Crystallines. The first two have brought major metamorphic changes. The earliest event (D₁) belonging to the Late Precambrian/Early Palaeozoic orogeny produced the early cleavage (S₁), parallel to the axial planes of isoclinal folds (F₁) which now trend NNE-SSW. Almandine garnet and sporadically occurring staurolite generated during this pre-Himalayan orogenic event reveal a growth under static condition. D₁ culminated with the intrusion of granitic lenses which granitised the adjacent metapelites. The second event (D₂) belonging to the Early Tertiary Himalayan orogeny refolded this sequence into another set of isoclinal folds (F₂) that are co-axial with F₁ and produced widespread crenulation cleavage and snowball garnet. The continued action of D₂ detached the crystallines from the ischolar_main zone and pushed them over the less metamorphosed unit of the Lesser Himalaya as thrust sheet. Chlorite trails of the retrograded garnet in the intensely mylonitized zone indicate southeasterly movement of the thrust sheet. The early stage of thrusting of the nappe synchronized with the progressive metamorphism in the interior region. Third (D₃) and fourth (D₄) events merely folded and crinkled these rocks.