Geochronology and REE geochemistry of zircon and garnet in pelitic gneisses from the Higo metamorphic terrane, Kyushu, Japan: Constraints on the timing of high–temperature metamorphism

摘要

The Higo metamorphic terrane in west–central Kyushu, southwest Japan, consists of a sequence of greenschist– to granulite–facies metamorphic rocks. This study reports the U–Pb Zrn ages and rare earth elements (REE) characteristics of Zrns and Grts from pelitic gneisses in the Higo metamorphic terrane to constrain its timing of high–temperature metamorphism. Based on the morphology, internal textures, occurrences, Th/U ratios, REE patterns and ages of Zrn, and the heavy REE (HREE) partition coefficient between Zrn and Grt, the results indicate that the prograde metamorphism occurred at ~ 120 Ma immediately after precursor deposition at 120–130 Ma and peak metamorphic conditions were attained during the period between 110 and 120 Ma. The ~ 110 Ma age might be indicative of the onset of cooling and exhumation after the peak metamorphic stage. Retrograde metamorphism continued at least to 105 Ma as obtained from Zrns in the granitic intrusion.