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

Abstract

Present study aims at understanding the genetic and tectonic relationship between the enclaves and enclosing granitoids, acidic volcanics and mafic dykes of the Ladakh plutonic complex. Similar rocks from Lhasa Block (Tibet) are also studied and compared. In terms of SiO₂ abundance, the enclaves vary in composition from basic to acidic but are predominantly andesitic-basalt. Mafic dykes intruding the Ladakh plutonic complex are of predominantly andesitic-basalt composition. Granitoids and acidic volcanics from Ladakh and Lhasa blocks are compositionally granodiorite, quartz monzonite and granite. They are predominantly meta-aluminous with slight peraluminous characters. The acidic volcanics, however, have K₂O/Na₂O > 1. All these rocks show calc-alkaline characteristics with high Al₂O₃ abundance, their rare earth elements (REE) and multi-element patterns depict enrichment of large ion lithophile elements (LILE)-light REE (LREE) and depletion of high field strength elements (HFSE) including Nb, P and Ti. It is suggested that the enclaves in Ladakh plutonic complex probably represent the initial pulses of magmatism, in response to intra-oceanic northward subduction of Neo-Tethyan ocean beneath an immature arc. Subsequently huge pulses of granitoids were intruded as the arc matured, sutured with southern continental margin of Eurasian plate and the lithosphere thickened. The granitoids in turn were cut by mafic dykes and acidic volcanics probably representing the last significant episode of subduction related magmatism in this region. It is suggested that the youngest, highly siliceous acidic volcanics may represent melts generated by partial melting and/or dehydration of upper part of subducted north Indian continental lithosphere and southern Eurasian active margin wedge, subsequent to the closing of Neo-Tethyan ocean and collision of Indian and Eurasian plates.