PALAEOPROTEROZOIC TECTONIC SET UP OF NORTH INDIAN CONTINENTAL MARGIN: CONSTRAINTS FROM THE GEOCHEMICAL ANALYSIS OF WANGTU GNEISSIC COMPLEX, LESSER HIMALAYA

The Wangtu Gneissic Complex (WGC) is the oldest gneissic complex exposed as a tectonic window in the Lesser Himalaya. WGC is considered to be the basement rocks which is still more or less in its indigenous realm. It comprises of foliated and nonfoliated granitoid, granite-gneiss, mica schist and metabasalts. Rocks from WGC are mylonitized and migmatized at several locations, and several mafic dykes have intruded the WGC. Geochemical data show that the metabasalts are continental tholeiitic in nature as they have higher SiO₂, K₂O and LREE'scontents. The geochemical data reveal the subduction-related magmatism but is unable to discern either it is because of anataxis of existing crust during the syn-to post collisional crust thickening or crustal melts in the post collisional regime. Since the Al₂O₃ > (Na₂O + K₂O + CaO) or A/CNK > 1.1; Most of the rock samples of WGC are peraluminous. Geochemical studies indicate that the granitoids of WGC are “S-type” and they are enriched in SiO₂ and K₂O, and hence these granitoids are thought to originate by the melting of pre-existing of metasedimentary rocks. Tectonic discrimination diagram and trace element concentration portray a collision/volcanic arc tectonic set-up. Such findings indicate the possibility of the northern boundary of the Precambrian North Indian Shield extends within the Himalayan belt, and pre-Himalayan orogeny has led to the destruction of the thick lithospheres of the Aravalli and Bundelkhand cratons as well as the northern margins of the Vindhyan metasediments. The subduction and melting of the Archaean crust and supracrustals may act as a source for the magma parental to the Palaeoproterozoic granites in the Himalaya.

The detrital ages of the zircons within the metasedimentary sequences from the lesser Himalayan rocks are ~3.3-3.1 Ga, 2.7-2.4 Ga, ~1.8-1.4 Ga, ~1 Ga [1]. All these detrital zircon ages are good indicators that the Precambrian lithological components from North Indian shield are possible provenance for meta-sedimentary units in the study areas. Proterozoic sediments are not much explored in the lesser Himalaya. Better constraints are required to find out the Proterozoic tectonic set up of North Indian continental margin, which is still a matter of conjecture.

Reference [1] Myrow, P.M. et al., (2010) Bulletin of the Geological Society of America.