High-field strength element geochemistry of mafic intrusive rocks from theBhagirathi and Yamuna valleys, Garhwal Himalaya, India

Incompatible high-field strength element geochemistry divides mafic intrusi ve 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 associat ed with the Central Crystallines, whereas, GMIR2 belongs to the Garhwal Gro up. They outcrop close to the Main Central Thrust (MCT) Zone and extend in a NW-SE direction, following the strike direction. Both types are metamorph osed to amphibolite facies and classified as subalkaline, high-Fe tholeiiti c basalts. They are enriched in large-ion lithophile and high-field strengt h elements relative to a primitive mantle source. GMIR2 has higher concentr ations of incompatible high-field strength elements than the GMIR1. Further , on multi-element spidergrams, GMIR1 samples do not show any elemental ano maly but GMIR2 samples show distinct negative Nb and Sr anomalies. GMIR1 mu lti-element and rare-earth element trends are similar to N-MORB, whereas, G MIR2 follows trends observed in the within plate basalts (continental flood basalts type). Discrimination diagrams, based on incompatible trace elemen ts, corroborate the N-MORB nature of GMIR1 and CFB (WPB) nature of GMIR2. G eochemical modeling indicates that these mafic intrusive rocks were derived by close-system fractional crystallization of depleted (for GMIR1) to vari ably enriched parental basalts (for GMIR2). Probably these two contrasting mafic rock types have been juxtaposed during the Himalayan orogeny. Protero zoic mafic rocks of similar geochemical characteristics have been reported from different parts of the northern Indian lithosphere. Observed compositi on suggests its genetic association with the sub-continental lithosphere ra ther than effect of crustal contamination.