Across- and along-arc geochemical variations of lava chemistry in the Sangihe arc: Various fluid and melt slab fluxes in response to slab temperature

The Sangihe arc, northern Indonesia, is an oceanic arc that is the western half of the only active arc–arc collision on Earth. To elucidate magma genesis and slab thermal structure in such a setting, we have obtained new geochemical data for lavas from the entire Sangihe arc. In the southern arc, the volcanic front lavas are enriched in fluid‐mobile elements, while the rear arc lavas are more enriched in melt‐mobile elements. The proportion of sediment versus altered oceanic crust in the slab component is only ∼20% but still larger in Sangihe than other arcs in the western Pacific such as Izu, suggesting more subduction of the thick sediments in the narrowing Molucca Sea. The slab component changes in character across the arc from low‐temperature fluid, through high‐temperature fluid, to partial melt during progressive subduction. The geochemical systematics, the estimated mass fraction of the slab component, and the inferred stability of accessory rutile, zircon, and phengite in the slab are all similar between the southern Sangihe and Izu arcs, indicating that the thermal structure of the slab is not affected by impending collision. In contrast, lavas from the dormant northern Sangihe arc show geochemical characteristics similar to the Quaternary rear arc rather than the Quaternary volcanic front lavas in the southern arc. This may be related to advanced collision in the northern arc that could have slowed the subduction rate and heated the slab in the Pliocene followed by cessation of volcanic activity in the Quaternary. Key Points Geochemical variation in Sangihe arc, where arc‐arc collision is ongoing Slab flux varies from low‐T fluid, high‐T fluid, to melt across southern arc No clear spatial geochemical variation in the north due to arc‐arc collision