酸性マグマ起源のざくう石

福島県山野川および山形県上田沢産のアルマンディン

抄録

Recently almandine-rich garnet from rhyolite and dacite has been remarked as a high-pressure mineral directly crystallized from an acid hydrous magma at depth. The mineralogy and genesis of almandine-garnets from Yamanogawa, Fukushima Prefecture and Kamitazawa, Yamagata Prefecture were described and discussed from this viewpoint. The former garnet occurs within a hornblende-dacite and the latter is found from an acid tuff. They are different from metamorphic garnets in the following points. (1) The inclusion is characterized by magmatic accessory minerals such as apatite, zircon, rutile, sphene, pyrite, and either magnetite or ilmenite. (2) The zoning patterns of both garnets show neither normal or reverse type but distinct oscillatory type (Figs. 9-14) which is similar to the zoning pattern of associated plagioclase phenocryst (Fig. 16). One of the most probable explanations of such oscillatory zoning may be a model of magma chamber rising step-by-step from depth which causes the decrease of load pressure and accordingly the repeated change of Mg, Ca and Ti. (3) Though slightly corroded, the garnets exhibit conspicuous euhedral form up to 3mm or more in diameter. (4) The almandine-rich composition is what is characteristic of magmatic or synthetic garnets (Fig. 15). The slightly Ca-rich composition of the Yamanogawa garnet may be corresponding to the bulk composition of host dacite.
Judging from these features with reference to recent literatures, the both Yamanogawa and Kamitazawa garnets are considered to be of a deep origin under a high pressure. The garnet-bearing calc-alkaline acid magmas may have been generated within the upper mantle or the lowest crust and then risen up to the surface through and along some cites of tectonic disturbance. Such environment may be suggested in the case of the Yamanogawa garnet-bearing dacite which occurs along the Yanagawa-Shirasawa tectonic zone developing between the Green Tuff region and the non-Green Tuff region and composing a part of the Morioka-Shirakawa tectonic line (Fig. 18). As for the Kamitazawa garnet-bearing acid tuff, it seems geologically significant that the rock occurs in an area of widespread acid volcano-plutonic formation including welded tuffs (Fig. 19).