PHENOCRYSTIC HORNBLENDES FROM TERTIARY ANDESITES AND DACITES, KAGAWA PREFECTURE, JAPAN

Abstract

Studies on volcanic hornblendes have been carried out with special reference to the relationship in chemistry to their host rocks from Kagawa Prefecture, Japan. The quantity of Si and AlIV in the hornblende seems to depend to a considerable extent on that of free silica in a magma represented by the rock. The groundmass contains about 4 times as much K₂O as the hornblende does, and the coefficient of partition of K and Na between the groundmass and the hornblende is 2.2 on an average. It is deduced from considerations of phase relations in the rocks, two-pyroxene geothermometer and the chemistry of hornblendes that the hornblendes ware crystallized at temperatures above 900°C from magmas probably undersaturated with water. A precipitation of some iron-rich phases together with the hornblende must be required to produce a typical cale-alkaline trend. Hornblende-controlled fractionation at a phenocrystic stage of magmatic crystallization must change the residual melt into a peraluminous composition.