Origin of Alkaline Basaltic Intrusive Rocks in an Exhumed Accretionary Complex: Implications for Past Petit-Spot Volcanism in the Ocean

Accreted basaltic rocks are expected to provide information on intraplate volcanism in the oceans. Basaltic rocks, originating from mid-ocean ridges, plateaus, and seamounts, have been reported from exhumed accretionary complexes. However, basaltic rocks related to petit-spot volcanism remain poorly documented. We examined basaltic intrusive rocks in an exhumed accretionary complex on Amami-Oshima Island, Ryukyu Arc. Basaltic sills intruded melange composed of seamount-derived micritic limestone and basaltic rocks, which deformed together with the mudstone-dominated melange in the subduction zone. A separate basaltic sill intruded pelagic chert before being incorporated as a melange block in the mudstone matrix during deformation in the subduction zone. Geochemical discrimination diagrams and trace element patterns indicate that the accreted dolerites and nonintrusive basalts likely originate from mid-ocean ridges and hotspots, respectively. On the other hand, the geochemical characteristics of basaltic sills are compatible with alkaline basalt with enriched in incompatible trace elements relative to other basaltic rocks of hotspot origin and may represent a low degree of partial melting from a deeper mantle source (>90 km). A higher ratio of gadolinium to ytterbium in the basaltic sills relative to hotspot or mid-ocean ridge-related basalt indicates both a deeper melting depth, and a magma source which upwelled through thick oceanic lithosphere far from the mid-ocean ridge. Based on the timing of basaltic intrusion and geochemical features, we suggest that the basaltic intrusive rocks could record past petit-spot volcanism in the oceanic plate.