Alkalic dolerites form many sheets intrusive into the Cretaceous formations widely developed in the Nemuro Peninsula. Sometimes the effects of differentiation in situ and pillow structures are observable in these bodies, and the following three groups are distinguished in the field: (1) differentiated dolerite-monzonite sheets, (2) undifferentiated dolerite sheets, and (3) pillow lava flows. The first group is characterized by the association of rock types ranging from picritic dolerite through dolerite and monzonite to syenite, as the result of gravitative separation of olivine and augite within the sheets. The second group is composed of dolerite only, and sometimes shows remarkable pillow structures in the central parts of the sheets. The third group probably represents offshoots from pillowed sheets of the second group.
Several rock types and their minerals were studied in detail, and some were chemically analyzed. The suite forms an “alkali series” with alkali-lime index of 49.0. Predominance of K20 and H20 are characteristic features that differ from alkalic rocks in other parts of Japan and the surrounding areas. The parent magma is inferred to be potash-rich olivine basalt, probably derived from mica-peridotite in the upper mantle under the continent.
At first, differentiation occurred at a deep level in the upper mantle, and alkali dolerite magma in a slightly advanced stage, ascended toward a magma reservoir at a higher level in the crust. As the Nemuro Formation was being deposited in the deep sea, the dolerite magma began its activity, intruded the well-stratified unconsolidated sediments, and formed pillow structures in the central portions of sheets. The high-temperature melts were in contact with water derived from wet sediments, but still retained their mobility. Columnar joints were formed in the more quickly chilled upper and lower portions where no movement was possible. No differentiation could occur in these thinner sheets.
After deposition of the Nemuro Formation was completed, more primitive parent magma from the deeper reservoir came up and intruded the already consolidated and dewatered sediments as thick sheets, generally more than 100 m in thickness. Since virtually no water was available in this case, pillow structures were not developed, and slower cooling favored differentiation in situ, resulting in the formation of several rock types within the sheets.