研究業績

The Tokoro Belt exposed in NE Hokkaido (Japan) represents part of a Late Cretaceous accretionary complex, which includes variously metamorphosed volcanic rocks that are interbedded with chert, lenticular limestone and some fore-arc sedimentary rocks. The Tokoro Belt is notably different from other Late Cretaceous accretionary complexes around the Pacific Rim because of widespread occurrence of basalts and volcaniclastic rocks in it. The Nikoro Group, characterized by widespread occurrence of volcanic rocks, is divided into western, eastern and southern sections based on the internal structure, geochemical affinities and metamorphic grades of their volcanic lithologies. OIB (ocean island basalt)-type volcanic rocks with low-grade metamorphic overprint predominate in the western and southern sections, whereas MORB (mid-ocean ridge basalt)- and OIA (ocean island alkaline basalt)-type rocks in the eastern section with partly high-pressure metamorphism make up the northern part of the eastern section. Trace element patterns display transitional trends from MORB to OIA geochemical affinities. OIB-type rocks display trace element characteristics similar to those of shield volcano lavas on Hawaii, rather than small and mainly alkaline, Polynesian hotspot lavas; furthermore, they show significant HREE (heavy rare earth element) enrichment probably caused by plume?ridge interaction. Widespread OIBs in the Tokoro Belt represents tectonic slices of a large (>80 km wide) Hawaiian-style, seamount shield volcano on the Izanagi oceanic plate that was accreted into the continental margin of Far East Asia in the Late Cretaceous.