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5: Petrology of Tarumae volcano
Erupted materials from the Tarumae Volcano are mostly andesite and low - silica decite. They emplaced mostly pyroclastic as dominant white and grey pumice, with minor amount of basaltic andesite scoria, and banded pumice. Among them, Ta-d2, Ta-c1, Ta-b0, and ejecta of 19th century contain juvenile scoria clasts. The lavas are only identified after 19th century eruptions as the lava domes. The juvenile pumice and scoria contains more than 20 % (in mode)of phenocrysts of plagioclase, orthopyroxene, clinopyroxene and iron oxide, and olivine in mafic ejecta except for Ta-d2. In Ta-c2 hornblende is rarely included. Scoria of Ta-d2 is characteristically depleted with phenocryst as less than 5 %.
The dominant ejecta from the Tarumae Volcano is andesite of SiO2=60 to 63 % (in weight). Especially Ta-c2, Ta-b, and Ta-a contain minor amount of basaltic andesite. Among each stage activities, ejecta become less silicic less than 60 % SiO2 in Stage 2 and Stage 3 (
Table 3). Harker diagrams show linear correlation in stages and eruptions respectively and they can be discriminated from each other. The Ta-d1 has SiO2=50.5 to 57.5 % while the Ta-d2 has SiO2=60 to 63.5 %. The First Stage eruptives are clearly distinctive from those of the other stages in view of potassium and magnesium contents ( Fig. 6). In the Second Stage, the Ta-c1 show narrow range of SiO2=55 to 57 % and has low potassium and magnesium contents similar to the Ta-d1. Contrastingly, the Ta-c2 and following stages show medium potassium and enriched in magnesium contents. Therefore chemical property of the magma had changed between the Second Stage. The Ta-c2 shows wide range of SiO2=51.8 to 63.2 % while the Ta-c3 has narrow range SiO2=57 to 58 %. In the Third Stage SiO2 ranges 55 to 63 % and has medium potassium content similar with the Ta-c2. As the magnesium content is less than the Ta-c, it enables us to discriminate from the Ta-c ( Fig. 6).