Petrology and Chemistry of the Cenozoic Volcanic Rocks in the Western Part of the San-in Region, Southwest Japan

Abstract

Volcanic rocks distributed in the western part of the San-in region belong to the so-called “Cenozoic alkali suite of Circum-Japan Sea region (TOMITA)”. Their distributions can be divided into three districts: Otsu-gun, Abu-gun and Mi-shima in Yamaguchi prefecture. The followings have been clarified by chemical analyses of 52 specimens and microscopic observations of many specimens of these volcanic rocks. In the Otsu District, volcanic activities of the tholeiitic rock series took place, followed by that of the alkali rock series. As a part of the latter series, the series of iron rich type has been found. In the Abu District, volcanic activities of the alkali rock series took place, followed by the activities of andesites of the calc-alkali rock series, which are formed by successive assimilation of the acidic rocks of the basement with alkali-olivine basalt magma. Large number of xenolithes of gneissose rocks have been found in a part of these lavas. Quartz xenocrysts are found in both of the rock series. In the Mi-shima District, activities of the tholeiitic rock series took place, followed by that of the alkali rock series. The latter rock series are rich in olivine, and picritic basalt is sometimes found. Most of these Cenozoic volcanic rocks erupted out after the Pleistocene Age, but some (Kasa-yama) in the Recent Age. The tholeiitic rock series in the Otsu District show higher Na + K than that in the Izu and Hakone Districts. Almost all of the alkali rock series analysed in this study are plotted in the field of less Fe than of the Circum-Japan Sea province, on the triangular diagram of Fe-Na+K-Mg. Especially, the alkali rock series in the Mi-shima District show high ratio of Mg:Fe. The calc-alkali rock series in the Abu District have similar chemical compositions as of the Chokai volcanic zone (KATSUI) and the Daisen volcanic zone (KURASAWA and TAKAHASHI). The results on the minor elements distributions in these rocks will be reported elsewhere.