A trachyte dike in the northern part of Shidara area, Aichi Prefecture, Japan.

Abstract

In the northern part of Shidara area, where acid Miocene volcanics are widely distributed, numerous alkali basalt dikes and sheets occur and some of them were studied by Kuno (1960). A trachyte dike also occur in the area and it is most alkaline among the rocks of the area. The trachyte consists of phenocrysts of ferrohedenbergite rimmed to sodic ferrohedenbergite, fayalite, arfvedsonite, plagioclase and alkali feldspar. Groundmass minerals are alkali feldspar sodic ferrohedenbergite, aenimgatite and titanomagnetite. The trachyte includes small xenoliths, which may have been trachybasalt as variolitic and labradorite core of plagioclase phenocryst are preserved. The xenoliths consist of titanaugite rimmed to aegirinaugite, ferroaugite, aegirinaugite, arfvedsonite, plagioclase, aenigmatite and titanomagnetite. Enrichment of acmite molecules in pyroxenes such as sodic ferrohedenbergite and aegirinaugite and crystallization of arfvedsonite, albite, alkali feldspar and aenigmatite may be due to reaction between trachyte including xenoliths and alkali-rich residual fluid at the latest stage. Crystallization trend of pyroxenes of the trachyte is different from that of the xenoliths. Trend of the former is ferrohedenbergite-sodic ferrohedenbergite and that of the latter is titanaugite-aegirinaugite. However, in the latest stage, it seems that sodic ferrohedenbergite and aegirinaugite have crystallized on similar condition which nearly corresponds to “no oxide field” proposed by Nicholls and Carmichael (1968).