Detrital heavy minerals from Lower Jurassic clastic rocks in the Joetsu area, central Japan: Paleo-Mesozoic tectonics in the East Asiancontinental margin constrained by limited chloritoid occurrences in Japan

摘要

Detrital chloritoids were extracted from the Lower Jurassic sandstones in the Joetsu area of central Japan. The discovery of detrital chloritoids in the Joetsu area, in addition to two previous reports, confirms their limited occurrence in the Jurassic strata of the Japanese islands. This finding emphasizes the importance of the denudation of chloritoid-yielding metamorphic belts in Jurassic provenance evolution, in addition to a change from an active volcanic arc to a dissected arc that has already been described. Possible sources for the detrital chloritoids from the Jurassic sandstones are the Permo– Triassic chloritoid-yielding metamorphic rocks distributed in dispersed tectonic zones (Hida, Unazuki, Ryuhozan and Hitachi Metamorphic Rocks), which are in fault contact with Permian to Jurassic accretionary complexes in the Japanese islands. This is because all of these pre-Jurassic chloritoid-yielding metamorphic rocks have a Carboniferous– Permian depositional age and a Permo–Triassic metamorphic age, whereas a Permian– Triassic metamorphic age on the Hitachi Metamorphic Rocks remains unreported. In addition, most metamorphic chloritoids imply a former stable land surface that has evolved into an unstable orogenic area. Therefore, the chloritoid-yielding metamorphic rocks might form a continuous metamorphic belt originating from a passive continental margin in East Asia. Evidence from paleontological and petrological studies indicates that the Permo– Triassic metamorphic belt relates to a collision between the Central Asian Orogenic Belt and the North China Craton. The evolution of the Permian–Jurassic provenance of Japanese detrital rocks indicates that the temporal changes in detritus should result from sequences of collision-related uplifting processes.