The Hidaka metamorphic terrane in the Meguro-Shoya area, Hokkaido, Japan is divided into four progressive metamorphic zones: A-biotite zone; B-cordierite zone; C-cordierite-K-feldspar zone; and, D-sillimanite-K-feldspar zone of the andalusite-sillimanite facies series type of metamorphism. The metamorphic grade ranges from the higher temperature part of the greenschist facies (zone A) through the amphibolite facies (zones B and C) to the lower temperature part of the granulite facies (zone D). The zone boundaries intersect the bedding planes at high angles. P-T conditions estimated are 450-550°C and 2 kbar for zone A, 550400°C and 2-2.5 kbar for zone B, 600450°C and 2.5-3 kbar for zone C and 650-750°C and 3-4 kbar for zone D. The metapelites of zone D were partially melted.At the later stage of the regional metamorphism which is early Oligocene to early Miocene in age, cordierite tonalite and biotite tonalite intrusives associated with segments of the highest grade rocks (zone D) were emplaced into the lower temperature part of the regional metamorphic rocks, giving rise to a contact metamorphic aureole. The thermally metamorphosed terrain (zone C') belongs to the arnphibolite facies and its P-T conditions are estimated to have been 550-700°C and 2 kbar.The P-T-t paths of the Hidaka metamorphism show a thickening-heating-uplifting process. The metamorphism is inferred to have taken place beneath an active island arc accompanied by partial melting of the crust. This paper first discusses the metamorphic facies and the P-T conditions of the Hidaka Key-words: andalusite-sillimanite type meta-west. morphism; Hidaka metamorphic belt; P-T-l path; orogenic belt

The southernmost Hidaka metamorphic belt consists mainly of cordierite tonalite intrusions and pelitic metamorphic rocks ranging from the greenschist to the granulite facies. Anatectic migmatites are common in the higher amphibolite and granulite facies zones. Compositional changes in major, rare earth elements and some other trace metals are so small that they are undetectable among the pelitic metamorphic rocks of zones A + B + C and D, but they are large enough to be detected in the higher am phibolite (zone D) to the granulite facies rocks (zone E). The enrichment of Fe, Mg, Na, Eu, and Sc, and the depletion of K, P, La, Ce, Nd, Cs and Rb are statistically significant in pelitic granulites, while heavy REEs are very variable. The chemical variation of pelitic granulite was derived from the accumulation of plagioclase +garnet. This suggests that more than 50-60% of the total volume of pelitic granulite was melted to produce a large amount of tonalitic magma, leaving pelitic granulite as a restite.Migmatites of the higher amphibolite facies are anatexites, and their K, P, Cs, Rb and light REE content is the same as that of lower grade metamorphic rocks. Migmatites of the higher amphibolite facies melted incipiently to segregate only a small amount of melt, and could not produce a large magmatic mass such as the cordierite tonalites.Cordierite tonalites are S-type granites, and their major elements, Cs, Rb and light REE concentra tions are similar to those of lower grade metamorphic rocks. The chemical variation of cordierite tonalites is explained by the extraction of plagioclase +garnet from a tonalitic magma and the variation of original sedimentary rocks. The small chemical difference between the cordierite tonalites and the lower grade metamorphic rocks suggests that the former was derived from a massive melting of meta pelites or that much of the restite is retained. The material migration among higher amphibolite facies rocks, pelitic granulites, migmatites and cordierite tonalites took place through mineral /melt interac tion in the lower crust.

A catastrophic landslide induced by the 2018 Hokkaido Eastern Iburi earthquake occurred on a slope of distal tephra deposits, which is the largest historic landslides in Japan with an area of 13.4 km 2 . Such landslides do not occur on steep slopes because the tephra deposits themselves do not form slopes steeper than the angle of repose, and occur on gentle slopes shallower than 30°. Geochemical data for the pyroclastic fall deposits indicate sliding surfaces in halloysite-rich weathered pumice fall(loam)could facilitate the effective transfer of land slide mass on gentle slopes.

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