Clinopyroxene of the ultrabasic rocks in the northern Kanto mountains, Central Japan

Abstract

Six clinopyroxenes of the ultrabasic rocks such as wehrlite, olivine clinopyroxenite, olivine bearing hornblende clinopyroxenite and clinopyroxene hornblendite have been chemically analyzed (Table 1). They are slightly iron rich comparing with those from the peridotite of Dun Mountain and of Horoman. The Ca: Mg: Fe ratios are from Ca44.9 Mg48.5 Fe6.6 to Ca44.2 Mg46.0 Fe9.8 and are plotted as shown in Fig. 1. The clinopyroxenes in the Hayachine ultramafic complex in the Kitakami mountains reported by Onuki (1963) are plotted in the field near the one of the northern Kanto mountains. Total Al contents on the basis of 6 oxygen atoms are from 0.126 to 0.244 and the proportions of Al of four fold coordinate are 6.25 to 8.30%. Ca-Tschermak' molecule calculated by the method proposed by Kushiro (1962) are variable. Total Al contents and the proportion of Al in Z position increase in accordance with enrichment of Fe''+Fe'''+Mn in the course of fractional crystallization as shown in Fig. 2. The proportions of Al in Z position vary in accordance with the amounts of total Al as shown in Fig. 3, however, it is clear that the ratio between amounts of AlIV and total amounts of Al decrease when the total Al contents in clinopyroxene increase. It is therefore, suggested that the enrichment of Al in magma is the most effective factor controlling the amount of Al in clinopyroxene especially in the case of ultrabasic intrusives in the northern Kanto mountains.