Rb-Sr Ages of Granitic Rocks Within the Lesser Himalayan Nappes, Kumaun, India

Abstract Whole-rock Rb-Sr isochron analyses of granitic components of the two prominent tectono-stratigraphic units, the Ramgarh and Almora Groups, in the Kumaun Lesser Himalaya yield two relatively coherent age clusters around 1800 Ma and 550 Ma, respectively. The older age corresponds to the porphyritic granites, quartz-porphyry and augen gneisses from the Ramgarh Nappe, the basal parts of the synformal Almora Nappe and the Askot-Dharamghar Klippen in the inner Lesser Himalaya, whereas the younger age is restricted to the massive Champawat Granodiorite - Almora Granite emplaced in the upper part of the Almora Nappe. The older rock seems to occur also in the root zone at the base of the Great Himalaya. The early Palaeozoic granites appear to be very widespread, extending both into the Himachal Pradesh and Nepal sectors. The spatial distribution of the two age components has been used to infer the possible relationships between the two lithotectonic units.