Abstract
Tertiary volcanic rocks in the Hamamasu area along the Japan Sea coast in northwestern Hokkaido are lithologically divided into three formations. These are, in ascending order, the Iwao, Horo and Hamamasu Formations. The Iwao Formation is composed mainly of lavas and pyroclastic rocks of basalt and andesite with subordinate amounts of debris flow deposits and fluvial deposits. The Formation is divided into six units, which are in ascending order, A, B, C, D, E and F units. K-Ar ages include 3.8±0.1 Ma for andesite from the A unit and basalts from the B and C units as well as an andesite dike intruding the B unit. The andesites of the D and F units are dated to be 3.7±0.1 Ma, and a basalt lava of the Hamamasu Formation to be 2.6±0.1 Ma.These stratigraphical results and K-Ar age determinations show that volcanic rocks from the Iwao Formation, and the dike were derived from a volcano which has been erupted during a relatively short period of 3.7 to 3.8 Ma in the eastern part of this area, and the volcano seems to have collapsed, resulting in debris flow deposits and fluvial deposits. After completion of the volcano, the fluvial deposits accumulated on the southern part of this area. Thereafter, at 2.6 Ma the basaltic lavas overlay the volcanic products formed during the 3.7-3.8 Ma volcanic activity, mainly at the southern and northern parts of this area.The geochemical characteristics of basalts from this area, different from those of the Pliocene to Quaternary basalts from the NE Japan arc and the Quaternary basalts from the Kurile arc, may suggest a different origin of the magma from those in the NE Japan and Kurile arcs.
