Intrusion and differentiation of the Tsubashiki diorite stock in the western part of Yamaguchi Prefecture, S.W. Japan. Formation of the stock through fractional crystallization and filter-pressing under a hydrous condition.

Abstract

The Tsubashiki diorite body in the western part of Yamaguchi Prefecture, Southwest Japan, is a small stock, with comagmatic dikes of various rocks. It is composed of melanocratic hornblende diorite and mesocratic hornblende quartz diorite, with hornblende-pyroxene diorite intruding them. Comagmatic mafic and felsic dikes are accompanied with the diorite stock. The mafic dikes are composed of pyroxene andesite, hornblende andesite and hornblende porphyrite. The felsic dikes are mainly composed of hornblende-bearing rocks of tonalite, granodiorite and granite, with minor quartz porphyry. Enclaves of andesitic and dioritic rocks occur in some felsic dikes. The SiO2 content ranges from 52 to 59 wt.% in the diorite stock, from 55 to 56 wt.% in the mafic dikes, from 60 to 74 wt.% in the felsic dikes and from 55 to 59 wt.% in the andesitic enclaves. The diorite stock, mafic dikes and the andesitic enclaves are rich in K2O, while the felsic dikes are poor in K2O and rich in Na2O. Whole rock K-Ar ages are 82.8±1.8 Ma for the hornblende-pyroxene diorite in the diorite stock and 77.3±1.7 Ma for the biotite-hornblende granodiorite in the felsic dike, indicating the last stage in the Sanyo-type plutonic activities during Cretaceous in the western Chugoku district. From field relations, modes of occurrence, petrographical features, K-Ar ages and chemical compositions, it is suggested that the Tsubashiki diorite stock was formed by intrusion of a basic andesitic magma under the condition of increase of water and by slight fractional crystallization of ferromagnesian minerals in the hydrous magma. It is also suggested that the mafic dikes were derived from the undifferentiated basic andesitic magma, and that the felsic dikes were formed through filter-pressing differentiation of the residual liquids with magma mixing during the process of cooling of the stock.