Mineralogy of the Orikabe plutonic complex, Kitakami Mountains, Northeast Japan.

Abstract

The Orikabe plutonic complex in the Kitakami Mountains is divided into Main and North plutons. The Main pluton is composed of three rock types; i.e., the Tokusenjo type (gabbro), Orikabe type (monzodiorite-monzogranite) and Sasamori type (granodiorite). The oxidation states of magmas around solidus are estimated from coexisting Fe-Ti oxides as follows; near the FMQ and NNO buffers in the Tokusenjo type, in the range from the FMQ to HM buffer in the Orikabe type, and intermediate between the NNO and HM buffer in the Sasamori type. The total pressure for the Orikabe and Sasamori type magmas at their solidus temperatures is estimated to be about 2-3 kb by hornblende geobarometer. Hornblende of the Sasamori type has cores rich in A1IV and Ti, whereas that of the Orikabe type is generally poor in them. It indicates that hornblende in the Orikabe type began to crystallize at lower temperature than that in the Sasamori type, which is concordant with microscopic observation of the Orikabe type rocks, suggesting crystallization of hornblende simultaneous with or later than biotite. The delay of hornblende appearance in the Orikabe type is due to lower water content of the magma than that of the Sasamori type. In gabbros of the Tokusenjo type, the coexistence of Fe-rich olivine and Ca-poor plagioclase and scarcity of hornblende are characteristic, indicating a H2O poor magma (less than 3 wt. %) of the Tokusenjo type. Chemical composition of the minerals in the Tokusenjo type rocks is continuous to that of the Orikabe type with increase in SiO2 of host rocks, suggesting continuous differentiation of magmas of the both types.