On the contact-metamorphic rocks from Iwaizumi, northern Kitakami-mountainland

Abstract

In the Iwaizumi district, northern Kitakami-mountainland, various contact-metamorphic rocks are found along the western margin of the Tanohata granodiorite which is subdivided into two main rock types, the Moichi type and the Otomo type. Petrochemically, especially with respect to K2O/Na2O ratio, the two types of granodiorite are remarkably different from each other, as shown in Fig. 6 (solid circles show Moichi type and open circles show Otomo type). In this respect it is interesting to note the metamorphic behavior in the contact aureoles of both Moichi granodiorite and Otomo granodiorite. The former is characterized generally by sillimanite-andalusite hornfels. Sillimanite and andalusite, however, are replaced by muscovite at the immediate contact where muscovite-banded gneiss is developed. On the other hand, at the contact of the Otomo granodiorite there is no banded gneiss and both sillimanite and andalusite remain stable. Sometimes metamorphosed chert with characteristic quartz-porphyroblasts occurs near the contact. Further, in the eastern aureole of the Otomo granodiorite, cordierite-rocks of the Orijarvi type are found in some places on shear zones which contain the Taro bedded cupriferous pyritic deposit.