北上山地，折壁複合深成岩体の累帯構造

Abstract

Early Cretaceous Orikabe plutonic complex, in South Kitakami Mountains, Northeast Japan, consists of two plutons: Main and North. The Main pluton is divided into three rock types: Tokusenjo, Orikabe, and Sasamori. In the Orikabe type, three subtypes 0-1, 0-2 and 0-3 are recognized. The North pluton is also composed of three rock types: Tsuwamonozawa, Tamogi and Murone. The rocks of the Tokusenjo, Orikabe and Murone types are Kf ?? Qz, while those of the Sasamori and Tamogi types are Qz>Kf. The Main pluton makes up a zoned pluton having more felsic composition outward and a little younger age inward. From the outside to the center of the pluton, rock types are arranged as follows: Tokusenjo, 0-1, 0-2, 0-3, and Sasamori. At each boundary of the rock types and subtypes, rock facies and modal composition change sharply, so that main mechanism of the formation of the zonal structure is considered due to multiple stage of the intrusion. The Tokusenjo type and Orikabe type might have been derived from the same original magma. But the Sasamori type had a different crystallization history from the Orikabe type, because the Sasamori type is characterized by euhedral hornblende and is lower in K, Rb, Ce, Nb, F than the Orikabe type in the same SiO2containing rocks. Major and trace element abundances of the Tokusenjo-type gabbro are similar to those of basalt of Niitsuki Formation, erupted a little earlier than the intrusion of the Orikabe complex. They are produced from the same origin of magmas, which have chemical characteristics of calcalkali basaltic magma of island arc or continental margin.