Origin of the greenstones in the Setogawa accretionary complex and their tectonic significance

Abstract

The Setogawa Terrane occupies the outer part of the Shimanto Belt in Shizuoka and western part of Yamanashi Prefectures, central Japan, and is composed of seven thrust sheets of middle Eocene to lower Miocene age. Greenstones in one of the sheets, the Takizawa thrust sheet, were geochemically studied for major and trace elements. The greenstones are classified into two types based on field occurrence: 1) slabs; and 2) blocks. Geochemically the slab-type greenstones are tholeiitic basalts, whereas block-type greenstones are both basalts and andesites. The slab-type greenstones have lower Nb/Y ratios and the chemical characteristics are similar to mid-ocean ridge basalts or to back-arc basin basalts. Alkalic rocks of the block-type greenstones have chemical affinities with hot spot basalts. The slab-type greenstones originated from ocean-floor basalts, whereas block-type ones are fragments of seamount, arc-volcano, and ocean-floor basalts. -from English summary