AGE OF METAMORPHISM IN THE LESSER HIMALAYA AND THE MAIN CENTRAL THRUST ZONE, GARHWAL INDIA - RESULTS OF ILLITE CRYSTALLINITY, AR-40-AR-39 FUSION AND K-AR STUDIES

Illite crystallinity data from the Lesser Himalaya of Garhwal show tha t the upper Paleocene-lower Eocene Subathu Formation, deposited immedi ately prior to or early in the Himalayan collision, has not suffered s ignificant regional metamorphism. The regional metamorphism in the upp er Precambrian-lower Palaeozoic Lesser Himalaya must therefore be prec ollisional. Illite crystallinity results from Lesser Himalayan fossili ferous Permian strata show grades of metamorphism intermediate between upper Paleocene-lower Eocene and Proterozoic-lower Palaeozoic strata indicating a pre-Permian regional metamorphism for the latter. K-Ar wh ole rock cooling ages provide supporting evidence for pre-collisional regional metamorphism in the Lesser Himalaya. Slates and phyllites bel ow the Main Central Thrust (MCT) show pre-Cenozoic whole rock ages, as old as Ordovician (486 Ma). Whilst resetting of K-Ar whole rock ages has occurred locally in pervasively cleaved Palaeozoic strata (near th rusts?), fracture cleaved Permian and upper Paleocene-lower Eocene sed iments give whole rock ages compatible with diagenesis. The illite cry stallinity results confirm that these sediments have not been heated a bove mica blocking temperatures. Muscovite Ar-40-Ar-39 and K-Ar minera l ages within the 5 km thick MCT zone are as young as 8 Ma indicating that temperatures of above similar to 350 degrees C were maintained in the MCT zone for over 10 Ma after high temperature (similar to 550 de grees C) shearing on the MCT. This heating did not affect the MCT foot wall Lesser Himalaya to any regional extent, where pre-Permian low gra de regional metamorphism has not been overprinted.