小笠原諸島父島, 三日月山火山岩類の角閃石石英デイサイト

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The hornblende quartz dacite is one of the very rare hornblende-bearing rocks in the Bonin Islands. The dacite has a high SiO_2 Content of 72.0 wt.% corresponding to rhyolite, but the Cr content is high, 50 ppm, a value of average andesite. It consists of phenocrysts of plagioclase (8.6 vol. %), quartz (3.9%), orthopyroxene (2.2%), green hornblende (1.8%), clinopyroxene (1.2%) and magnetite (0.4%) in the groundmass of feldspar, pyroxene, amphibole, iron ore and glass. The majority of the hornblende phenocrysts are magnesio-hornblende with 9.29-5.67% Al_2O_3,1.12〜0.45% TiO_2,1.77〜0.93% Na_2O, <0.05% Cr_2O_3 and 58.5〜68.4 Mg#, which are similar to those in diorite from Palau. The clinopyroxene phenocrysts have Mg# and Cr_2O_3 as high as 84.4 and 0.45%, respectively, suggesting that they have crystallized from a more primitive magma with higher MgO and Cr, whereas the orthopyroxenes are mostly hypersthene. The dacite also contains plagioclases having unusually high An contents of 86.8 to 42.6,despite its high SiO_2. The hornblende quartz dacite seems to have derived from a high-Mg andesite magma crystallizing clinopyroxene prior to orthopyroxene, which is different from most boninite magmas that have crystallized protoenstatite and/or bronzite following olivine. Crystallization of the magnesio-hornblendes suggests that the hornblende quartz dacite has been cooled slowly in a magma chamber at depth, in the sharp contrast with rapid quenching from >900℃ of the boninite series rocks.
静岡大学の論文

小笠原諸島父島, 三日月山火山岩類の角閃石石英デイサイト

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