Abstract
K-Ar ages and chemical compositions of the Cenozoic volcanic rocks from Hamamasu area at western margin of central Hokkaido have been determined with the purpose of obtaining constraints on the tectono-magmatic evolution at a junction between the Kuril and Northeast Honshu arcs. The periods of eruption from this area is distinguished into the following four stages of island arc volcanism : (1) the early Miocene (19 Ma) ; (2) middle to late Miocene (14∼7 Ma) ; (3) late Miocene (7∼6 Ma) ; and (4) Pliocene (4∼2 Ma). The early to late Miocene (19-7 Ma) volcanic rocks have more enriched isotopic ratios (higher 87Sr/86Sr and lower 143Nd/144Nd) and lower Zr/Nb ratio than those of the late Miocene to Pliocene (7-2 Ma) volcanics. The geochemical trends from 19 Ma to 2 Ma in volcanic rocks suggest that the sites of magma generation beneath Hamamasu moved from the lithosphere to the asthenosphere. The early Miocene volcanism along the western margin of central Hokkaido including Hamamasu area is characterized by the bimodal volcanic rocks. We propose that the volcanism may have been resulted from the collision between the Eurasian Plate and the Okhotsk Plate, followed by the dextral strike-slip faulting associated with the formation of the Kuril Basin. The change from an enriched to a depleted source region during late Miocene time is correlated with an inactive interval of the volcanism during 6-4 Ma. The Pliocene volcanism is attributed to the upwelling of asthenosphere in the arc-arc junction.
