[S-CG55] Oceanic plate as inputs to subduction zone: from oceanic spreading ridge to subduction trench

[SCG55-P05] Petrological features and formation processes of lower crustal cumulate xenoliths from Oki-Dogo Island: A possibility of mantle plume origin

*Rio Higuchi¹, Yuji Ichiyama¹, Akihiro Tamura², Tomoaki Morishita² (1.Chiba University, 2.Faculty of Geosciences and Civil Engineering, Kanazawa University)

Keywords:Backarc-spreading, Backarc-spreading of the Japan Sea, Xenolith, Mantle plume, Lower crust, Southwest Japan

In Oki-Dogo Island, the northern part of Shimane Prefecture, Pliocene alkali basalts often include ultramafic and mafic xenoliths (e.g., Aoki, 1977; Takahashi, 1976). Takahashi(1976) reconstructed a petrologic structure from crust to upper mantle beneath Oki-Dogo Island, based on the lithofacies of the xenoliths. Abe et al. (2003) investigated lherzolitic mantle peridotite xenoliths from Oki-Dogo Island and revealed that they are characterized by Fe- and rare-earth element (REE)-enrichment and the presence of aluminous spinel. They also proposed that the Oki-Dogo peridotite xenoliths are involved with the Middle Miocene backarc spreading of the Japan Sea. In contrast to attention to the mantle peridotite xenoliths, mafic and ultramafic xenoliths derived from the lower crust have not yet been studied. In this study, the petrographic observation and major- and trace-element analyses for the lower crustal cumulate xenoliths, olivine-gabbro, orthopyroxene-gabbro, granulite, clinopyroxenite, and wehrlite were performed to examine the petrological features and formation processes and to reveal their source mantle composition and tectonic implications for the circum-Japan Sea region. The lower crustal xenoliths exhibit the cumulate texture of poikilitic plagioclase with olivine and clinopyroxene oikocrysts, and the estimated equilibrium temperatures (>700℃) indicate that they formed in the lower crust. Olivine Mg# [=Mg/(Mg+Fe)] in the cumulates is lower than those of MORB (0.7–0.8), at a given NiO content, and spinel Cr# [=Cr/(Cr+Al)] is significantly low (<0.1). The REE patterns of clinopyroxene show light REE-enrichment. These petrological characteristics indicate that the lower crust cumulates were crystallized from the magma that was produced by low degrees of partial melting of the source mantle. The estimated REE patterns of the parental magma of the cumulates are different from those of basalts formed during the Japan Sea backarc-spreading in the Middle Miocene but resemble those of the Late Miocene-Pliocene alkali basalts, formed by mantle plumes in the post-backarc-spreading magmatism. The lherzolite xenoliths can be the source mantle for the estimated parental magma of lower crustal cumulates. The Oki-Dogo spinel- lherzolite xenoliths are possibly enriched mantle material derived from the Late Miocene-Pliocene mantle plumes.