草津白根火山，白根火砕丘群，弓池マールおよび逢ノ峰火砕丘の岩石学的特徴

抄録

We report petrographic characteristics and whole-rock chemical compositions of the Stage 3 (after 16 ka) eruption products of Kusatsu-Shirane Volcano, central Japan. The Stage 3 eruption products analyzed in this study are from the Shirane Pyroclastic Cone Group (SPCG), the Ainomine Pyroclastic Cone, and the Yumi-ike Maar, which are all located in the northern summit area of Kusatsu-Shirane Volcano. All lavas and juvenile pyroclasts of the Stage 3 eruption products contain phenocrysts of plagioclase, orthopyroxene, clinopyroxene, and opaque minerals. In addition to these minerals, some rocks contain rare olivine, quartz, and biotite phenocrysts. The lavas that were erupted during the early activity of the SPCG (the Hebei-ike, Kagusa, and Shirane-higashi lavas) have wide variations in SiO₂ content from andesitic to dacitic and in some outcrops exhibit a heterogeneous mingled appearance. In contrast, the lavas and pyroclasts that were erupted during the later stage of the SPCG (Shirane Pyroclastic Deposits and the Mizugama Lava Dome), as well as the Ainomine and the Yumi-ike Pyroclastic Deposits, are homogeneous in appearance and have andesitic compositions. Irrespective of the relative eruption ages, the analyzed Stage 3 eruption products are all associated with the calc-alkaline and medium-K series.