Geophysical and geochemical constraints on the origin of Holocene intraplate volcanism in East Asia

A number of Holocene volcanoes in East Asia - Jeju, Ulleungdo, Tianchi, Longgang, Jingbohu, Erkeshan and Wudalianchi - are located far (600-1500 km) from the nearest subduction zone. The origin of these intraplate volcanoes remains unclear, and mechanisms proposed to explain their origin include plume activity and subduction processes with or without slab fluid involvement. Here we evaluate the feasibility of these mechanisms. We present an analysis of available geophysical data, including slab geometry models and the full-waveform FWEA18 tomography model, as well as statistical tests on a compilation of geochemical data. High-resolution tomography data provide no evidence for a deep-seated mantle plume. Instead, the tomography shows that Tianchi, Longgang, Jingbohu, Erkeshan and Wudalianchi are located above edges of the Pacific slab, which is stagnant near the 660 km discontinuity under East Asia. The tomography also shows that Jeju is situated above a section of the Philippine Sea slab that has subducted to the 410 km discontinuity. While the intraplate volcanoes are underlain by subducted slabs, their geochemical signatures do not support melting via slab metasomatism typical of subduction zones. Instead, the Holocene intraplate volcanoes are alkaline and have trace element compositions comparable to those of ocean island basalts. Given the absence of geophysical or geochemical evidence for plume activity or slab metasomatism, we propose that volcanism has been generated by decompression melting associated with convective upwellings at the edges of the Pacific and Philippine Sea slabs. Tectonic reconstructions suggest that the Pacific slab may have been stagnant in the mantle transition zone for millions of years, so we speculate that localised convection at Pacific slab edges was triggered by changes in Western Pacific subduction dynamics during late Neogene-Quaternary time. The geophysical and geochemical data also suggest that Quaternary rollback of the Philippine slab might be responsible for volcanism at Jeju, which is located at the leading edge of the Philippine Sea slab.