Zircon U-Pb and fission-track ages of granitic rocks from the Nojima Fault and its vicinity: A new horizon for the LA-ICP-MS U-Pb dating method

摘要

野島断層とその周辺の花崗岩質岩から得たジルコンを用い，FT年代とLA-ICP-MSによるU-Pb年代を求めた．その結果，52～78 MaのFT年代と83～88 MaのU-Pb（238U-206Pb）年代が得られた．今回の結果と既存の放射年代結果から，淡路島北端部の花崗岩質岩が貫入した時代から現在に至るまでの冷却史を明らかにした．今回，U-Pb年代を求めるために，FT年代測定用に準備したジルコンを用いるとともに，Fish Canyon TuffをU-Pb年代の補正に用いた．その結果，Buluk Member Tuffで精度と信頼性の高いU-Pb年代（16.3±0.1 Ma）が得られた．国内外でジルコンのFTに関する論文は多数あるが，今回用いた方法により，FT年代測定用に準備したジルコンを再利用し，ジルコンの晶出年代を付加することに道を拓くことができたと思われる． Zircon U-Pb dating by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) and zircon fission-track (FT) dating were performed on granitic rocks from the Nojima Fault and its vicinity on northern Awaji Island, Hyogo Prefecture, Japan. The analyses yielded zircon FT ages of 52-78 Ma, which are in agreement with previously reported zircon FT ages. Zircon U-Pb (238U-206Pb) ages of 83-88 Ma were also determined. By combining these results with existing radiometric age data, we estimated the entire cooling history of the granitic basement on northern Awaji Island, spanning the period from the time of intrusion to the present. U-Pb ages were determined from zircons that were prepared for FT dating. Moreover, U-Pb ages were corrected based on analyses of Fish Canyon Tuff zircons as a standard material. As a result, a precise and accurate age of 16.3±0.1 Ma was obtained for the Buluk Member Tuff. This result demonstrates that the effect of common Pb is minimal even when analyzing zircons that are chemically etched, and that the Fish Canyon Tuff, which is a well-known age standard for FT dating, can be used as a standard for U-Pb dating. Many previous studies have reported on zircon FT analyses performed throughout the world. The method applied in the present study should pave the way for adding information on the crystallization age of the studied rocks by analyzing the zircons prepared previously for FT dating.