Magmatic Processes of the Upper Cretaceous Susuma–Nagaho Plutonic Complex, Southwest Japan: Its Role on Crustal Growth and Recycling in Active Continental Margins

Abstract

Magmatic processes in the active continental margins are one of the important issues to understand the evolution of the continental crust. The Cretaceous Susuma–Nagaho plutonic complex, southwest Japan, is situated at the continental arc, and made up of gabbro, quartz diorite to granodiorite, and granite. According to the field occurrence, they are coeval intrusive rocks, and the biotite K–Ar ages of the granodiorite and granite are approximately 93 Ma, corresponding to the period of a magmatic flare-up in southwest Japan. Based on the whole-rock chemical analyses including Sr–Nd isotopic compositions, the granodiorite magma has been formed through fractional crystallization of basaltic magmas, whereas the origin of granite magma involved partial melting of the continental crust. The gabbro contains calcium-rich plagioclase (An > 90) and the presence of early crystallized hornblende, indicating its derivation from a hydrous basaltic magma. Such basaltic magma intruded into the middle to lower crust and supplied the heat energy necessary for crustal partial melting and granitic magma formation. The fractional crystallization and crustal melting took place at the same time, playing an important role in the crustal growth and differentiation during the magmatic flare-up event.