日高変成帯南東部,角閃石トーナル岩のRb-Srアイソクロン年代 : ピーク変成作用の時期について Rb-Sr isochron ages for hornblende tonalite from the southeastern part of the Hidaka metamorphic belt, Hokkaido, Japan : Implication for timing of peak metamorphism

日高変成帯南東部には, 砂泥質変成岩類を貫く花崗岩類が広く分布する。花崗岩類は, 菫青石トーナル岩と角閃石トーナル岩に区分され, 多くの場合後者が前者を貫く。角閃石トーナル岩のRb-Sr全岩・黒雲母年代は40.5±0.3 Maで, 岩体の冷却年代を示している。一方, 岩体の固結・冷却年代を示すRb-Sr全岩年代は鉱物年代より約10 Ma古く, 51.2±3.7 Maである。両年代の閉止温度を考慮し岩体の冷却速度を求めるとおよそ30〜40℃/Maとなる。菫青石トーナル岩は, 低P/Tで特徴づけられる日高変成帯の広域変成作用をすでに被っていた泥質片麻岩に貫入し, 岩体の周辺では母岩に熱変成を与えている。角閃石トーナル岩は, 泥質片麻岩および菫青石トーナル岩に貫入していることから, 日高変成作用がピークに達した時期は少なくとも51 Maより古くなる。

The southeastern part of the Hidaka metamorphic belt consists mainly of granific rocks and small amounts of metamorphosed pelitic and psammitic rocks. The granitic rocks are divided into two types: cordierite tonalite and hornblende tonalite. In many cases, the hornblende tonalite intrudes the cordierite tonalite. The Rb-Sr internal isochron connecting with biotite, whole rock and felsic fracfion for the hornblende tonalite indicates an age of 40.5±0.3 Ma. The inclination line is defined largely by the biotite because of its high ^<87>Rb/^<86>Sr ratio. Therefore, the internal isochron could be identical with the biotite isochron in the Rb-Sr system. The age of 40 Ma represents the passing time of the hornblende tonalite through the closure temperature of biotite (300℃). On the other hand, the Rb-Sr whole rock isochron age for the hornblende tonalite is 51.2±3.6 Ma (MSWD=0.02) and initial ^<87>Sr/^<86>Sr ratio is 0.70452±0.00004. The age of 51 Ma is regarded as the solidification age for the hornblende tonalite. On the basis of the closure temperature, the cooling rate is 40℃ to 30℃/Ma at the temperature above 300℃. The hornblende tonalite intrudes the pelific gneiss. The cordierite tonalite is cut by the hornblende tonalite, and intrudes the pelitic gneiss. The cordierite tonalite magma gives thermal effect to the host gneiss, which had already undergone the low P/T Hidaka metamorphism. Hence, the timing of the peak metamorphism of the Hidaka metamorphic belt should be older than c. 51 Ma.