Abstract
Choshi Peninsula, is one of the very limited localities in the Kanto Plain where pre-Cretaceous strata are exposed and thus is a significant location for considering the geological connection of SW Japan and NE Japan. Pre-Cretaceous Atagoyama Group is composed of clastic sediments, chert and limestone, and considered to be correlative with Jurassic accretionary terrane which is widely distributed in SW Japan. We described and dated granitic clasts from the conglomerate in the Atagoyama Group, and discussed their significance. The granitic clasts are medium-grained hornblende-biotite granodiorite to tonalite. They contain primary magnetite grains, which cause high magnetic susceptibility showing magnetite-series characteristics. The K-Ar ages of two clasts are 255 Ma (biotite) and 270 Ma (hornblende). Such Permian granitic clasts can be correlative with the Kinshozan quartzdiorite constituting Paleo-Ryoke terrane forming a nappe structure on the Sambagawa metamorphic belt and with the granitic clasts in Permian Usuginu-type conglomerate in the South Kitakami and Kurosegawa terranes.
