Many volcanic rocks in the Miocene age, are known in the green tuff region of Shimane prefecture, the coast of the Sea of Japan.
There are two volcanic rock series : calc-alkaline and tholelitic. The rock series tend to vary with time and stratigraphic level in this region. Both series include basalt, andesite, dacite and rhyolite.
From petrochemical point of view, based on the alkali content, these rocks are classified into the following three types : lower, middle and higher types in K_2O and K_2O＋<Na>_2O content.
The first type occur on the Oda Miocene sedmentary basin underlain by Sangun metamorphic rocks. The second one occur on the Izumo and Matsue Miocene sedimentary basin underlain by Paleogene granite batholith. The last one occurs on the Shimane peninsula district. The basement rocks of the Miocene sediments in this district are composed of rocks which belong to the Hida-Oki metamorphic rocks in the western part and Paleogene granite batholith in the eastern part. The regionality on the petrochemical nature of these volcainc rocks may result from the difference of these basement rocks.
It may be given as a conclusion that the K_2O and K_2O＋<Na>_2O content of volcanic rocks in this region tend to increase with the advancing development of continental-type crust.