Petrology of the Nishidohira cortlandtitic mass in the southern Abukuma Mountains, Northeast Japan.

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  • 南部阿武隈山地,西堂平コートランド岩質岩体の岩石学
  • 南部阿武隈山地,西堂平コートランド岩質岩体の岩石学〔英文〕
  • ナンブ アブクマ サンチ ニシ ドウダイラ コート ランドガンシツガンタイ ノ

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Abstract

The Nishidohira cortlandtitic mass of probable Late Mesozoic age in the southern Abukuma Mountains has been characterized on the basis of field evidence, microscopic observations and chemical analyses of rocks and minerals. Though the mass is small, it is composed of various rock types.<br> Cortlandtite, quartz hornblende gabbro, monzonite and syenite are products of clinopy-roxene dominated fractionation of a picrite-basaltic magma. Whereas amphibole-dominated fractionation played an important role in the formation of hornblende gabbro, quartz diorite and granite from a fairly MgO-poor basaltic magma. The two rock groups are probably not comagmatic, but might have originated from a common parental material enriched in K and Rb and depleted in Ni. The two effective fractionations had occurred before the intrusion to the present position.<br> The cortlandtite has crystallized under conditions of 7-9 kb>Ptotal>5±2 kb, PH<sub>2O</sub>≥2-3 kb, and had equilibrated at temperatures from 1000 to less than 750°C. Prevailing high PH<sub>2O</sub> and slow cooling of the mass brought about considerably low equilibration temperatures. When the almost congealed cortlandtite had intruded at about 5±2 kb, the latest liquid of the clinopyroxene-dominated fractionation was segregated to form the syenite and monzonite.

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