(Tertiary TiO2-rich tholeiite from northern part of Motegi district of Tochigi Prefecture, northeast Japan.)

Abstract

Tholeiitic basalts characterized by high TiO2 content were found in the NE Japan arc. These basalts (Motokozawa basalts) occur as lava and small dykes in the lower Miocene formation. Chemical compositions of 9 basalt specimens, 10 clinopyroxenes, 6 plagioclase and 10 Fe-Ti oxides are given. The main constituent minerals are plagioclase, clinopyroxene (augite, ferroaugite, subcalcic augite and pigeonite) and Fe-Ti oxide. The basalts belong to high-alkali tholeiite or quartz tholeiite of Kuno (1968) and Yoder & Tilley (1962). On the basis of petrography and chemistry of the rocks and minerals, the authors suggest that the Motokozawa basalts are produced by the fractional crystallization from the TiO2-rich and K2O-poor primary magma generated in the upper mantle enriched in Ti and depleted in K through a previous partial melting under hydrous conditions.-K.O.