Abstract
The Shoboji Diorite distributes sporadically in the southernmost area of the Motai Metamorphic Rocks. Constituting mafic minerals are composed of clinopyroxene, hornblende and small amount of biotite. The Diorite is massive and heterogeneous, and comprising mainly diorite with subordinate gabbro, tonalite and gabbroic cumulate. Field evidences indicate that the age of the Diorite is post-Motai Metamorphic Rocks and pre-Tobigamori Formation (Upper Devonian). The K-Ar ages of hornblende have the narrow range from 446 to 432 Ma with one exception. The ages are slightly younger than those of the banded amphibolites of the Matsugataira-Motai Metamorphic rocks and the mafic to ultramafic rocks of the Hayachine Complex. The hornblende K-Ar ages indicate that the Diorite belongs to the igneous activities of the Late Ordovician to the boundary of Ordovician-Silurian age. Tonalitic rocks exposed along the Isawa-gawa River at the foot of Mt. Yakeishi-dake in the Ou Mountains, and the Hikami granite in the Ofunato district, may also belong to the products of the same igneous activity. These igneous activities at the Late Ordovician characterize that of the subduction tectonics, and the same igneous activities had continued to Siluro-Devonian and Carboniferous. Further, the characteristics of these activities suggest that the South Kitakami Belt at that time had situated in the island arc or active continental margin along the Yangtze massif in northern border of the Gondwanaland.
