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 (n = 10, MSWD = 2.5). One sample from the UBG gives an upper intercept age of 1895 ± 22 Ma (n = 15, MSWD = 0.82), whereas another sample does not give any upper intercept age, but indicates magmatism from c. 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 c. 100 Ma during the Proterozoic. This arc magmatism is related to the formation of the Columbia supercontinent.Supplementary material: LA–ICP-MS calibration and adjusted major element data used for computing the isometric log-ratio transformations are available at https://doi.org/10.6084/m9.figshare.c.4272158
The Lichi volcanic rocks crop out along the Main Boundary Thrust (MBT) of the Arunachal Himalaya, northeast India are comprised primarily of sub‐alkaline basalt (SiO2 ranges from 48.66 to 50.62 wt%), with minor trachy‐andesite (SiO2 ranges from 57% to 61.42 wt%). The distribution of incompatible trace elements, low concentrations of large‐ion lithophile elements, and REE behaviour suggests an enriched nature of the studied rocks and limited influence of the crustal contamination during their evolution. Linear distribution trends of major oxide, consistent patterns of trace elements, and REEs on primitive mantle‐ and chondrite‐normalized diagrams suggest the cogenetic nature of mafic basaltic rocks and intermediate trachy‐andesite. The variation in Ti/Y (256.00–469.95) and Zr/Y (7.65–36.96) ratios indicates that garnet was present in the source. The well‐defined geochemical trends on variation diagram and the sub‐parallel trends on the chondrite‐normalized REE patterns suggest that fractional crystallization processes have played a major role in the evolution of parental magma. Petrogenetic modelling implies that the sub‐alkaline basalts and intermediate rocks were derived from low‐degree partial melting (1%–5%) of a garnet bearing peridotite source. The melt further evolved by fractional crystallization. We conclude that the geochemical attributes of the Lichi volcanics of the Arunachal Himalaya imply their emplacement in a continental extensional regime and possibly resulted from the Cretaceous Kerguelen plume activities.
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