Hot, volatile-poor, and oxidized magmatism above the stagnant Pacific plate in Eastern China in the Cenozoic

Abstract

Eastern China has experienced widespread and voluminous, basaltic to andesitic intraplate magmatism in the Cenozoic. Seismic tomography and kinematic reconstructions show that magmas have erupted above the stagnant Pacific plate, which currently extends within the mantle transition zone for >1500 km beyond the active arc. Seismic studies also show in intriguing detail that low-velocity wave anomalies reach from the mantle transition zone or below to major volcanic centers, suggesting an intimate relation between the magmatism and components derived from the mantle-transition zone and possibly from deeper levels. Here, we provide new petrological, mineral chemical, and whole-rock geochemical data showing that Cenozoic basaltic andesitic and andesitic magmas that have erupted above the present-day edge of the subducted Pacific plate are largely unfractionated, ≥1130-1160±15-50 °C hot, mantle-derived melts that were volatile-poor (undegassed melt CO2 and H2O of ~2400-3100±500 ppm and ~0.5-0.6±0.4-1.1 wt%), and relatively oxidized (~FMQ to ~FMQ+1.3±1.0). We further infer that the magmas were derived by partial melting of oxidized, H2O- and CO2-poor eclogite-rich sources (~FMQ-1.0 to ~FMQ). We use this to suggest that the low-velocity seismic anomalies below Eastern China largely reflect the presence of oxidized and enriched, hot and not necessarily particularly hydrous mantle domains as common interpretation has suggested. This is crucial information for quantifying material flow above and around the subducted Pacific plate and for constraining residence times of subducted components in Eastern Asia and globally.