K-Ar ages of the igneous rocks in the Senmaya-Kesennuma area, southern Kitakami Mountains

K-Ar ages of six hornblendes and two biotites were determined for the igneous rocks in the area around Senmaya and Kesennuma, southern Kitakami Mountains. The Niitsuki Formation, mainly composed of andesitic-basaltic volcanic rocks, is exposed in the eastern part. The Orikabe Plutonic Complex, characterized by quartz monzodiorite and granodiorite, intrudes into the Niitsuki Formation, and is intruded by porphyrite dike swarms in the northern part. The Senmaya Tonalitic Pluton, mainly composed of homogeneous tonalite, is widely exposed in the western part. Hornblende ages from andesitic fragments of the Niitsuki Formation, a granodiorite of the Orikabe Plutonic Complex, and a hornblende porphyrite dike are 121, 120 and 107 Ma, respectively. Hornblende and biotite ages for the Senmaya Tonalitic Pluton range from 105 to 108 Ma and from 109 to 112 Ma, respectively. The hornblende ages for the Niitsuki Formation and the Orikabe Plutonic Complex imply their close relationship, and they are typical K-Ar ages for Cretaceous volcanic and plutonic rocks in the Kitakami Mountains. On the other hand, the hornblende and biotite ages for the Senmaya Tonalitic Pluton and porphyrite dikes are the youngest among the plutonic rocks in the Kitakami Mountains, indicating they are the products of the later igneous activity. The obtained K-Ar ages in this area imply a shift of plutonic activity to the western continental side after the main plutonism in the Kitakami Mountains in the Early Cretaceous.