Timing and duration of regional metamorphism in the Kagasawa and Unazuki areas, Hida metamorphic complex, southwest Japan

Abstract

U–Pb zircon geochronology of two granite intrusions containing bluish‑tint K-feldspars and two biotite-hornblende gneisses in the Kagasawa area, situated at the center part of the Hida metamorphic complex, southwest Japan, reveals the early Triassic low P/T metamorphism in the Hida metamorphic complex (251–247Ma). The granite intrusions into the Hida gneiss occurred at two different stages (235.4±1.6Ma and 183.6±1.8Ma) and thermally affected the Hida gneiss. In the Unazuki area, situated at the northeast margin of the Hida metamorphic complex, a quartzo-feldspathic schist and a biotite granite containing a quartzo-feldspathic schist as xenolith yielded U–Pb zircon ages of 258.0±2.3Ma and 253.0±1.9Ma, respectively, which indicates that the medium P/T metamorphism in the Unazuki area occurred in late Permian. The similarity between the geochemical compositions in whole rocks and zircons of the quartzo-feldspathic schist and the granite suggests that felsic volcanics were deposited on basement rocks at 258Ma and, after affecting the medium P/T metamorphism, residual felsic magma was exhumed while taking the Unazuki schists as a xenolith at 253Ma. Although the time interval between protolith formation and post-peak regional metamorphism in the Unazuki area (0.8–9.2Ma) is similar to that in the Kagasawa area (1–8.6Ma), the different timing of the regional metamorphism indicates that the protolith of the Unazuki metasediments was deposited not on the proximal Hida metamorphic complex but on ancient rocks containing Eoarchean-Paleoproterozoic components of the North China Craton.