Short note on ultramafic granulites in the Ongul Islands area, East Antarctica

概要

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Ultramafic granulites in the East and West Ongul Islands area are grouped into two types, A and B. One of the fundamental criteria of the grouping is that type A does not yield plagioclase and type B does. Type A is characterized by the abundance of orthopyroxene and clinopyroxene and by the lack of plagioclase and garnet. It occasionally yields olivine. Type B is characterized by the presence of plagioclase and/or garnet and by the poverty in pyroxenes. In common cases, type A tends to occur as massive isolated blocks or pods in the surrounding gneisses, while type B as concordant sheets or layers showing a distinctly foliated structure. Between these two types, chemical characteristics of main constituent minerals such as orthopyroxene, clinopyroxene and hornblende are different from each other. Those in type A are comparatively more magnesian and less aluminous. Metamorphic temperature estimated from the pairs of orthopyroxene and clinopyroxene of both types is as high as around 800℃. Pressure condition is calculated based upon the associations of garnet, pyroxenes and plagioclase of type B to give the values around 7.5kb. The essential difference between the two types is inferred to reflect the difference in petrological characters of original rocks.
国立極地研究所の論文