Major element oxide analyses of Miocene to Quaternary igneous rocks from eastern Shimane Prefecture and Oki Dozen Island show that although arc-related volcanism is dominant in this area, alkaline and tholeiitic rocks are also widespread.
The oldest samples (Dolerites and Omori Formation) are a suite of tholeiitic to mildly calc-alkaline eruptives and intrusives, the latter mutually related by low-pressure fractional crystallization. Younger rocks of the Matsue Formation are alkaline to transitional basalts, some containing conspicuous biotite, with accordingly high K_2O/Na_2O, Volcanic rocks of Oki Dozen Island are an alkali basalt-trachyte suite, related by low-pressure fractionation. The lowermost Pliocene Wakurayama Andesite is succeeded by alkaline to transitional basalts sampled from south of Yasugi City and from Daikon Jima Island.
The oldest tholeiitic rocks coincide with opening of the Japan Sea, and their chemistry is comparable with that from the initial magmatic stages of other ensialic marginal basins. Younger rocks have source regions variably contaminated either by fluids derived from the downgoing oceanic crust beneath the area, or result from hybridisation of magma with the overlying mantle wedge.