Petrogenesis of the granitoid rocks from Askot crystallines,Kumaun Himalaya |
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| Authors: | D Rameshwar Rao Rajesh Sharma |
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| Institution: | 1.Wadia Institute of Himalayan Geology,Dehradun,India |
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| Abstract: | The Askot crystallines form a doubly plunging synformal belt and occurs as a detached crystalline belt or klippen in the vast
sedimentary terrain lying between Central crystallines towards north and the Almora crystallines to the south. It is dominated
by granite gneiss and augen gneiss, and also comprise of metapelites, migmatites and basic intrusives. In this paper, the
geochemical studies of the granite gneiss and augen gneiss from the Askot crystallines, Kumaun Himalaya were carried out in
order to understand their origin and evolution.
The granite gneiss is generally foliated, with less foliated and porphyritic variety seen in the core part. The K-feldspar
shows Carlsbad twinning, while plagioclases show complex twinning. They show euhedral zircon and apatite along with titanite
as accessory minerals. The granite gneiss is moderately evolved (Mg# ∼50) and has granodiorite composition with metaluminous,
calc-alkaline trends. They show higher concentration of Ti, Ca, Mg and low abundance of ∑REE (∼165 ppm) in comparison to augen
gneiss. They show volcanic arc signatures and compare well with Lateorogenic granites of Proterozoic times distributed world
wide. These calc-alkaline granites appear derived from a Paleoproterozoic mafic/intermediate lower-crust reservoir probably
involving arc magma underplating. Granite gneiss is also peraluminous with molar A/CNK>1.1, and the heterogeneity of granite
gneiss can be explained with the precursor melts, experiencing assimilation during up-rise through crust or contamination
of source itself involving sediments from the subduction zone. |
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