Direct dating of folding events by 40Ar/39Ar analysis of synkinematic muscovite from flexural-slip planes |
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| Institution: | 1. State Key Laboratory of Geological Processes and Mineral Resources and Institute of Earth Sciences, China University of Geosciences, Beijing 100083, PR China;2. CSIRO Earth Science and Resource Engineering, PO Box 1130, Bentley, WA 6102, Australia;3. School of Earth and Environment, The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia;4. Department of Applied Geology, Curtin University, Bentley, WA 6845, Australia;5. Department of Geosciences, National Taiwan University, Taipei 106, Taiwan;6. Geological Survey of Norway-NGU, 7491 Trondheim, Norway;7. Department of Geology and Mineral Resources Engineering, Norwegian University of Science and Technology-NTNU, 7491 Trondheim, Norway;1. Key Laboratory of Mineral Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, No. 19, Beitucheng Western Road, Chaoyang District, 100029, Beijing, PR China;2. Beijing Institute of Geology for Mineral Resources, 100012, Beijing, PR China;3. Key Laboratory of Earth and Planetary Physics, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, 100029, PR China;1. GFZ German Research Centre for Geosciences, Telegrafenberg, Potsdam, 14473, Germany;2. Department of Earth and Planetary Science, University of California, Berkeley, CA 94720, USA;3. CERMES – Laboratoire Navier Ecole des Ponts Paros Teche, 6-8 Av. Blaise Pascal, Cite Descartes Champs sur Marne, 77455, Marne la Vallee Cedex 2, France;1. Université des Antilles, Géosciences Montpellier, UMR 5243, Campus de Fouillole, 97120 Pointe à Pitre, France;2. Université Rennes 1, Géosciences Rennes, bat. 15 – Campus de Beaulieu, 263 Av du général Leclerc, BP 74205, 35042 Rennes Cedex, France;3. Université de Montpellier, Géosciences Montpellier, UMR CNRS 5243, Campus Triolet, Place E. Bataillon, 34000 Montpellier, France;4. GeoRessources, Université de Lorraine/CNRS/CREGU. ENSG, 2 rue du Doyen Marcel Roubault Vandoeuvre-lès-Nancy F-54518, France;1. A.A. Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the Russian Academy of Sciences, Novosibirsk, Russia;2. Novosibirsk State University, Russia;3. O.Yu. Schmidt Institute of Physics of the Earth, Russian Academy of Sciences, Moscow, Russia;4. B.N. Eltsyn Kyrgyz-Russian Slavic University, Bishkek, Kyrgyzstan;5. The University of Texas at el Paso, USA;6. Nature Research Centre, Vilnius, Lithuania |
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| Abstract: | Timing of folding is usually dated indirectly, with limited isotopic dating studies reported in the literature. The present study investigated the timing of intracontinental, multi-stage folding in Upper Proterozoic sandstone, limestone, and marble near Beijing, North China, and adjacent regions. Detailed field investigations with microstructural, backscattered electron (BSE) images and electron microprobe analyses indicate that authigenic muscovite and sericite crystallized parallel to stretching lineations/striations or along thin flexural-slip surfaces, both developed during the complex deformation history of the study area, involving repeated compressional, extensional and strike-slip episodes. Muscovite/sericite separates from interlayer-slip surfaces along the limbs and from dilatant sites in the hinges of folded sandstones yield muscovite 40Ar/39Ar plateau ages of ～158–159 Ma, whereas those from folded marble and limestone samples yield ages of 156 ± 1 Ma. Muscovite from thin flexural-slip planes on fold limbs and hinges yields ages within analytical error of ～155–165 Ma. Further muscovite samples collected from extensionally folded limestone and strike-slip drag folds yield younger ages of 128–125 Ma with well-defined plateaus. To assess the potential influence of the detrital mica component of the host rock on the age data, two additional muscovite samples were investigated, one from a folded upper Proterozoic–Cambrian sandstone outside the Western Hills of Beijing and one from a folded sandstone sampled 20 cm from folding-related slip planes. Muscovite separates from these samples yield significantly older ages of 575 ± 2 Ma and 587 ± 2 Ma, suggesting that the timing of folding can be directly determined using the 40Ar/39Ar method. This approach enables the identification and dating of distinct deformation events that occur during multi-stage regional folding. 40Ar/39Ar dating can be used to constrain the timing of muscovite and sericite growth at moderate to low temperatures (<400 °C) during folding, yielding well-defined plateau ages and thereby the age of deformation in the upper crust. |
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| Keywords: | Folding Muscovite Direct isotopic dating |
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