Comparison in geochemical characteristics and genesis models of different boninites between Qilian Orogen and Izu-Bonin arc system

Boninites are characterized by high-Si (＞52 wt.%), high-Mg (＞8 wt.%), and low-Ti (＜0.5 wt.%). Boninite is thought to be originated from the partial melting of the refractory mantle induced by fluids released from the subducting seafloor during the subduction initiation. Therefore, study on petrogenesis of boninite is of great significance to further understand the geodynamic mechanism of subduction initiation. Although contribution of subducted materials for the boninitic magma source is significant as people commonly believed, the varying enriched extents of incompatible elements among different boninites reflect the complex physicochemical properties of subducting slab-derived fluids for the formation of boninites. In this study, we compared boninite from the Izu-Bonin-Mariana (IBM) arc system and the North Qilian orogenic belt, and found many geochemical differences between them. Compared to Izu-Bonin boninites, boninite from the North Qilian orogeny does not show U-shaped rare earth element (REE) pattern or enriched light REEs and Zr-Hf, while both of them have highly varied ratios of fluid-soluble element to incompatible element (e.g. Ba/La) and high (⁸⁷Sr/⁸⁶Sr)_i values. These characteristics reflect the contribution of slab-derived fluids / melts to the magma source for Qilian/Izu-Bonin boninite, respectively. Different from the Izu-Bonin boninite, the Qilian boninite is likely to be produced in a mature subduction system with back-arc spreading centers. Combined with previous studies, we proposed two potential ways for the formation of the Qilian boninite unrelated to the seafloor subduction initiation: (1) the back-arc lithosphere extension and the hot mantle upwelling provided a suitable temperature and pressure for the formation of boninite magmas, with the contribution of the hydrated/serpentinized mantle; (2) the corner flow carried the residual peridotite of the back-arc mantle into the sub-arc/fore-arc mantle, which can be melted and possibly induce dehydration of the subducting slab again to produce boninite magmas.