岡山県布賀に産する高温型スカルンを形成した火成岩類の岩石学的研究

Abstract

Many dikes varying from monzodiorite to quartz monzonite occur in limestone at Fuka, Okayama Prefecture. From the modes of occurrence and chemical compositions of the dike rocks, it is likely that the rocks are products of the same magmatic origin and intruded into the limestone in the following two stages. In the first stage, a monzodiorite dike intruded and formed gehlenite-spurrite skarns as high-temperature metasomatic products. The dike is composed mainly of orthopyroxene, clinopyroxene, and plagioclase. The metasomatic temperatures using the two-pyroxene geothermometer range from 860°C to 930°C, which is the highest skarnization temperature among the skarns reported in the world. In the second stage, a melanocratic quartz monzonite dike first intruded, and formed the same skarns as the monzodiorite dike. The dike is composed mainly of plagioclase, K-feldspar, quartz, biotite, clinopyroxene, and hornblende. The geothermometric temperatures using coexisting hornblendes range from 690°C to 800°C. In succession, a leucocratic quartz monzonite dike intruded, and formed low temperature skarns containing Ca-bearing minerals such as garnet, vesuvianite, and wollastonite.The monzodiorite dike characteristically contains large amounts of biotite and pyroxene hornfelses as xenoliths. The pyroxene hornfels is composed mainly of olivine, orthopyroxene, clinopyroxene, and plagioclase. For the pyroxene hornfels, the metamorphic temperatures ranging from 900°C to 1090°C were estimated from the two-pyroxene geothermometer. The pyroxene hornfels would, therefore, be formed by partial melting of biotite hornfels before high temperature skarnization.