Dating of altered mafic intrusions by applying a zircon fission track thermochronometer to baked country rock,and implications for the timing of volcanic activity during the opening of the Japan Sea |
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| Authors: | Hiroyuki Hoshi Hideki Iwano Tohru Danhara Naoyoshi Iwata |
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| Institution: | 1. Department of Earth Sciences, Aichi University of Education, Kariya, Aichi, Japan;2. Kyoto Fission‐Track Co., Ltd, Kita‐ku, Kyoto, Japan;3. Department of Earth and Environmental Sciences, Faculty of Science, Yamagata University, Yamagata, Japan |
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| Abstract: | Once a mafic intrusive rock has become altered, it is generally difficult to obtain a reliable intrusion age using conventional isotopic dating methods. To overcome this problem, this study used zircon fission track (ZFT) thermochronometry to determine the timing of crystallization of altered mafic intrusions. ZFT dating was carried out on samples of baked granite country rock adjacent to dolerite dikes (5–10 m thick) in the Takato area of central Japan. Three granite samples collected within 8 mm of a dike contact yielded consistent ZFT ages of 17–16 Ma, with confined track lengths indicative of the complete annealing of pre‐existing tracks by reheating due to dike intrusion. An older ZFT age was obtained for one granite sample collected within 20 mm of the contact, but confined track length measurements indicate that this is an incompletely reset age that lies between the ZFT age of the unbaked granitic country rocks (ca. 55 Ma) and the emplacement age of the dike. Petrographic examinations suggest that post‐intrusion hydrothermal activity did not influence the ZFT ages. We conclude that the 17–16 Ma ZFT age represents the emplacement age of the dikes. Our results show that ZFT dating of baked country rock is an effective tool for dating altered mafic intrusions, for which other dating techniques are not applicable. In the eastern part of Southwest Japan, dispersed volcanic activity occurred in the late Early to early Middle Miocene (18–15 Ma), and the volcanic belt extended into the forearc. This pulse of activity was possibly related to the injection of asthenospheric material into the trench‐side mantle wedge beneath the Japan arc. We also present young apatite fission track ages (ca. 4 Ma) that may reflect a Middle Miocene or later thermal event associated with local magmatic activity near the Takato area. |
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| Keywords: | apatite dolerite fission track dating fission track thermochronometry geochronology Japan Sea opening mafic intrusion Miocene Southwest Japan zircon |
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