The origin of arc basalts: New advances and remaining questions

Abstract

Whether arc magmatism occurs above oceanic subduction zones is the forefront of studies on convergent plate margins. The most important petrologic issue related to the evolution of arc systems is the origin of arc magmatism, among which arc basalts are the most important one because they provide insights into mantle enrichment mechanism and crust-mantle interaction at oceanic subduction zones. Fluids or melts released either by dehydration or by melting of subducting oceanic slab infiltrate and metasomatize the overlying mantle wedge at varying depth, leading to the formation of source regions of arc basalts. Such processes make most of arc basalts commonly enriched in large ion lithosphile elements and light rare earth elements, but depleted in high-field strength elements and heavy rare earth elements. Small amounts of arc basalts are characterized by relatively high Nb contents or by Nb enrichment. Rare basalts with compositions similar to ocean island basalts or mid-ocean ridge basalt also occur in arc systems. For these peculiar rocks, it remains debated whether their source is affected by subduction-related components. During their ascent and before their eruption, arc basaltic magmas are subjected to crystal fractionation, mixing and crustal contamination. In addition to the contribution of subducting slab components to the mantle source of arc basalts, the materials above the subducting slab at forearc depths would have been transported either by drag or by subduction erosion into the subarc mantle and into the source of arc magmas. Heats and materials brought by corner flows also play important roles in the generation of arc basalts. Despite the important progresses made in recent studies of arc basalts, further efforts are needed to investigate subarc mantle metasomatism, material recycling, the formation of arc magma sources, geodynamic mechanism in generating arc basalts, and their implicationd s for the initiation of plate tectonics on Earth.