Emplacement of Granitic Rocks in the Outer Zone of Southwest Japan and Geological Significance

Abstract

With respect to the modes of occurrence, petrographical and chemical characteristics, and the responses during tectonic movements, the Southwestern Outer Zone-type Miocene granitic rocks, which are mainly within the Shimanto major belt in the outer zone of Southwest Japan, have characteristics different from either the synkinematic granitic rocks of the Hidaka major belt in northern Japan or the trondhjemitic rocks of the green tuff region in northeastern Japan, though the developmental stages of the Shimanto, Hidaka and Green Tuff orogenies are about contemporaneous with Alpine orogeny in relation to tectonic events on a world-wide basis. The Southwestern Outer Zone-type granitic rocks have not been affected either by significant regional metamorphism or by post-emplacement deformation. The kinds of xenoliths that occur in the granitic rocks, and the contamination-trends of the rocks, considered in the light of experimental studies on rock melting make it likely that the magmas were generated by early partial melting of felsic materials or geo-synclinal sediments plus later assimilation and contamination. The granitic rocks of the Southwestern Outer Zone-type were formed by post-kinematic acid plutonism of the Shimanto orogeny, and represent one of a series of products of the Alpine orogenic movement in the Circum-Pacific regions.