Late Triassic A-type granite boulders in Lower Cretaceous conglomerate of the Hida belt, Japan: Their origin and bearing on the Yamato tectonic line in Far East Asia

Abstract

To identify the origin of the Hida belt in central Japan, geochemistry and U–Pb age of zircons were analyzed for the extra-large granitoid boulders in the Lower Cretaceous fluvial conglomerate of the Jinzu Group. This study clarified that the boulders of granitoids have geochemistry of typical A-type granite, as characterized by high Nb + Y and high Ta + Yb values. U–Pb ages of igneous zircons from three individual granite boulders are concentrated at ca. 220 Ma (Late Triassic). As to Late Triassic A-type granites, there is no corresponding body previously recognized within Japan, whereas identical A-type granites occur in the eastern Songliao block on the immediate west of the Jiamusi block in NE China. The large size of boulders and the fluvial facies of the hosting conglomerate indicate their origin in the Hida belt per se, suggesting the cryptic and/or past occurrence of A-type granite, rather than in NE China. Together with the eastern Songliao block (China), Laoelin-Grodekov (LG) belt (Primorye, Russia), and Yamato Ridge (Japan Sea), the Hida belt forms a unique domain on the immediate west of GSC in Far East Asia, which is characterized commonly by the co-occurrence of Permo-Triassic and Jurassic granitoids, and probably with Late Triassic A-type granite. These confirm that Hida belt represents an allochthonous unit tectonically emplaced onto the rest of Japan, which is composed of the Phanerozoic subduction-related orogenic belt (Nipponides) developed along the Pacific side of Greater South China (GSC) since the Cambrian. For emphasizing a major geotectonic boundary of the Mesozoic granitoid provinces in Far East Asia, we propose the Yamato tectonic line between the easternmost Central Asian orogenic belt and GSC/Nipponides, which is traced for up to 3000 km from the Russia/China/North Korea border to SW Japan.