Permian tectonic evolution of the proto-Japan and East Asia: Insights from detrital zircon geochronology and crystal morphology

Abstract

Combination analyses of detrital zircon ages and morphology from Permian sandstones in Japan provide evidence of orogenic evolution caused by collision in East Asia and reveal their paleogeographic setting. This study compares published and newly measured data from Permian fore-arc accretionary complexes (Akiyoshi Belt) and back-arc basin deposits (Maizuru Belt) in the Inner Zone of Southwest Japan. The fore-arc deposits and lower part of the back-arc deposits consist mainly of approximately 300–250 Ma zircon grains that are close to the depositional ages and are generally angular to subangular. These sediments formed around the Late Paleozoic volcanic arc isolated from the older continental crust, and their source rocks were exposed immediately prior to their erosion and transport and were transported directly to the depositional site in a relatively short time. In contrast, upper part of the back-arc deposits generally contains approximately 2,700–1,800 and 500–250 Ma zircon grains, which are commonly older than their depositional ages and are rounded. These grains were supplied from multiple sources, such as the Paleozoic volcanic arc and Precambrian continental block, and were affected by long-distance transport and a strong abrasion effect during transport. Comprehensive detrital zircon data emphasize that the oceanic arc–back-arc basin–continental margin system formed along the eastern margin of East Asia, and that the volcanic arc collided with the eastern margin of the Sino-Korean Block during the Permian. The variation in detrital zircon ages and shapes of back-arc deposits provides a constraint for modeling the East Asian Permian collision. Detritus derived from continental crust was captured by a back-arc basin and blocked by the uplift zone of the volcanic arc. However, a small amount of detritus originating from the continental crust was supplied to the fore-arc.