Early Miocene Fore-Arc Magmas Derived From Subcontinental Lithospheric Mantle During the Japan Sea Opening: Geochemistry of the Ishimoriyama and Iritono Volcanic Rocks in the Iwaki District, NE Japan

Abstract

During the Miocene opening of the Japan Sea, volcanic activity expanded greatly toward the trench due to the injection of the hot asthenosphere into the mantle wedge. The Ishimoriyama and Iritono volcanic rocks, both erupted at around 17.5 Ma in the Iwaki district on the Pacific coast of NE Japan, are products of this event. Ishimoriyama is a small composite volcano comprising calc-alkaline basaltic to andesitic volcaniclastic rocks. Iritono is a monogenetic volcano composed of low-K aphyric pillow basalts with high TiO₂ contents. The Pb isotopic compositions of volcanic rocks from both volcanoes (²⁰⁶Pb/²⁰⁴Pb = 18.39–18.40, ²⁰⁷Pb/²⁰⁴Pb = 15.61–15.62, and ²⁰⁸Pb/²⁰⁴Pb = 38.53–38.57) are more radiogenic than the Indian MORB-like trend defined by other Miocene volcanic rocks in the fore-arc region of NE Japan and overlap those of the most enriched Japan Sea Miocene basalts. The variation among the Japan Sea basalts can be explained by the mixing of depleted Indian MORB-like mantle and enriched preexisting subcontinental lithospheric mantle (SCLM). Therefore, the Ishimoriyama and Iritono volcanic rocks were derived from SCLM, and the reason why magma was generated by partial melting of the SCLM, which does not normally melt spontaneously, was due to the injection of the hot asthenosphere. The trace element abundances in the Ishimoriyama volcanic rocks match those in normal arc-type volcanic rocks, but their compositional variations suggest the fractionation of large amounts of amphibole from basaltic andesite magma. Because this differentiation process requires a high pH₂O in the parental magma, the source could have been hydrous SCLM. In contrast, the Iritono volcanic rocks are depleted in fluid-mobile elements and have convex rare earth element patterns peaking at Sm. Accordingly, this magma formed when a residual SCLM domain was re-melted by the hot asthenospheric injection. These distinct geochemical differences between two nearly contemporaneous volcanic rocks only 20 km apart indicate that the SCLM was heterogeneous, with adjacent hydrous and residual domains.