Journal of the Faculty of Science, Hokkaido University. Series 4, Geology and mineralogy;Vol.XVIII, No.4


Relict Clinopyroxenes in the Mikabu Greenstones, Oshika and Kosetokyo Districts, Nagano Prefecture, Central Japan

Watanabe, Teruo;Kawachi, Yosuke;Yuasa, Makoto

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Magma types and spilitization process of the metamorphosed greenstones (Mikabu greenstones) in the Oshika and Kosetokyo districts were examined on the basis of the chemical composition of rocks and relict clinopyroxenes. The alkali contents of the rocks are plotted in both alkaline and tholeiite basalt fields. However, most greenstones with high alkali contents have less TiO2 and P2O5 than typical alkali basalts. One of the rocks carries the clinopyroxene with high Si and low Ti contents as in tholeiite basalts. These features may be interpreted as due to spilitization. It is suggested that most of the original rocks of the Mikabu greenstones of these districts, especially of Kosetokyo district, show an oceanic tholeiite affinity, though some an alkali basaltic affinity.