Revised stratigraphy and zircon U-Pb age data of the Hitachi metamorphic formations in the southern Abukuma Mountains, and reconstruction of the basement of the Northeast Japan Arc before the opening of the Japan Sea

Abstract

We revised the stratigraphy of the Hitachi metamorphic district, based on supplementary U-Pb SHRIMP zircon ages and a review of the geochronological data available for this district. Early to late Cambrian formations and intrusives are grouped into the Cambrian Hitachi volcano-plutonic complex, which consists of the Akazawa Formation, the Tamadare Formation, meta-granites and meta-porphyry. Post-Silurian mica gneiss that occurs near the Tamadare Formation is newly defined as the Takizawa Formation, distinct from the Tamadare Formation. The Daioin and Ayukawa formations are early Carboniferous and early Permian in age, respectively. The contact between the Hitachi volcano-plutonic complex and the Daioin Formation is marked by a prolonged hiatus. There are no geochronological data available for the Omika Formation.Zircon U-Pb age data of the Nishidohira Formation indicate deposition during the Early Cretaceous. The Nishidohira Formation was intruded by Ma granitoids and was metamorphosed during -Ma under conditions of the amphibolite facies, kyanite-sillimanite zone. Sedimentation, metamorphism and exhumation, and intrusion of granitic magma in the terrain of the Nishidohira Formation occurred over a short period of time during the Cretaceous. A similar rapid metamorphism and exhumation is also recorded in the Takanuki metamorphic rocks of the Abukuma Belt.We reconstructed the basement of the Northeast Japan Arc for the period before the opening of the Japan Sea, assuming that the Median Tectonic Line had a linear trend from the Southwest Japan Arc to the Northeast Japan Arc during the Paleogene. After the reconstruction, the Paleozoic Hitachi Formations and the Abukuma Belt move closer to the Jiamusi and Khanka blocks of Northeast China. This result indicates that the Cambrian Hitachi volcano-plutonic complex was originally marginal part of the North China Block.