Ordovician backarc‐basin metadolerite and metabasalt of the South Kitakami Terrane, Northeast Japan. (July 2016) - Uchino, Takayuki
Abstract: Basement rocks that occur along the northern margin of the South Kitakami Terrane in Japan consist of Ordovician ultramafic rocks (Hayachine ultramafic complex), gneissose amphibolite (Kuromoriyama amphibolite), and mafic rocks (Kagura igneous rocks, KIR). The KIR are composed of metagabbro, metadolerite, metabasalt, and minor felsic–intermediate dikes. Although the KIR contain green hornblende due to metamorphism of greenschist to epidote–amphibolite facies, they rarely retain primary brown hornblende. Approximately 30% of the metabasalt shows porphyritic textures, with phenocrysts of saussuritized plagioclase and/or altered mafic minerals. The geochemistry of the common metadolerite and metabasalt of the KIR shows a tholeiite trend, a low TiO2 content, and high Th/Nb and Ti/V ratios. The KIR are therefore indicative of a supra‐subduction zone tectonic setting, which implies a backarc origin (as also indicated by discrimination diagrams). Trace element patterns of the KIR resemble those of the backarc‐basin basalt of the Japan and Yamato basins in the Japan Sea. We propose that the KIR formed during backarc spreading from the Ordovician to Early Silurian. This view is supported by the geochemical data, the tectonic setting of the Hayachine ultramafic rocks, and the provenance of clastics within Silurian sedimentary rocks. Abstract : We show the tectonic model of the South Kitakami Terrane, Northeast Japan, for the Ordovician to Early Silurian, based on the geochemistry of the Kagura igneous rocks. The Kagura igneous rocks developed at the margin of the continental crust during the Ordovician to Early Silurian backarc spreading. Orthoquartzite clasts and Precambrian detrital zircon grains were derived from the continental basement of either the continental crust or the island arc, and deposited in the backarc basin.