Field Mapping and Zircon U–Pb Dating of the Early Paleozoic Amphibolite-Facies Paragneiss in the South Kitakami Belt, NE Japan: Implications for the Sedimentary and Metamorphic History of Proto-Japan

Abstract

The early Paleozoic tectonic setting of proto-Japan remains poorly constrained. In the South Kitakami Belt in Northeast Japan, Ordovician–early Silurian basement rocks and overlying Silurian–Cretaceous shallow marine sediments are widely distributed. The basement rocks include the Hikami Granitoids and the associated Tsubonosawa Metamorphic Rock (TMR), which experienced high-temperature and low-pressure (high-T/P) metamorphism. The TMR comprises biotite gneiss and migmatite derived from a terrigenous clastic protolith. Previously reported radiometric ages of the TMR vary widely, ranging from 450 to 180 Ma. To better understand the origin of the TMR, we produced a detailed geological map and obtained zircon U–Pb ages from biotite gneiss and leucosome in migmatite. The youngest cluster ages are ca. 470 Ma for the gneiss and ca. 440 Ma for the leucosome. Notably, the younger leucosome ages are derived from overgrowths on zircon rims. Since the leucosome is considered to have formed by anatexis of the TMR protolith, the ca. 440 Ma age indicates the metamorphic age of the TMR. In contrast, the ca. 470 Ma age from detrital zircons in the gneiss constrains the depositional age of the TMR protolith to the Middle Ordovician or later. Our data suggest that the protolith formed on the continental shelf during the Middle Ordovician and subsequently experienced high-T/P metamorphism due to intrusion of the Hikami Granitoids, resulting in formation of the gneiss during the latest Ordovician–earliest Silurian. The complex present-day distribution of the TMR reflects folding of a xenolitic TMR sheet incorporated into the Hikami Granitoids, followed by offset and displacement along strike-slip faults with minor vertical components.