関東山地の四万十帯のホルンフェルス : 塩山市落合付近の産状と変成鉱物の化学組成
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概要
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Thermal metamorphic aureole of the Tokuwa Granodiorite observed near Ochiai, Enzan City is divided into three zones, Biotite hornfels, Cordierite biotite hornfels, and Garnet biotite hornfels zones. Characteristic mineral assemblages are described as follows. Alkali-feldspar and oligoclase are the feldspathic assemblage in the garnet and cordierite zones. Alkali-feldspar tends to be orthoclase in the garnet zone. White mica population in the biotite zone has a wide variety ranging from Phengite-leucophylite series to Tri-octahedral end-member. The phengite-leucophylite change occurrs by the Tschermak substitution and Tri-octahedral end-member change occurrs by the FeMg and Fe^<3+> Al substitution. The progressive abundance of tri-octahedral end-member in white mica is the characteristic feature of the studied area. The reaction takes place consuming chlorite in the source rocks of the Shimanto belt. Progressive crystalization of biotites shows that those in the cordierite zone formed first, those in the biotite zone second, and those in the garnet zone last, which indicates that the garnet zone was reheated by local dykes of the Tokuwa Granodiorite. Thermal metamorphic reaction from the biotite to cordierite zones is shown in equation, which was originaly proposed by Yuasa (1976). Cordierite in the studied area has X_<Mg> value of 0.4 and in anatectic origin of their formation, which is apparently higher in metamorphic temperature than that of Hanno Formation. From the reason, hornfels near Ochiai is not considered a supply area of gravels to the Hanno Formation. Using garnet-biotite geothermometer, it is found that garnets in the early stage are formed at 600℃, and those in the late stage at 700℃. The 600℃ may be the regional equilibrium temperature and the 700℃ may be the end temperature when local dykes reheated the garnet zone.
- 城西大学の論文
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