Geochronological constraints on Paleoproterozoic crustal evolution and regional correlations of the northern Outer Hebridean Lewisian complex,Scotland |
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| Affiliation: | 1. Swedish Museum of Natural History, Box 50 007, SE-104 05 Stockholm, Sweden;2. Geological Museum, Copenhagen University, DK-1350 Copenhagen, Denmark;1. Key Laboratory of Orogenic Belts and Crustal Evolution, Ministry of Education of China, Beijing 100871, China;2. School of Earth and Space Sciences, Peking University, Beijing 100871, China;3. Institute of Fluid Physics, China Academy of Engineering Physics, Mianyang 621900, China;4. Mineral Physics Institute, Stony Brook University, Stony Brook, NY 11794-2100, USA;5. Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China;6. High Pressure Science and Engineering Center and Department of Physics and Astronomy, University of Nevada, Las Vegas, NV 89154, USA;1. CSIRO Mineral Resources, Private Bag 10, Clayton South, VIC 3169, Australia;2. Australian Synchrotron, 800 Blackburn Road, Clayton, VIC 3168, Australia;3. Department of Earth Sciences, Natural History Museum, Cromwell Road, London SW7 5BD, United Kingdom;4. CSIRO Manufacturing, Private Bag 10, Clayton South, VIC 3169, Australia;1. Department of Applied Geology, The Institute for Geoscience Research (TIGeR), Curtin University, GPO Box U1987, Perth, WA 6845, Australia;2. Laboratory for Crustal Petrology, Department of Geology, University of Maryland, College Park, MD 20742-4211, USA;3. British Geological Survey, West Mains Road, Edinburgh EH9 3LA, UK;4. Earth System Science Research Centre, Institute for Geosciences, University of Mainz, Becherweg 21, D–55099 Mainz, Germany |
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| Abstract: | Ion-microprobe U–Th–Pb geochronological data are presented for four samples from Paleoproterozoic belts in the Lewisian of the northern Outer Hebrides, north-west Scotland. Two of these samples, a tonalite sheet associated with the South Harris igneous complex, and a psammite from the Leverburgh metasupracrustal belt, South Harris, yield zircons with a dominant ca. 1.87 Ga age. These are interpreted as the igneous crystallisation age for the tonalite and the source rock for the psammite, and their age concordance suggests that the latter was developed in an arc basin sequence, derived largely from contemporaneous igneous rocks, and buried during collision, which resulted in documented >1.83 Ga high-grade metamorphism. A diorite from the Paleoproterozoic shear zone at the northern tip of Lewis has a probable 2.7–2.8 Ga protolith age, although its zircons have strongly been affected by Pb-loss during later events culminating in development of low Th/U overgrowths at ca. 1.86 Ga. Zircons from a tonalite from Berneray in the Sound of Harris yield an Archean crystallisation age of ca. 2.83 Ga, with no indication of later disturbance, thus providing a southern limit to the region affected by Paleoproterozoic tectonothermal events. The Paleoproterozoic arc in South Harris represents a major tectonic boundary (active margin) in the Lewisian of the Outer Hebrides, possibly correlated with the Laxford or Gairloch shear zones of the mainland Lewisian. Contrasts in the flanking region geology and geochronology, possibly reflecting lateral heterogeneities, may be introduced by major thrusts and/or extensional faults (e.g. the Outer Isles fault) developed between the shear zones. On a broader regional scale, evidence for a magmatic arc in the Lewisian is consistent with the tectonic style of other ca. 1.9 Ga Paleoproterozoic collisional orogens throughout Laurentia–Fennoscandia, suggesting a reappraisal of the formerly proposed intracratonic evolution of the Lewisian at this time. |
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