Early Cretaceous (~ 140 Ma) aluminous A-type granites in the Tethyan Himalaya, Tibet: Products of crust-mantle interaction during lithospheric extension

Abstract

A-type granites have been the focus of considerable research due to their distinctive major- and trace-element signatures and tectonic significance. However, their petrogenesis, magmatic source and tectonic setting remain controversial, particularly for aluminous A-type granites. The earliest Cretaceous (ca. 140Ma) Comei granite in the eastern Tethyan Himalaya is associated with coeval oceanic island basalt (OIB)-type mafic lava, and has A-type granite geochemical characteristics including high 10,000×Ga/Al (up to 6), FeOtotal/MgO (4.6–6.1) and (Na2O+K2O)/Al2O3 (0.50–0.61) ratios but low CaO (0.6–1.6wt%) and Na2O (1.8–2.6wt%) contents. The Comei granite also has variable peraluminous compositions (A/CNK=1.00–1.36) along with zircon δ18O, εNd(t) and initial 87Sr/86Sr values of 8.2‰ to 9.3‰, −13.0 to −12.4 and 0.7238 to 0.7295, respectively. This range of compositions can be interpreted as the interaction between high-temperature upwelling OIB type basaltic magmas and a shallow crustal (<5kbar) metapelitic source. The Comei granite and coeval OIB type basaltic rock could represent the earliest stage (145–140Ma) of a large igneous event in eastern Tethyan Himalaya, which may well have been triggered by pre-breakup lithospheric extension prior to the arrival of the Kerguelen plume head.