Geochemical evolution of peraluminous paleoproterozoic bandal orthogneiss NW, Himalaya, Himachal Pradesh, India: implications for the ancient crustal growth in the Himalaya

Abstract

Paleoproterozoic (1900±100 Ma) granites along a 2000 km linear belt in the Lesser Himalaya are significant in terms of their tectonic setting and emplacement as they form the basement for younger sedimentary cover rocks. The geochemistry and petrogenesis of one granitic body (Bandal Orthogneiss, 1904±70 Ma) from the Lesser Himalayan belt has been studied and correlated with the other plutons in the belt. The Paleoproterozoic Bandal orthogneiss, covering ∼500 sq. km surface area, occupies the core of the Kulu-Rampur window, Himachal Pradesh, NW Lesser Himalaya. The orthogneiss is S-type, peraluminous and can be subdivided into two suites: a muscovite-biotite granite [Suite I, coarse grained porphyritic gneiss (CPG) and fine grained gneiss (FG)] and a tourmaline-muscovite granite [Suite II, leucogranite, (LG)]. The two mica granites (Suite I) form the bulk of the Bandal orthogneiss whereas the tourmaline-bearing granite, which cuts across suite I, occurs in the central part of the main pluton. Comparison of their bulk composition with melt experimental data suggest formation of LG from a water-undersaturated magma of minimum-melt composition at pressures around 3–4 kbar whereas the tourmaline-free granites may have been derived by melting due to biotite dehydration at pressures >5 kbar. High concentrations of MgO+FeO, Ba/Sr, Rb/Sr and Ti of the two-mica granites reflect higher extents of melting derived from biotite dehydration melting of a metasedimentary source. Tourmaline leucogranites with higher Rb/Sr and low Ba/Sr ratios represent low-fraction melts extracted from their protoliths under conditions of low water activity. The striking similarities in terms of the geological setting, geochemical characteristics and the age data between Bandal orthogneisses and the adjoining Wangtu Gneisses suggest that they were formed under similar tectono-magmatic environment during Paleoproterozic period. The wide spread occurrence of Paleoproterozoic gneisses in the Lesser Himalayan belt strongly suggest that they represent the granitic basement on which the younger sedimentary rocks were deposited. The geochemical studies on the overlying sedimentary rocks (Chail and Jutogh formations) in the Lesser Himalaya suggest that the huge volumes of granitic magmatism played a significant role in changing the crustal composition of the Himalayan rocks from mafic to felsic during the post-Archean.