PETROLOGY OF THE KINKASAN GRANITIC BODY, KITAKAMI MOUNTAINS

Abstract

The Kinkasan granitic body, Kitakami mountains, was studied from the petrochemical and mineralogical point of view. From field and microscopic observation, this body is divided into four rock types of A, B, C and D, whose modal compositions are shouwn in Table 1 and Figure 3. A and D types are schistose and protoclastic. Variation trends on the SiO2-oxides diagrams are generally smooth (Figure 4). Mineral chemistry of biotites, hornblendes and magnetites in all rock types shows approximately similar characteristics, suggesting they are co-magmatic. But in the strict sence, petrochemistry and mineralogy of B and C types are very similar, while those of A type are different from B and C types, and moreover those of D type lie between A and B-C types though its SiO2 content is the highest of the four types. Judging from these petrochemical and mineralogical evidences, D type was not differentiated from C type (i.e. in order of A-B-C-D), but from A type directly. The rock types A, B and C were differentiated in a magma chamber located at about 15km in depth. At the upper part of the chamber, about 10km in depth, C type magma was generated by differentiation. At first A type has uprised to the present place, located at about 3.5 to 5km in depth, and differentiated D type in situ. After these two types were deformed by tectonic movement, B and C types were intruded into A type rocks successively. Cr2O3 content in minerals is controlled by that in bulk rocks, while it is more concentrated in magnetites. Behavior of Zn in mafic silicate minerals agrees with the conclusion of Kato (1970), and Zn content in magnetites is roughly proportional to that in the associating biotites (Figure 7) and hornblendes.