The Tomari volcanic rocks were products of the Middle Miocene volcanic front of the NE Japan arc. The volcanic activity has been divided into four stages with five lithologies, as assessed from surface geology. Eruptive rocks of Stage I (oldest) are the Kanatsuyama Andesite, the lower and upper units of Stage II are the Gassan Andesite and Tomari Basalt, respectively, Stage III comprises the Takahoko Andesite, and Stage IV is the Fukkoshi Andesite. Coring investigation was conducted on the northwestern side of the exposed Tomari volcanic rocks on central Shimokita Peninsula, Aomori Prefecture, in the northern NE Japan arc during 2014-2016. In the SN-010 core, the Tomari volcanic rocks occur from the bottom of the core at 1484 m below the surface to 308 m below the surface; the lower boundary of the volcanic rocks was not penetrated in the SN-010 borehole. We collected eight samples from the core and described their petrological characteristics to correlate the volcanic stages of the Tomari volcanic rocks between the core and surface geology. Modal compositions, especially abundant olivine phenocrysts and the occurrence of chromium-spinel, define the Tomari Basalt as a rock with a unique signature within the exposed Tomari volcanic rocks. The Tomari Basalt includes a primitive basalt characterized by high whole-rock contents of MgO, Cr, and Ni compared with the other Tomari volcanic rocks. The SN-010 core samples at a depth of ~1003 m have similar petrological characteristics to those of the Tomari Basalt, except that olivine phenocrysts in the core samples are perfectly altered. LILE/HFSE ratios, such as Ba/Nb, change below 1197.86 m (deep core samples) and above 1003.35 m (shallow core samples) in the SN-010 core. Ratios for shallow core samples are similar to those for surface samples. HFSE/HFSE ratios, such as Zr/Nb, show similar values among deep, shallow, and surface samples, indicating a cognate source mantle. These findings imply changes in the slab-derived component during magma generation associated with the opening of the Japan Sea.
