関東山地の四万十帯のホルンフェルス : 塩山市一之瀬付近の産状と変成鉱物の化学組成
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Thermal metamorphic aureole of the Hirose Granodiorite observed near Ichinose, Enzan City is divided into three zones ; biotite hornfels zone, cordierite I hornfels zone, and cordierite II hornfels zone. Characteristic mineral assemblages are described as follows. In the biotite zone, plagioclase of 35% abundance is composed of andesine and oligoclase, the former of which is a relict mineral. Quartz is the second abundance with 30%. Alkali-feldspar (+albite) decreases drastically from 34% in the unmetamorphic zone to 15%. The alkali-feldspar together with chlorite and other clay minerals contributes to form muscovite of 10-11% and biotite of 5%. The muscovite in the biotite zone has a wide variety ranging from phengite-leucophyllite species to tri-octahedral member. The phengite-leucophyllite change occurrs by the Tschermak's substitution and tri-octahedral change occurrs by the biotite substitution. In the cordierite zone, quartz is 28-29%, Alkali-feldspar and oligoclase are both 25% in abundance. Alkali-feldspar tends to be orthoclase in this zone. Muscovite decreases to -5% and biotite increases to 9-13%. Consuming muscovite and plagioclase, metamorphic minerals such as biotite, orthoclase, cordierite, and garnet are formed in this zone. Progressive crystallization of biotite and muscovite is well shown by their solidification index and phengite trend, both of which indicate apparent chemical zoning from the contact of the Hirose Granodiorite. Using garnet-biotite geothermometer, it is found that core of garnet in K56 is formed at 650-710℃, and the rim at 530-600℃. The rapid retrogressive metamorphism may have occurred near the contact by some tectonic disturbance. Cordierite in the studied area has wide X_<Mg> values and in variety origin of their formation, which is apparently higher in metamorphic temperature than that of Hanno Formation. From these data, the hornfels near Ichinose village is not considered an exact supply area for gravels in the Hanno Formation.
- 城西大学の論文
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