Permo-Triassic and Early-Middle Jurassic granitoid clasts from the Jurassic conglomerates in the Mino terrane, central Japan

Abstract

CHIME (chemical Th-U-total Pb isochron method) has been applied to determine the ages of seven granitoid clasts (5 granite, 1 mylonitized granite and 1 granodiorite) in the Jurassic Ohgitani, Bandokoro and Tsukiyozawa conglomerates in the Mino terrane, central Japan. CHIME dating is also applied to detrital zircons in the Jurassic sandstone in the vicinity of the Ohgitani conglomerate and matrix sandstone of the Ohgitani conglomerate. Results of CHIME show that the monazites from the three granite clasts yield isochron ages of ca. 250 Ma (259±7 Ma, 245±4 Ma and 228±39 Ma). Apparent CHIME zircon ages of the other 2 granite and 1 mylonitized granite clasts also show ca. 250 Ma. On the other hand, monazites in a granodiorite clast indicate younger isochron age of ca. 180 Ma than the other granitoid clasts. The CHIME ages from the granitoid clasts indicate emplacement of ca. 250 Ma granites and ca. 180 Ma granodiorite. Detrital zircons from the Jurassic sandstone yield ages ranging from 2031 to 183 Ma, and some age data gathered ca. 250 Ma. The CHIME ages of detrital zircon suggest that Proterozoic (ca. 2000 Ma), Paleozoic (ca. 450, 350 and 250 Ma) and Early-Middle Jurassic (ca. 180 Ma) thermal events in a provenance of Jurassic clastic rocks of the Mino terrane. Both results from the granitoid clasts and sandstones especially suggest ca. 250 and 180 Ma igneous activities in the provenance of the Mino terrane. Occurrence of ca. 250 and 180 Ma granitoid reminds us of igneous activity at eastern margin of the Asian continent, and the Hida terrane which is continental affinity in the Japanese islands.