大浦複合体 舞鶴市・大浦半島の層状貫入岩体についての岩石学的研究

Abstract

The Oura complex is a series of layered mafic and ultramafic plutonites exposed over 23 square kilometres in the northern area of Maizuru city. On the basis of their field relationships the complex can be divided into two units; (1) a mafic unit comprising a sequence of gabbroic and pyroxenitic piles, about 4, 000 metres thick, and (2) an ultramafic unit composed of several bodies of dunite with a small amount of pyroxenite which intruded into the mafic unit. The order in which the essential minerals in the mafic unit separated from the magma is clinopyroxene, plagioclase, and magnetite. Rocks in the ultramafic unit are exclusively made up of olivine, chromite, and clinopyroxene. The chemical composition of silicate minerals is estimated from their optical properties. Curves are drawn to show the order of the separation of the essential minerals in the mafic unit and their compositional changes as crystallization proceeded. The general trend of the curves from more magnesian to more iron-rich for the clinopyroxene and more calcic to more sodic for the plagioclase is evident. The composition of the magma at the about 2, 000 metres high in the layered sequence of the mafic unit is estimated from the composition of the pore material. It is similar to an andesitic composition, oversaturated in silica, comparatively rich in soda and alumina, intermediate in total iron, and low in magnesia. The weakly iron-enriched fractionation trend of the Oura magma, in contrast to the strongly iron-enriched one in some other layered intrusions, may be explained by the relatively small amount of plagioclase which precipitated in the early and middle stages of the layered sequence.