Petrology of the Amami plutonic rocks, Kagoshima Prefecture, Japan.

Abstract

The Amami plutonic rocks occurring in the Amami-O-shima Islands. Kagoshima Prefecture consist of six small bodies; Kasari, Ichi, Kachiura, Koniya, Ankyaba and Uke. These bodies distributed over the islands and mainly composed of adamellite and granodiorite with minor amounts of tonalite, diorite and gabbro. They are divided into peraluminous and metaluminus groups (P and M) based on their mineral assemblage, major and minor chemical compositions. K-Ar ages and Sr isotopic features. Kasari, Ichi, Koniya and Ankyaba bodies comprise the P group, and others the M group. The Kachiura and Uke bodies of the M group gave K-Ar ages of 17.23+ or -0.22 Ma and 110.3+ or -1.2 Ma, respectively. Age from the Kachiura body is similar to that of granitoids in outer zone of southwest Japan (OZSWJ) rather than that of the P group. Mineralogical and geochemical characteristics and the initial Sr ratio of the Kachiura body are similar to those of I-type granitoids of OZSWJ. Muscovite contents and A/CNK ratios in the P group increase continuously from the back arc side to the trench side, and their initial Sr ratios also show the same trend except for the Ankyaba body. The original magmas on the trench side were more affected by pelite-rich continental materials than those on the back arc side. Such spatial variation is also recognized in granitoids of OZSWJ. The Amami plutonic rocks have similar chemical and petrological characteristics to granitoids of OZSWJ, however, ages of the plutonic rocks are different. Common petrological characteristics between both rocks may have resulted from similar tectonic condition.