A burst of middle-late Jurassic high-Mg andesitic magmatism in central Tibet: types, magmatic process, and subduction dynamics

High-Mg andesites (HMAs) formed at convergent plate margins are of special importance in deciphering subduction processes and dynamics. Although abundant, diverse Middle-Late Jurassic (ca. 170–148 Ma) HMAs have been documented in Central Tibet (SW China), and their detailed spatiotemporal evolution and exact genetic and dynamic connections with the subduction of the Bangong-Nujiang Tethyan Ocean (BNTO) are not well constrained. Here, we report newly discovered 157–155 Ma sanukitic HMA dykes in the Qieli Tso ophiolite, central Tibet. Geochemical characteristics and simulation suggest they were generated by low-degree melting (3–6%) of sediment melt-fluxed mantle wedge at 1.8–2.2 GPa, 1230–1260°C, resembling those of sanukite from SW Japan. Comprehensive geochemical analysis of the HMAs in central Tibet divides them into three types with distinctive magmatic processes: (1) boninitic HMAs, generated by melting of fluxed depleted mantle; (2) sanukitic HMAs, generated by melting of sediment melt-fluxed mantle; and (3) adakitic HMAs, generated by melting of subducted slab and lower crust. Moreover, significant spatiotemporal variation is displayed among these HMAs according to their current geography and inferred tectonic settings, i.e. ca. 170–160 Ma FAB-boninitic magmatism in the southern forearc region, ca. 169–155 Ma dominant sanukitic magmatism in the northern continental arc region, and ca. 155–148 Ma dominant adakitic magmatism in between them. These variations attest to a northward and then southward migration of the magmatism front. This migration, together with other geological evidence, is consistent with successive transitions of subduction dynamics of BNTO, likely from subduction initiation through ridge-trench collision to ridge subduction. The prolonged high heat flux during the whole transitioning process results in a burst of high-Mg magmatism.