No abstract

Small-volume plutons of Early to Late Cretaceous ages are widely distributed in the Yamizo Mountains, central Japan. These plutons consist predominantly of granitoids, classified into hornblende gabbro, quartz diorite, hornblende-biotite granodiorite and coarse-grained biotite granite. The quartz diorite (52-64 wt% of SiO 2 ) is characterized by a high Sr content (606-769 p.p.m.) associated with a low Y (13-27 p.p.m.) and heavy rare earth element content (Yb content of 1.19-2.13 p.p.m.). On the Sr/Y versus Y diagram, this rock type mainly plots in the adakite and Archean high-Al tonalite, trondhjemite and granodiorite (TTG) field. Together with its initial Sr isotopic ratios, which range from 0.7038 to 0.7046, these data suggest that quartz diorite originated as slab melts. However, geochemical calculations assuming either eclogite or garnet amphibolite as the source material do not support this suggestion. Instead, the chemical compositions of quartz diorite are better explained by the fractional crystallization of hornblende, plagioclase and biotite from a primitive, basaltic melt in a magma chamber. In this case, the formation of the associated hornblende gabbro can also be explained by the accumulation of hornblende and plagioclase. Adakitic rocks of Early Cretaceous ages have also been reported in the Tamba Belt of the inner zone of southwest Japan, located ca 500 km west of the Yamizo Mountains. These rocks can be correlated to the adakitic rocks in the Yamizo Mountains based on the geology, petrography, geochemistry and radiometric ages. Therefore, we propose the possibility that the Early Cretaceous adakitic rocks in the inner zone of southwest Japan were produced by fractional crystallization from basaltic arc magmas generated by a partial melting of metasomatized wedge mantle peridotite.

平成 25 年 8 月 30 日受付，平成 26 年 12 月 31 日受理，平成 27 年 3 月 20 日早期公開，担当編集委員亀井淳志特集号ゲスト編集 委員) (独)海洋研究開発機構地球内部ダイナミクス領域，〒237 0061 横須賀市夏島町 2 15  現所属千葉工業大学工学部，〒275 0016 千葉県習志野市津田沼 2 17 1   新潟大学大学院自然科学研究科，〒950 2181 新潟市西区五十嵐二の町 8050 † 岩手大学教育学部，〒020 8550 盛岡市上田 3 18 8 † † 山形大学理学部，〒990 8560 山形市小白川町一丁目 4 12  東北大学理学部，〒980 8578 仙台市青葉区荒巻字青葉 6 3