Magma processes and genesis of the Cretaceous Kawara granodiorite Ushikiri-yama body (Ushikiri-yama granodiorite), North Kyushu, SW Japan

Abstract

The Cretaceous Ushikiri-yama granodiorite is exposed in the northern part of Tagawa city (Fukuoka Prefecture, SW Japan) and is part of the North Kyushu batholith. Three types of dikes with varying compositions are associated with the Ushikiri-yama granodiorite: porphyritic fine-grained diorite (Pr Fine Di), porphyritic fine-grained tonalite (Pr Fine To), and porphyritic fine-grained granite (Pr Fine Gr). The granodiorite intrudes the pre-Cretaceous basement rocks as an isolated stock, which can be divided into two bodies (North and South bodies), separated by a fine-grained granodiorite unit with a hypidiomorphic granular texture. The South body, which contains magmatic epidote, was intruded by the North body. Higher initial Sr isotope ratios of the South body, relative to the North body, indicate that sedimentary rocks were assimilated in the magma of the South body during its emplacement in the middle/upper crust. Geochemical data indicate that the Pr Fine To resulted from mixing of the magmas that produced the South body and the Pr Fine Di, with minor crustal contamination by host sedimentary rocks. Fractional crystallization of plagioclase, biotite, and hornblende in the South body resulted in the differentiation of the evolved Pr Fine Gr. In addition, Pr Fine Di has a sanukitic high-Mg andesite composition that is similar to Cretaceous high-Mg diorite bodies from the North Kyushu batholith. Overall, the results indicate that a variety of igneous processes including magma mixing, fractional crystallization, and assimilation with the host rocks, were responsible for the formation of the Ushikiri-yama granodiorite.