Triassic high-Mg andesitic magmatism induced by sediment melt-peridotite interactions in the central Tibetan Plateau

Abstract

High-Mg andesitic rocks (HMAs) at convergent margins can provide crucial insights to the generation of the continental crust and the deep crust-mantle interactions. Here, we report a series of high-Mg diorites in the central Tibetan Plateau, which principally intrude into the Triassic accretionary complexes of the the Zhiduo-Yushu mélange and the central Hohxil-Bayan Har-Songpan-Garzê (HBSG) terrane. The central HBSG diorites yield zircon UPb ages of 209–210 Ma, while the Zhiduo-Yushu diorites give relatively older zircon UPb ages of 217–218 Ma. Both of them exhibit the sanukitic HMAs affinities with relatively low ratios of La/Yb and Sr/Y, but the Zhiduo-Yushu diorites have lower Ti-in-zircon temperatures than those of the central HBSG diorites. Geochemical and isotopic signatures indicate that both of them might be formed by partial melting of Triassic turbidites and the subsequent melt-peridotite interactions during a low-pressure and high-temperature condition, although the proportion of sediments involved is different. Our latest data, combined with some data from previously published studies, show that the Triassic HMAs in the central Tibetan Plateau formed in three episodes: 219–229 Ma, 217–218 Ma, and 209–215 Ma. And they show a northeastward younger trend and a significant compositional transition from adakitic HMAs to sanukitic HMAs. These features lead us to propose that melting of the hot subducted oceanic crust and the subsequent melt-peridotite interactions generated the adakitic HMAs, while upwelling of asthenosphere mantle triggered by slab rollback could sustainably provide a sufficiently high-temperature condition for the genesis of the Triassic sanukitic HMAs. And the slab rollback induced the northeastward migration of the arc magmatism in the Triassic accretionary complexes.