DIFFERENTIATION OF OTAKI DOLERITE SILL, YAMAGATA PREFECTURE

The Otaki dolerite sill, which is about 30 m thick and well-differentiated sill, intruded into the Otaki formation, northern margin of the Shinjo basin, northeastern Japan in the Miocene age. After the intrusion, a magma having about 25 per cent of plagioclase and small amounts of augite and olivine phenocrysts and about 75 per cent of liquid formed many segregation veins (up to 40cm thick) parallel to the contact by fractionation in situ. The compositions of dolerites and segregation veins vary widely with fractionation, showing the clear increase of SiO2, total FeO, Na2O and K2O and the decrease of Al2O3, CaO and MgO. The trends of the crystallization of the Otaki dolerite magma follow those of high-alumina basalt series of olivine-tholeiite series. Petrographically and chemically, there were essentially not the three basalt zones, but the high-alumina basalt zone only asociating with tremendous amounts of calc-alkaline andesites and dacites in northeastern Japan during the lower Miocene to upper Pliocene time. Local alkali basalt and tholeiite were of rare exception. Lateral distributions of the tholeiite, high-alumina basalt and alkali basalt zones crossing the Pacific to Japan Sea side would be established at the early Pleistocene time.