Tectonic evolution of the Early Paleozoic proto-East Asian continental margin: Inference from Paleozoic metamorphic rocks in the South Kitakami belt, northeast Japan

To constrain the incipient Pacific-type orogeny and tectonic processes in the Early Paleozoic proto-East Asian continental margin, the Motai-Matsugadaira-Yamagami (MMY) metamorphic rocks in the South Kitakami belt, northeast Japan were investigated. They are divided into two different types: amphibolite-facies rocks associated with serpentinite and blueschist-facies rocks associated with pelitic and psammitic schists. Three geochemical groups are identified from the MMY metamorphic rocks. Groups 1 and 2 resemble geochemical characteristics of mid-ocean ridge basalt and continental arc rocks, respectively. Group 3 exhibits considerable depletion of highly incompatible elements, which is caused by the high degree of partial melting of a hot mantle plume. The zircon U-Pb ages of Group 1 indicate that the protoliths experienced amphibolite-facies metamorphism soon after their formation in the Early Ordovician. Group 2 exhibits a coeval zircon U-Pb age with Group 1. The age distribution of detrital zircons in the MMY psammitic schists shows a peak of 500-400 Ma, the presence of Archean to Neoproterozoic zircons, and the youngest Late Devonian zircon. The following model is proposed for the tectonic evolution of the proto-East Asian continental margin: (1) the formation of an arc in the eastern margin of the South China craton in the Cambrian to Ordovician; (2) the subduction of a spreading axis and an oceanic plateau at the same time as the continental arc formation; (3) the consumption and subduction of arc materials by tectonic erosion; and (4) the formation of the Carboniferous accretionary complex and high-pressure metamorphic rocks under steady oceanic plate subduction. The proposed tectonic evolution model may also be applicable to equivalent Early Paleozoic rocks in southwest Japan.