Origin of Basaltic Intrusive Rocks in the Chichibu Accretionary Complex of Amami-Oshima Island, Ryukyu Arc

概要

Basaltic rocks in exhumed accretionary complexes are crucial for understanding past intraplate volcanism in the oceans. Geological and geochemical studies suggest that accreted basaltic rocks are derived from oceanic crust, plateau, and seamounts. However, the origin of basaltic intrusive rocks in exhumed accretionary complexes is controversial. In this study, I examined basaltic sills and dikes at three localities (Asanzaki, Oganeku, and Naon) in the Chichibu accretionary complex of Amami- Oshima Island, Ryukyu Arc.

　At Asanzaki, basaltic sills emplaced into mixed rocks composed of micritic limestone, massive and basaltic pillow lava and breccia. These rock assemblages constitute blocks in the mudstone matrix. At Oganeku, the basaltic sill penetrated the block of pelagic radiolarian chert, which was in turn incorporated into the mudstone matrix with the blocks of pillow basalt breccia and dolerite. Geochemical discrimination diagrams indicate that the basaltic sills at Asanzaki and Oganeku are alkaline basalt, whereas other basaltic rocks show a wide range of compositions between mid-ocean ridge basalt and alkaline basalt. Trace element concentrations suggest that basaltic sills are more enriched in incompatible trace elements than other basaltic rocks, possibly representing a low degree of partial melting from a deeper mantle source. These geological and geochemical features suggest that the alkaline basaltic sills emplaced after the deposition of pelagic chert and the mixing of micritic limestone and basalt on the flank of the seamount but prior to the mixing of pelagic and terrigenous mudstone associated with the mélange formation in the subduction zone. Therefore, the basaltic sills may represent petit spot volcanism, or post-shield and rejuvenated stage activity at the hotspot in oceanic plate.

　At Naon, two basaltic dikes cut massive basalt, reddish chert, varicolored shale, and grey chert. The basaltic dikes yielded 40Ar/39Ar ages of 16.37±0.14 Ma and 16.51±0.10 Ma and exhibit reversed magnetic polarity. Correlation with the geomagnetic polarity timescale suggests that igneous activity occurred in the forearc region during reversed polarity subchrons from C5Cn.1r (16.261–6.351 Ma) to C5Cn.2r (16.532–16.434 Ma). Trace element patterns of the basaltic dikes are characterized by enrichment of incompatible trace and light rare earth elements, possibly representing a low degree of partial melting from a deeper mantle source. The age and trace element patterns of the basaltic dikes are comparable to those of alkaline basaltic intrusive rocks distributed in the Outer Zone of Southwest Japan and northern Ryukyu Arc, which could represent Middle Miocene forearc alkaline magmatism in association with the subduction of young and warm Shikoku Basin. The discovery of alkaline basaltic dikes at Naon suggests that the distribution of Middle Miocene forearc alkaline magmatism may extend to the Amami-Oshima Island in central Ryukyu Arc. Hence, the northern end of the Kyushu-Palau Ridge could have been located at the south of Amami-Oshima Island during 16.5–16.3 Ma, then moved eastward to the current location.