Early Cretaceous (100–105 Ma) Adakitic magmatism in the Dachagou area, northern Lhasa terrane, Tibet: implications for the Bangong–Nujiang Ocean subduction and slab break-off

Abstract

We performed U–Pb zircon dating and whole-rock major and trace element geochemical analyses of monzonites and granodiorites from the Dachagou area, northern Lhasa terrane, Tibet. Zircon U–Pb dating of two plutons yielded magmatic crystallization ages of 100 and 105 Ma, indicating that the intrusive rocks were emplaced in the Early Cretaceous. The monzonites and granodiorites are characterized by high Al2O3 and Sr contents, low Yb and Y contents, and high Sr/Y ratios, and secondarily by low K2O contents, high Mg # values, and relatively high Cr and Ni abundances. Major and trace element geochemical analyses indicate that the rocks have adakitic signatures, suggesting generation by partial melting of an oceanic slab. However, there are some differences between the monzonites and granodiorites, which imply that the monzonitic magmas were contaminated by enriched mantle and crustal components and that the granodioritic magmas were contaminated by enriched mantle melts without a significant crustal contribution. Based on these data and the regional tectonic setting, we propose that the oceanic slab break-off model best explains adakitic magma generation in the Dachagou area. Identification of adakites in the area provides important evidence of slab break-off during the southward subduction of the Bangong–Nujiang Ocean plate.