Metagabbros of the Gangdese arc root, south Tibet: Implications for the growth of continental crust

Abstract

The Gangdese batholith of the southern Tibet is regarded as a principal component of the pre-Himalayan collision, Andean-type continental margin. However, the Gangdese arc root remains poorly understood because most of previous studies concentrated on granitoids from the middle and upper crust of the arc. In this paper, we report new petrochemical and geochronological characteristics of metagabbros occurred as a part of the Late Cretaceous Lilong batholith in the eastern Gangdese arc. The gabbros with protolith ages of ∼82–95Ma, underwent slightly late (68–77Ma) granulite-facies metamorphism and nearly coeval anatexis, typical of lower arc crusts. The metagabbros and diorites of the Lilong batholith show geochemical features of juvenile crust in continental magmatic arcs, and an original magmatic differentiation trend, with gabbros formed as the cumulates, whereas diorites as the crystalline products of evolved magmas. The Lilong batholith forms an arc crustal section with a possible thickness of 20–30km, and may have an overall andesitic composition, providing a support for the “andesite model” of continental crust growth.