Geochemistry and geodynamic implications of magmatic rocks from the Trans-Himalayan arc

Present study aims at understanding the genetic and tectonic relationship b etween the enclaves and enclosing granitoids, acidic volcanics and mafic dy kes of the Ladakh plutonic complex. Similar rocks from Lhasa Block (Tibet) are also studied and compared. In terms of SiO2 abundance, the enclaves var y in composition from basic to acidic but are predominantly andesitic-basal t. Mafic dykes intruding the Ladakh plutonic complex are of predominantly a ndesitic-basalt composition. Granitoids and acidic volcanics from Ladakh an d Lhasa blocks are compositionally granodiorite, quarts monzonite and grani te. They are predominantly meta-aluminous with slight peraluminous characte rs. The acidic volcanics, however, have K2O/Na2O > 1. All these rocks show calc-alkaline characteristics with high Al2O3 abundance, their rare earth e lements (REE) and multi-element patterns depict enrichment of large ion lit hophile elements (LILE)-light REE (LREE) and depletion of high field streng th elements (HFSE) including Nb, P and Ti. It is suggested that the enclaves in Ladakh plutonic complex probably repre sent the initial pulses of magmatism, in response to intra-oceanic northwar d subduction of Neo-Tethyan ocean beneath an immature are. Subsequently hug e pulses of granitoids were intruded as the are matured, sutured with south ern continental margin of Eurasian plate and the lithosphere thickened. The granitoids in turn were cut by mafic dykes and acidic volcanics probably r epresenting the last significant episode of subduction related magmatism in this region. It is suggested that the youngest, highly siliceous acidic vo lcanics may represent melts generated by partial melting and/or dehydration of upper part of subducted north Indian continental lithosphere and southe rn Eurasian active margin wedge, subsequent to the closing of Neo-Tethyan o cean and collision of Indian and Eurasian plates.