Studies of minerals and rocks occurring in the Yagoshi mine and in its environs (V)

Abstract

In the vicinity of the Yagoshi-mine, Iwate Prefecture in northern Japan, there occur some lamprophyric rocks such as beerbachite, orbite, gabbro-porphyrite, malchite or the like. The mode of occurrence of, microscopic structure of, and Radioralian remain in the beerbachite reveal that this rock is not a dyke-rock but a kind of hornfels metamorphosed by dioritic magma. The field relations of the other lamprophyric rocks also show that they are not dyke-rocks but xenolithic ones in the same plutonic mass. In these lamprophyric rocks, there are always phenocrystic crystals of plagioclase; however, they are not such real phenocrysts as seen in igneous rocks (Figs. 11-14). In this study, special attention was paid to these pseudo-phenocrystic plagoclases and the following features were especially noticed: (1) the phenocrystic plagioclases are xenomorphic; (2) they often contain small patches of the fine grained pseudo-ground-mass: (3) the refractive index of the plagioclase of the groundmass is higher than that of the phenocrystic plagioclase; (4) the phenocrystic plagioclases always show remarkable zonal structures with decreasing basicity outward. It is evident from these facts that the plagioclase materials were successively supplied from the surrounding dioritic magma into the xenolith and thus the phenocrystic plagioclases grew zonally, in some parts the hornfels materials being included in patches. Consequently the origin of the lamprophyric rocks of this region can be attributed to the contact-metamorphism or the contamination caused by the intruding dioritic magma. The historical development of lamprophyre concept is briefly stated, and it is suggested that some of the so-called “lampro-phyres” from other localities may be of such xenolithic origin, and not dyke-rocks of igneous origin as primarily defined by C. W. von Gümbel.