Abstract
The bore hole HO-1 dolerite sill, which is about 120m thick and well-differentiated sill, intruded into the Okuzu formation (middle Miocene), about 5km NNW of Odate city, northeastern Japan in the middle Miocene age, after the formation of the Kuroko ore deposits in this district. After the intrusion, a magma having about 10 per cent by volume of olivine and auigte phenocrysts formed several segregation veins composed of soda monoznite and/or soda syenite subparallel to the contact by fractionation in situ. Thirteen representative rocks have been analyzed chemically (Table 1). Petrographically, the HO-1 dolerite belongs to the calc-alkalic basalt having high-alkali tholeiite (Kuno, 1968b) or olivine tholeiite (Yoder and Tilley, 1962) in composition. The compositions of olivine dolerite-soda syenite suite vary widely with the fractionation, showing the clear increase of SiO2 and Na2O and the decrease of CaO and MgO. The crystallization trend of the dolerite magma does not follow that of calc-alkalic series, but follows alkalic series. Such unusual trend would be attributable to the presence of small amounts of water in the magma.
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