Early Permian rhyolite within a Jurassic accretionary complex in the NorthKitakami Belt and its attribution

Abstract

Mega-porphyritic rhyolite, ca. 2 km long and ca.150 m wide, occurs in the Nakatsugawa Complex of a Jurassic accretionary complex in the southwestern North Kitakami Belt, Kitakami Massif. The rhyolite has been considered as one of the Early Cretaceous dikes occurring ubiquitously in the massif. However, it is proved to be an early Permian body based on ca. 280 Ma obtained by zircon U–Pb dating. Considering that the rhyolite including abundant potassium feldspar is unlikely to a component of an accretionary complex and that many tectonic blocks such as the Ordovician ultramafic and plutonic rocks or the Paleozoic high-P/T schists occur in the Nedamo Belt located south of the North Kitakami Belt, the rhyolite is likewise thought to be a tectonic block emplaced into the accretionary complexes by a post-Jurassic tectonic movement. This mega-porphyritic rhyolite is possible to be a fragment of the late Paleozoic island-arc igneous bodies, which is almost missing in the present Japanese islands.