Age and geochemistry of the Paleoproterozoic Bhatwari Gneiss of Garhwal Lesser Himalaya, NW India: implications for the pre-Himalayan magmatic history of the Lesser Himalayan basement rocks

Published on 2018-10-19T10:07:37Z (GMT) by
The Bhatwari Gneiss of Bhagirathi Valley in the Garhwal Himalaya is a Paleoproterozoic crystalline rock from the Inner Lesser Himalayan Sequence. On the basis of field and petrographic analyses, we have classified the Bhatwari Gneiss into two parts: the Lower Bhatwari Gneiss (LBG) and the Upper Bhatwari Gneiss (UBG). The geochemical signatures of these rocks suggest a monzonitic protolith for the LBG and a granitic protolith for the UBG. The UBG has a calc-alkaline S-type granitoid protolith, whereas the LBG has an alkaline I-type granitoid protolith; the UBG is more fractionated. The trace element concentrations suggest a volcanic arc setting for the LBG and a within-plate setting for the UBG. The U–Pb geochronology of one sample from the LBG gives an upper intercept age of 1988±12 Ma (<i>n</i>=10, MSWD=2.5). One sample from the UBG gives an upper intercept age of 1895±22 Ma (<i>n</i>=15, MSWD=0.82), whereas another sample does not give any upper intercept age, but indicates magmatism from <i>c</i>. 1940 to 1840 Ma. Based on these ages, we infer that the Bhatwari Gneiss has evolved due to arc magmatism and related back-arc rifting over a time period of <i>c</i>. 100 Ma during the Proterozoic. This arc magmatism is related to the formation of the Columbia supercontinent.

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Sen, Aranya; Sen, Koushik; Srivastava, Hari B.; Singhal, Saurabh; Phukon, Purbajyoti (2018): Age and geochemistry of the Paleoproterozoic Bhatwari Gneiss of Garhwal Lesser Himalaya, NW India: implications for the pre-Himalayan magmatic history of the Lesser Himalayan basement rocks. Geological Society of London. Collection.