Petrology of alkali-dolerite of Oga peninsula, Akita Prefecture

Abstract

An alkali-dolerite dyke with a segregation vein of trachyandesitic composition in the central part intruded into the preTertiary granodiorite and the tuff breccia of the Nishi-Oga formation of lower Miocene at the western end of Oga peninsula. The alkali-dolerite, trachyandesite and clinopyroxene (titanaugite) are chemically analysed and their detailed petrographic description is given. The crystallization differentiation in an undersaturated alkali-dolerite magma yielded a small amount (0.5% of total volume) of oversaturated trachyandesite. The alkali-lime index of the suite is 54.3, and is higher than that of the average Japanese alkalic rocks (51.3). From its geographical position and petrographic character, the Oga alkali-dolerite constitutes the eastern margin of the Circum Japan Sea alkalic rock province at the northeastern Japan.