能郷白山接触変成帯中のメラナイトおよびファッサ輝石

Abstract

Melanite and fassaite are found from thermally metamorphosed rocks in the Nogo-Hakusan area, central Japan. The modes of occurrence of the melanite are divided into the following three: A) in white vein in the basic hornfels in the pyroxene hornfels facies zone. B-1) in dark fragment in the upper amphibolite facies zone. B-2) in dark fragment and the reaction zone around the dark fragment in the pyroxene hornfels facies zone. The fassaite occurs with the melanite in the pyroxene hornfels facies zone (A and B-2). The dark fragments which were originally basaltic rocks have been subjected to the K-metasomatism accompanying the contact metamorphism. TiO2, content of the melanite attains up to ca. 19 wt%. The correlation between color of the melanite and TiO2 content is observed. The compositional gap between grandite and the melanite is not recognized. Some grains in the dark fragments show sharp decrease of TiO2 content from cores to rims. The melanite in this area is relatively enriched in Al and depleted in Fe3+. Al2O3 content of the fassaite attains up to ca. 14.5 wt%. Total Al and Al (IV) have weak positive correlation with Ti. The Al-Ti diagram of the melanite in this area suggests that Ti replace Al. The Fe3+-Ti diagram indicates that there is no correlation between Fe3+ and Ti in the case of Ti > 0.5. These imply that the principal substitution is Ti (VI)+Fe3+ (VI)+Al(IV) ?? 2Al (VI)+Si (IV) when 0 0.5. In the dark fragment, Ti content of melanite decreases from core to rim. The amount of melanite of the dark fragment in the pyroxene hornfels facies zone is larger than that in the upper amphibolite facies zone, while the amount of biotite is smaller. These suggest that the formation of melanite under increasing temperature. Symplectic melanite sometimes occurs with K-feld-spar as the result of the decomposition of biotite with increasing temperature.