Geochemical and geological implications of in-situ basalts and petit-spot basalts since Late Cretaceous accretionary complexes in Japan.

Accreted basalts into the accretionary complex are generally interpreted as allochthonous rocks originated from oceanic island basalt (OIB), mid-ocean ridge basalt (MORB), and petit-spot submarine basalt. Accreted basalt, therefore, provide the information about the type of past igneous activities occurred on the subducted oceanic plate. In-situ basalts, which is designated by MORB geochemical suite, are not allochthonous but reported from accretionary complexes. They erupt into terrigenous pelitic rocks in contact with chilled margins. The Late Cretaceous accretionary complex in Japan exposes a systematic distribution of in-situ basalts in eruption age. In-situ basalts in Hidaka Belt, Central Hokkaido, show younger age (60 Ma, Miyashita et al., 1997), while ones in Amami Oshima Island, southern Japan, show older age (90 Ma, Osozawa et al., 1983). These characteristics plausibly result from the drifting of the Izanagi-Pacific ridge subduction along paleo Japan continental arc during the Late Cretaceous (Kiminami et al., 1994). Accreted petit-spot submarine volcanoes, as in the other case of in-situ basalts, are recently reported from the Late Cretaceous accretionary complex (Kiminami et al., 2017; Ikeda and Goto, 2018). The petit-spot magmas as well as in-situ basalt magmas, furthermore, originate from asthenosphere (Hirano et al., 2006; Machida et al., 2017). Hence, both of in-situ basalt and accreted petit-spot basalt in the Late Cretaceous accretionary complex must significantly record the geochemical feature of shallow mantle after the Late Cretaceous. Miyazaki et al., (2016) proposed the stationary boundary of Indian- and Pacific-domains in upper mantle below the western Pacific plate on the basis of a few in-situ basalts and present petit-spot basalts. Here, we report more temporal and new geochemical information to define the boundary between Indian- and Pacific-domains in shallower mantle using in-situ basalts and potential accreted petit-spots.