PROVENANCE OF ORTHOQUARTZITE CLASTS IN THE LATE JURASSIC AND EARLY CRETACEOUS TETORI GROUP, JAPAN

There are many strata containing quartzite clasts in the Japanese Islands. In the present, there exist no orthoquartzite sequences distributed in Japan. Orthoquartzite sequences are commonly found in upper Precambrian and lower Paleozoic sequences in East Asia. For this reason, many geologists have long considered that orthoquartzite clasts in the Japanese Islands were originated from neighboring countries such as China and Korea. This study deals with the provenance of orthoquartzite clasts in conglomerate beds of the Late Jurassic and Early Cretaceous Tetori Group, northern central Japan. The Tetori Group is divided into three subgroups: they are the Kuzuryu, the Itoshiro and the Akaiwa subgroups, in ascending order. The composition of gravelly clasts in the Tetori Group changes from the granite-dominated type to the orthoquartzite-dominated type through the chert-dominated type with decreasing time. A total of about 440 quartzite clasts were collected from seven conglomerate layers. These layers are from the Yambara and the Okura formations, which is the lowermost part of the Itoshiro and the Akaiwa subgroups, respectively. To identify the source of orthoquartzite clasts in the Tetori Group we have also studied orthoquartzite sequences mainly distributed in central eastern Korea. Texturally and geochemically (trace and rare earth elements), the orthoquartzite clasts of the Tetori Group are very similar to orthoquartzites of the lower Paleozoic sequence, Korea. This result coincides with the previous paleomagnetic and paleocurrent studies on the Tetori Group. Therefore, this study indicates that northern central Japan was connected with central eastern Korea before the opening of the East (Japan) Sea in the Miocene.