The petrogenesis and geological significance of the Late Triassic andesite and dioritic xenolith in the Riwanchaka area, central Qiangtang, northern Tibet

The Qiangtang terrane can be divided into northern and southern terranes by the Longmu-Shuanghu-Lancangjiang Suture Zone (LSLSZ). Late Triassic magmatic rocks are widely exposed on both sides of LSLSZ, coeval with exhumation of the metamorphic rocks. Recently, Late Triassic andesite and dioritic xenolith were identified in the Riwanchaka area of central Qiangtang. New U-Pb zircon ages and geochemical and Hf isotopic data for andesite and dioritic xenolith are reported in this paper. Crystallized and inherited zircons from the andesites yielded ages of 223.9±1.3Ma and 364.7±1.9Ma, with initial ε_Hf(t) values being -5.94 to -4.14 and +7.22 to +8.69, respectively. Together with recently published data, the authors hold that the dioritic xenoliths were derived from the mantle wedge in an oceanic subduction setting of Late Devonian while the andesites were derived from mixing between Southern Qiangtang crust-derived melt and Late Devonian dioritic juvenile crust under a post-collisional extension setting by slab breakoff. The presence of dioritic xenoliths in the andesites further supports the viewpoint of the existence of a subduction zone as an important site for the growth of continental crust.