Alkali Amphibole-Bearing Metamorphic Rocks in the Kamuikotan Belt of the South-Western Part of Asahikawa, Central Hokkaido

Abstract

The Kamuikotan metamorphic rocks in the southwestern part of Asahikawa city, Hokkaido, are divided into the following three parts, as shown in Fig. 2, not only by their geological features but also by their metamorphic characters : 1. weakly metamorphosed basic volcanics. 2. black schists and alkali amphibole bearing green schist. 3. black schists and green schists. They are arranged from east to west in order mentioned above.The first part (zone I) is characterized by the presence of chlorite and pumpellyite, the second part (zone II) is characterized by the presence of lawsonite and alkali amphibole, and the third part (zone III) is characterized by the presence of epidote and blue green hornblende. Recrystallization is also advanced in this order. Between each part there occur large bodies of serpentinite, running to N-S direction, which agree with the general trend of the schistosity, many faults and sheared planes in this district. However, such a general trend is modified by another fault group of E-W direction, and the resultant blockwized structure is characteristic. In the second part, many small bodies of serpentinite are Seen among the schists. According to the observation under the microscope, alkali amphibole under consideration seems to have crystallized in intimate relation with such relic minerals as pyroxene or brown hornblende, as shown in Fig. 5. It was confirmed that one of relic clinopyroxene contains 1.4% of Na2O. The original materials may be important for the growth of alkali amphibole, and the process forming the alkali amphibole may be considered as schematically illustrated in Fig. 6. The chemical analyses of the rocks in the present district are shown in Table 1. From this data, it is clear that sodium content is rather poor in comparison with normal alkaline basalt. Fe2O3/FeO + Fe2O3 ratios are rather high as compared with those in the basic metamorhic rocks in the regions of the non-glaucophanitic metamorphism.