Timing of some plutonic intrusions and tectonics in the Hida Mountain Range: An application of LA-ICP-MS U-Pb dating on zircons

摘要

飛騨山脈のいくつかの深成岩に対し，LA-ICP-MSによるジルコンのU-Pb年代測定を行った．対象とした試料はIto and Tanaka（1999）により，高瀬川断層を挟んで冷却年代に顕著な差が見られた試料である．U-Pb年代測定の結果，高瀬川断層の西側に分布する奥黒部花崗岩で65.1 ± 1.6 Ma，同断層の東側に分布する大白沢花崗岩で64.7 ± 2.3 Maが得られたことから，両者は約65 Maのほぼ同時期に貫入したことが分った．大白沢花崗岩にはマイロナイト構造が認められ，2 Ma以降の急激な断層運動による隆起・削剥を反映したものと推定した．高瀬川断層の東側に分布する金沢花崗閃緑岩のU-Pb年代として2.15 ± 0.15 Maが得られ，同花崗閃緑岩は第四紀に貫入した深成岩であることが分った．飛騨山脈において，放射年代測定により確認された，地表に露出する第四紀の深成岩は，滝谷花崗閃緑岩についで二例目となる． Laser ablation inductively coupled plasma mass spectrometry was used to perform U-Pb dating on zircons from granitic rocks in the Hida Mountain Range, central Japan. Samples were targeted for dating that showed a marked difference in cooling age across the Takasegawa Fault (Ito and Tanaka, 1999). The Okukurobe granite to the west of the fault, and the Oshirasawa granite to the east of the fault, yielded ages of 65.1 ± 1.6 Ma and 64.7 ± 2.3 Ma, respectively. These ages imply that both granites were intruded at the same time (ca. 65 Ma). The Oshirasawa granite has a mylonitic texture, which probably reflects rapid uplift and erosion by faulting in the past 2 Myr. The Kanazawa granodiorite to the east of the Takasegawa Fault was dated as 2.15 ± 0.15 Ma, indicating that this pluton was intruded in the Quaternary. In addition to the Takidani granodiorite, this is the second confirmed occurrence of an outcropping Quaternary pluton in the Hida Mountain Range.