紀伊半島大峯花こう岩質岩から見いだされたMgに富む火成包有物

概要

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Magnesian igneous enclaves showing sub-angular shape and up to 2 m in the maximum dimension were found from granodiorite in the Dorogawa pluton, the northernmost pluton of the Ohmine granitic rocks in Kii peninsula, southwest Japan. Igneous enclaves are composed mainly of plagioclase and hornblende, as phenocrysts, and of plagioclase, hornblende, quartz, and biotite as groundmass minerals. They also contain hydrous mafic mineral aggregates consisting of chlorite, actinolite, and biotite with rare chromian spinel microcrystals. Their whole rock compositions were characterized by high MgO content (5.8-7.0 wt%), low FeO*/MgO ratio (0.80-0.92), high Cr (256-346 ppm), and Ni (81-138 ppm) contents. Patterns of trace element abundances are close to those of the high-Mg andesite (HMA) of the Setouchi volcanic rocks. The compositional range of microcrystals of chromian spinel within hydrous mafic mineral aggregates is similar to those of included in olivine or orthopyroxene phenocrysts of the Setouchi HMA. These geochemical and mineralogical characteristics of the igneous enclaves suggest the activity of HMA magma closely in space-time with the intrusion of the Dorogawa pluton.
日本鉱物科学会の論文
2005-01-30