On alkaline rocks of the Takakusayama district, Shizuoka

Abstract

The alkaline rocks of the Takakusayama district, Shizouka Prefecture, central Japan, form flows, dikes and shallow intrusive bodies in the Paleogene and the lower Miocene formations and are possibly middle Miocene. The rocks vary from undersaturated alkali olivine basalt and analcite-titan-augite-olivine dolerite through mugearite to oversaturated trachyte and sodatrachyte.
Main constituent minerals of these rocks are plagioclase, alkali feldspar, olivine, clinopyroxene, alkali amphibole, kaersutite, iron ores and analcite Clinopyroxenes vary from diopsidic augite through titanaugite to salite in the basic rocks but only augite is observed in acidic ones. Alkali amphibole, possibly arfvedosonite, is found in soda trachytic rocks, whereas kaersutite occurslocally in basic rocks. Interstitial analcite indicates that it crystallized at the latest stage of the magmatic crystallization.
Sixteen rocks of this district were chemically analysed. They have a range from 45 to 65 per cent SiO₂ and are characterized by high TiO₂, Al₂O₃, Na₂O and H₂O+contents and low ∑FeO and K₂O contents. These features are briefly discussed in comparison with the alkaline intrusive rocks in the other districts of Japan and Sakhalin.