中九州肥後変成帯のざくろ石角閃岩

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In the zone II of the Higo metamorphic terrane garnet amphibolites were found. Wet chemical analyses were made on 3 amphibolites, and electron microprobe analyses were made on 9 garnets and 11 hornblendes. Porphyroblastic garnets (up to 1 cm in diameter) in two garnet amphibolites are traversed by many parallel cracks which are partly filled by biotite flakes. Garnets in another garnet-bearing amphibolite are small in size and irregular in form due to the invasion and penetration of hornblende and plagioclase. The garnets are almandine-rich and do not show a systematic chemical zoning. They are plotted within a field of amphibolite facies garnets in diagram Pyr-Alm-(Grd+Sp). Hornblendes are chemically similar to those from the Gosaisho-Takanuki amphibolites in the central Abukuma plateau, and Al^<Vl> and Si of the hornblendes studied are plotted below the 5 kb line after RAASE (1974). In diagram Ca-Mg-(Fe_t+Mn), the tie lines for hornblende-garnet pairs from different garnet amphibolites trend in almost the same direction. This may indicate that hornblende and garnet were recrystallized in equilibrium during the Higo metamorphism, being contrary to the replacement appearance of garnet by hornblende and plagioclase. The Higo metamorphic terrane underwent a low-pressure regional metamorphism has been correlated with the Ryoke metamorphic belt in Honshu. The Higo, however, is different from the Ryoke proper in containing more abundant limestone and basic and ultrabasic rocks and garnet amphibolite. From the occurrence and mineral assemblage it is suggested that garnets in the Higo amphibolites are relict minerals of products by an earlier medium-pressure metamorphism prior to the later Higo metamorphism of the low-pressure type. On the other hand, it can be interpreted as indicating the so-called pluri-facial character of the Higo metamorphism.
日本地質学会の論文

中九州肥後変成帯のざくろ石角閃岩

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