PETROLOGY OF THE TABASHINE PLUTONIC COMPLEX, SOUTHERN KITAKAMI MOUNTAINS, JAPAN

The Early Cretaceous Tabashine plutonic complex (TPC) in the southern Kitakami Mountains includes a range of intermediate to felsic rocks, and is divided into the North and South plutons on the basis of petrographical and petrochemical features. Rocks of the North pluton contain larger amounts of pyroxene and are slightly richer in K2O but poorer in Al2O3 than those of the South pluton. On the other hand, rocks of the South pluton scarcely contain pyroxene and are characterized by euhedral crystals of hornblende. A possible petrogenetic interpretation is that they were derived from a common original magma, but underwent different evolutional paths as a consequence of different water content in magma.     The North pluton shows normal continuous zoning, having more felsic composition with decreasing amounts of mafic enclaves inward. Such a kind of compositional zoning may be explained by a combined effect of differentiation occurring in a deeper chamber and mixing between mafic and felsic magmas rather than in situ crystal fractionation.     The TPC is accompanied by a gabbroic mass, quartz-dioritic and tonalitic intrusions, and the Bunatoge volcanics. The gabbroic mass and the TPC are characterized by high K2O content and their emplacement occurred within a short time period. On the other hand, the Bunatoge volcanics and the quartz-dioritic and tonalitic intrusions are distinctly lower in K2O contents. The Bunatoge volcanics erupted prior to the emplacement of the TPC, whereas the quartz-dioritic and tonalitic intrusions were emplaced later than the TPC. Thus, the Early Cretaceous magmatism in the study area is characterized by a complex variation in K2O content with respect to time.