On the Hanawa pluton and surrounding metamorphic rocks in the southern Abukuma mountains

The Cretaceous Hanawa pluton and neighboring metamorphic rocks are described from the geological and petrological viewpoints. The Hanawa pluton consists of, in order of formation, fine-grained quartz diorite and tonalite, foliated tonalite and granodiorite, and main granodioritic rocks. The granodioritic rocks are subdivided into porphyritic type, large sphene-bearing type, mylonitic type, and fine-grained equigranular type. In general the younger the rocks are, the richer in potash feldspar and quartz and the poorer in mafic minerals. Chemical compositions of 13 granodioritic rocks vary within a rather narrow range (SiO2 67-73%), and are similar to those of average Japanese granitic rocks. Mg/(Mg+Fe) ratio of biotite and An content of plagioclase decrease toward the southwestern side of the granodioritic pluton. High-grade metamorphic rocks including orthopyroxene-bearing granulite are distributed far away from the Hanawa pluton, whereas medium-grade metamorphic rocks near or in the pluton. This finding suggests that the intrusion of the Hanawa pluton does not play an important role in the formation of the high-grade metamorphic rocks.