ON THE TABITO COMPOSITE MASS IN THE ABUKUMA PLATEAU

Abstract

The Tabito composite mass in the southern Abukuma plateau, consisting of a granitic body and small gabbroic masses, is studied petrographically and petrochemically by the present author. The granitic body is mainly composed of granodiorite and quartz diorite and is divided into three rock types, namely Myojin-ishi, Komuro and Iritabyuto. The Myojin-ishi type has prismatic hornblendes characteristically and often contains a small amount of cummingtonite and pyroxenes. The Komuro type does not contain cummingtonite and pyroxenes, and is intruded by many leucocratic veins derived from the Iritabyuto type. The Iritabyuto type is the most leucocratic of the three and contains dark inclusions ubiquitously. Four gabbroic masses consist mainly of hornblende gabbro with a small amount of cortlandtite and gabbro pegmatite. Modal analyses on 53 rocks and new chemical analyses on 41 rocks are given. As to modal compositions and variation trends on the SiO2-Al2O3, SiO2-MgO diagrams, there is a discontinuity between the Iritabyuto type rocks and other rocks. Dark inclusions are always more mafic than the host rocks.