日高変成帯主帯に分布する高変成度角閃岩類の原岩推定および北部日高帯緑色岩類との比較(<特集>日高衝突帯研究の最近の進歩(1)-その深部過程と上昇過程)  [in Japanese] 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(<SPECIAL ISSUE>Recent Advances in Research of the Hidaka Collision Zone, Hokkaido, Northern Japan: Deep-Seated Processes and Tectonic Uplift (Part 1))  [in Japanese]

北海道の中軸部に分布する日高帯の南部地域には,変成岩類と深成岩類が広く分布し,日高変成帯主帯と定義されている.主帯には,角閃岩相〜グラニュライト相の変成作用を被った角閃岩類(主帯角閃岩類)が分布する.一方,日高帯北部地域は,主に堆積岩類から構成され,しばしば緑色岩類(北部日高帯緑色岩類)を含む.本論では,同位体組成を含む全岩化学組成を基に,主帯角閃岩類の原岩を推定し,北部日高帯緑色岩類と比較した.主帯角閃岩類は,主成分・微量・希土類元素組成およびNd同位体比初生値から,北部日高帯緑色岩類は主成分元素および微量元素組成からN-MORB起源であると考えられる.両者のSr同位体初生値は,同位体的相互作用の影響を受け一般的なMORB組成よりも高いが,両者の間で調和的な値を示す.したがって,主帯角閃岩類と北部日高帯緑色岩類の原岩は,N-MORB起源の同一の火成作用によって形成された可能性が高い.

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.