U-Pb and Pb-Pb zircon dating of the older orthogneiss suite in the Silvretta nappe,eastern Alps: Cadomian magmatism in the upper Austro-Alpine realm |
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| Authors: | Bernhard Müller Urs Stefan Klötzli Markus Flisch |
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| Institution: | (1) Institute of Isotope Geology and Mineral Resources, ETH-Zentrum, NO-Building, CH-8092 Zürich, Switzerland;(2) Department of Geology, Laboratory for Geochronology and Isotope Geology, University of Vienna, Franz Grill Strasse 9, A-1030 Vienna, Austria;(3) Brückfeldstr. 23, CH-3012 Berne, Switzerland |
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| Abstract: | Two orthogneiss suites dominate the Silvretta nappe. Primary crystallization of the larger suite ( younger orthogneisses ) is assumed to be Ordovician in age. The second, adjacent magmatic suite consists of older, alkaline to calc-alkaline, ultrabasic, basic to intermediate and granitic rocks known as older orthogneisses. U-Pb data of multigrain zircon fractions, as well as single zircon stepwise evaporation 207Pb/206Pb results suggest a latest Proterozoic to early Cambrian intrusion age for the protoliths of the older orthogneisses as both dating methods yield early Cambrian crystallization ages of 526±7 and 519±7 Ma for an alkaline granite gneiss; similar results were obtained for two neighbouring calc-alkaline orthogneisses (207Pb/206Pb ages of 533 ± 4 and 568 ± 6 Ma, respectively). The crystal habitus corresponds to P5, S19 and S9 zircons of magmatic origin. Whole-rock initial Sr isotope ratios indicate a primitive source. The igneous protoliths of these older orthogneisses represent a fragment of a Cadomian (Pan-African) crust found in places within the basement of the European Hercynides. |
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| Keywords: | Eastern Alps Magmatism Alkaline rocks Geochronology U-Pb Zircon Cadomian Pan-African |
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