A peculiar amphibolite rich in deep vivid green-colored amphibole porphyroblast was found as a boulder in the Chiroro River of Hidaka Town, Hokkaido, Japan. The amphibolite mainly consists of deep vivid green-colored amphibole (porphyroblast), colorless amphibole (matrix) and plagioclase with a small amount of chlorite, epidote and chromian spinel. Deep vivid green-colored amphibole porphyroblast is more than 20 modal % in the rock. Chromian spinel rarely occurs as inclusions in deep vivid green-colored amphibole porphyroblast. The Cr₂O₃ content of the deep vivid green-colored domain is usually ∼1 wt% (up to 2.5 wt% nearby chromian spinel) whereas that of colorless amphibole in the matrix is low, < 0.1 wt%. Deep vivid green-colored amphibole porphyroblast is heterogeneous in chemical compositions within each grain and is divided into three domains as follows: (1) high-Cr₂O₃ (∼1 wt%, up to 2.5 wt%) and Al₂O₃ (> 10 wt%) green-colored domain, (2) relatively high-Cr₂O₃ (∼1 wt%) and low- Al₂O₃ (< 10 w%) green-colored domain and (3) low-Cr₂O₃ (< 0.1wt%) and Al₂O₃ (< 10 wt%) colorless domain. Chondrite-normalized rare earth element (REE) patterns of both deep vivid green-colored amphibole porphyroblast and colorless amphibole in the matrix are characterized by low light REE with positive Eu anomaly. The Cr#[= Cr/(Cr + Al) atomic ratio] and TiO₂ content of chromian spinel in the amphibolite are similar to those in a serpentinite complex of the Kamuikotan Zone. The amphibolite was formed by either amphibolitization of a rock containing chromian spinel derived from the surrounding serpentinite complex or extensively metasomatized serpentinite due to interaction with surrounding metamorphic rocks.

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We determined the Sr, Nd and Pb isotopic compositions of volcanic rocks from the Inaniwadake volcanic area and adjacent Nanashigure volcano, which are located in the northern part of the volcanic front of the NE Japan arc. Volcanic rocks of the Inaniwadake district consist of Inaniwadake (Pliocene) and Joboji (late Miocene) tholeiitic rocks. The Inaniwadake tholeiitic rocks are relatively constant in isotopic composition and less radiogenic in Sr and more radiogenic in Nd isotopic composition (⁸⁷Sr/⁸⁶Sr 0.7041-0.7044, ¹⁴³Nd/¹⁴⁴Nd 0.5128-0.5129) than the Joboji tholeiitic rocks (0.7045-0.7053, 0.5126-0.5127). The Sr and Nd isotopic ratios of the Joboji tholeiitic rocks correlate negatively and positively, respectively, with SiO₂ wt%, indicating a magma mixing process.
     The isotopic compositions of both tholeiites and Nanashigure calc-alkaline rocks were compared with those of other late Cenozoic volcanic rocks on the volcanic front of the NE Japan arc. There are systematic isotopic differences between the volcanic rocks from north of Zao (northern group including the volcanic rocks of Inaniwadake and Nanashigure) and those from south of Azuma volcano (southern group). The volcanic rocks of Joboji and Nanashigure are the most radiogenic in Sr and Pb isotopes of all of the rocks of the northern group, indicating the contribution of a component that is radiogenic in the Pb isotope to the mantle source for the isotopic variations of this group.

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