著者

黒田 直

出版者

日本地質学会

雑誌

地質学論集 (ISSN:03858545)

巻号頁・発行日

no.13, pp.381-387, 1976-12-30

被引用文献数

1

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Nanzaki Volcano (34°36'20"N, 138°50'E), consisting of older scoria cone and younger lava flow, is a very small volcano which erupted on a limited abrasion platform of Pleistocene age. Both the lava and scoria are basanitic, and include a small number of xenoliths of quartz gabbro changed to quartz monzonite and related xenocrysts, e.g. quartz rimmed with augite micrograins and hypersthene enclosed by augite. The basanitic lava contains more augite phenocrysts with hour-glass structure than olivine (Fo 85) phenocrysts with Cr-spinel inclusions, and plagioclase (An 75, 80) laths and a quantity of interstitial natrolitic mineral in the groundmass. Chemically this lava is characterized by the presence of normative nepheline (2-8%), the dominance of normative diopside over normative olivine and high Cr content (about 400 ppm). The chemical composition of this lava is close to basanites and basanitoids of Grenada Island, the Lesser Antilles Arc, except for slightly lower SiO_2 (43-44%) and slightly higher iron oxides (10-11% as FeO). During and after the reaction between quartz gabbro xenoliths and a magrna which formed the basanitic lava, augite and the groundmass plagioclase crystallized dominantly, and then natrolitic mineral occurred in the lava. According to the experimental data on the plagioclase series under high water pressure, the late crystallization of plagioclase in the basanitic lava may indicate high water pressure of a magma which formed the lava.