Mineral Composition of Trachybasalts from Northern Kyushu - With Special Reference on the Petrogenesis of Amphibole-bearing Rocks

Abstract

In order to study the origin of amphibole in alkali basalts, the writer has statistically treated the mineral compositions of basaltic rccks from northern Kyushu, belonging to the “Cenozoic alkali suite of Circum-Japan Sea Region (TOMITA 1935)”. It is always the case that the amphibole-bearing rock is accompanied by the amphibole-free basalt, and the latter is intruded or overlaid by the former. Hence the basalts are classified into the olivine trachybasalt～trachyandesite without amphibole phenocryst (III_b and IV_b types as defined by H. KUNO), and the amphibole-bearing olivine trachybasalt～trachyandesite (VIII_b and IX_b types). Significant differences are noticed between these two types with respect to modal olivine as modal magnetite. Petrochemistry of these two types shows no significant differences between the two types with respect to their ratios. FeO + Fe₂O₃/MgO + FeO + Fe₂O₃, or Fe₂O₃ + TiO₂/FeO + Fe₂O₃ + TiO₂, which are regarded as the indicator of the content of magnetite (+ilmenite). Within the amphibole-bearing rocks, there is a negative correlation between the contents of amphibole and magnetite + ilmenite. From these considerations, the writer is lead to the conclusion that the amphibole is formed by the crystallization of magma during which the iron and titanium components concentrate into the ampoiboles, and consequently the separation of ore minerals is restricted.