High-Field strength element geochemistry of mafic intrusive rocks from the Bhagirathi and Yamuna Valleys, Garhwal Himalaya, India

Abstract

Incompatible high-field strength element geochemistry divides mafic intrusive rocks of Garhwal Himalaya, exposed in the Bhagirathi and Yamuna valleys, into two distinct types, viz. older Garhwal Mafic Intrusive Rocks (GMIR1) and younger Garhwal Mafic Intrusive Rocks (GMIR2). GMIR1 is mainly associated with the Central Crystallines, whereas, GMIR2 belongs to the Garhwal Group. They outcrop close to the Main Central Thrust (MCT) Zone and extend in a NW-SE direction, following the strike direction. Both types are metamorphosed to amphibolite facies and classified as sub-alkaline, high-Fe tholeiitic basalts. They are enriched in large-ion lithophile and high-field strength elements relative to a primitive mantle source. GMIR2 has higher concentrations of incompatible high-field strength elements than the GMIR1. Further, on multi-element spidergrams, GMIR1 samples do not show any elemental anomaly but GMIR2 samples show distinct negative Nb and Sr anomalies. GMIR1 multi-element and rare-earth element trends are similar to N-MORB, whereas, GMIR2 follows trends observed in the within plate basalts (continental flood basalts type). Discrimination diagrams, based on incompatible trace elements, corroborate the N-MORB nature of GMIR1 and CFB (WPB) nature of GMIR2. Geochemical modeling indicates that these mafic intrusive rocks were derived by close-system fractional crystallization of depleted (for GMIR1) to variably enriched parental basalts (for GMIR2). Probably these two contrasting mafic rock types have been juxtaposed during the Himalayan orogeny. Proterozoic mafic rocks of similar geochemical characteristics have been reported from different parts of the northern Indian lithosphere. Observed composition suggests its genetic association with the sub-continental lithosphere rather than effect of crustal contamination.