Anatomy of Shaku–dake high–Mg diorite, southwest Japan: Lithofacies variations and growth process of high–Mg diorite stock

Abstract

The Shaku–dake body represents a two–pyroxene diorite body (TPD) located in the Northern Kyushu region in southwest Japan and is a member of the Cretaceous Northern Kyushu batholith. Layered structures and feeder dikes in the wall of the TPD body are recognized as magma plumbing systems that result from magma pulses. The magma pulses were likely provided from localized anomalous spots, which have been recognized as the horizontal variations of modal, major, and trace element compositions in the TPD body. Vertical variation in the TPD body indicates irregular changes, of which differentiated facies rich in modal quartz and K–feldspar appear at altitudes of 300–450 m; however, mafic facies rich in modal pyroxenes and MgO, Cr, and Ni contents are also present at the same altitude. Field observation, petrography, and geochemical features suggest that the TPD body could have been formed by multiple magma injections from several sites. On the other hand, the exsolution texture of clinopyroxene appears in the center of the TPD body, indicating slow cooling. Overall, the multiple magma injections prolonged the supra–solidus state. A sheet–on–sheet intrusion model was the most plausible mechanism for addition of andesitic magma to the magma reservoir. Multiple magma pulses may have been involved in the formation of the TPD magma chamber, where the wall of the TPD body cooled quickly and left a layered structure; additionally, the magma pulses may have caused slow cooling in the TPD body interior.