Protoliths of the high-grade amphibolites from the main zone of the Hidaka metamorphic belt in Hokkaido, northern Japan and comparison with greenstones in the northern Hidaka Belt

Kawanami, S.; Nakano, N.; Osanai, Y.; Kagami, H.; Owada, M.

Chishitsugaku Zasshi = Journal of the Geological Society of Japan 112(11): 639-653

2006


Accession: 019816461

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Abstract
The Hidaka belt is situated in the central part of Hokkaido island, northern Japan. The southern part of the belt is composed mainly of Tertiary metamorphic and plutonic rocks defined as the Main Zone of the Hidaka metamorphic belt. Various types of amphibolites, which were metamorphosed under amphibolite-to granulite-facies conditions, are distributed in the Main Zone. The northern part of the belt consists mainly of Cretaceous to Palaeogene sedimentary rocks including non-or weekly-to highly-metamorphosed mafic rocks (referred to as Northern Hidaka greenstones in present study). In this study, we discuss whole rock chemistry including isotopic compositions of the Southern Hidaka amphibolites to realize their protoliths, and we also compare the results with those of the Northern Hidaka greenstones. The various discrimination diagrams using major and trace element compositions suggest N-MORB magmatic characteristics for the Southern Hidaka amphibolites. REEs and NdI values also show this feature. Major and trace element compositions of the Northern Hidaka greenstones show similar characteristics with the Southern Hidaka amphibolites (N-MORB). SrI values of the amphibolites and the greenstones are slightly higher than those of the representative N-MORB composition, possibly due to the isotope interaction between original magma and seawater or sedimentary rock (or sediment). It is noted that the SrI values of both the amphibolite and the greenstones are similar to each other. Based on the N-MORB-like composition of major, trace elements, REEs and NdI values for the amphibolites and major and trace elements for the greenstones as well as their similar SrI values, it is a possible to consider that the protoliths of the Southern Hidaka amphibolites and the Northern Hidaka greenstones were derived from a common N-MORB magmatism.