Potassium/argon ages of Cenozoic igneous rocks from eastern Shimane Prefecture—Oki Dozen Island, Southwest Japan and the Japan Sea opening

Abstract

New K/Ar dates for igneous rocks of the Shimane-Oki district of SW Japan span the Middle Miocene to Late Pleistocene interval. They largely confirm the stratigraphy of this region as indicated by Kobayashi et al. [1982. Geological Map of Shimane Prefecture (1:200,000). Department of Geology, Shimane University (in Japanese with English abstract)]. The three dated Middle Miocene units (Omori and Ushikiri Formations and Dolerites) represent tholeiitic activity coincident with and perhaps causally related to the opening of the Japan Sea. Ages for the Ushikiri and Matsue Formations indicate little overlap of igneous events in contrast to conclusion of other workers. Matsue Formation basalts are compositionally diverse yet have identical eruptive ages. This, and the eruption of calc-alkalic and alkalic rocks throughout t the Pliocene-Pleistocene argue for derivation of magma from a variety of sources Subduction influence was probably provided by the Pacific Plate until the Upper Miocene, then the Philippines Sea Plate, although this aspect requires further work. Alkalic volcanism on Oki Dozen Island largely preceded eruption of alkalic volcanics on neighbouring Oki Dogo Island.