Volume 38: January - December 1997

Issue 4: April 1997

Abstract

Petrological and isotopic studies on Palaeozoic high-pressure granulites, Gory Sowie Mts, Polish Sudetes

  • Petrological and isotopic studies on Palaeozoic high-pressure granulites, Gory Sowie Mts, Polish Sudetes

    • PJ. O'Brien, A. Kroner2, P. Jaeckel2,3, E. Hegner4, A. Zelazniewicz5 and R. Kryza6 1Bayerisches Geoinstitut, Universitat Bayreuth, Postfach 101251, D-95440 Bayreuth, Germany and 2Institut fur Geowissenschaften, Universitat Mainz, Postfach 3980, D-55099 Mainz, Germany3Max-Planck-Institut fur Chemie, Postfach 3060, D-55020 Mainz, Germany4Institute fur Geochemie, Universitat Tubingen, Wilhelmstrasse 56, D-72074 Tubingen, Germany5Institute of Geological Sciences, Polish Academy of Sciences, UL. Podwale 75, PL 50-449 Wroclaw, Poland6Geological Institute, University of Wroclaw, UL. Cybulskiego 30, PL 50-205 Wroclaw, PolandCorresponding author Email: patrick.obrien@uni-bayreuth.de

    ABSTRACT

    Granulites of intermediate to felsic composition were investigated from the Gory Sowie (Owl Mts) Massif of the central Sudetes of SW Poland. The strongly deformed granulites are in tectonic contact with ortho- and paragneisses that underwent polyphase deformation and metamorphism before the late Devonian. The metamorphic evolution is best exhibited by intermediate granulites that once contained the primary assemblage ternary feldspar-garnet-clinopyroxene-quartz-kyanite-rutile, from which conditions of 900-1000oC, 15-20 kbar were deduced. A medium-pressure granulite facies overprint (775-910oC, 6.5-8.5 kbar) is documented with the growth of orthopyroxene at the expense of garnet and clinopyroxene and the unmixing of the ternary feldspar to produce antiperthite. U-Pb and Pb-Pb isotopic ratios were measured by the vapour digestion and single grain evaporation techniques for two distinct zircon populations recognized in these samples. Detrital grains yielded 207Pb/206Pb ages between 752+-22 and 2154.7+-1.4 Ma, whereas 19 analyses of grain fractions and single grains of multi-faceted zircons interpreted, from their characteristic spherical 'football' shape, as newly crystallized metamorphic grains, provided a mean 207Pb/206Pb age of 401.5+-0.9 Ma. Values of [epsilon]Nd determined for the time of metamorphism range from -4 to -11, indicating highly heterogeneous protoliths. Nd model ages of 1.4-1.8 Ga reflect a mixture of Archaean and Proterozoic crustal material in the granulite protoliths as for other rocks of the Variscan basement. The zircon age is identical to a recently reported Sm-Nd mineral age of 402+-3 Ma for garnet peridotite associated with the granulites, and suggests formation of the metamorphic zircons at high-pressure granulite facies conditions. The age of 402 Ma is significantly older than ages determined for high-pressure felsic granulites from other parts of the Bohemian Massif and suggests (1) a long period of high temperatures at depth in a thickened crustal wedge, (2) growth of new zircon at different metamorphic stages (high-, medium- or low-pressure granulite facies) in different areas, or (3) spatially separated structural units that underwent the same anomalous high-pressure-high-temperature conditions at different times.

    Keywords: high-pressure granulite; Sudetes; Bohemian Massif; zircon dating; ternary feldspar

    Pages: 433 - 456

    Part of the OUP Journal of Petrology WWW service

    General Information

    Click here to register with OUP.

    This page is maintained by OUP admin

    Last updated 29 Apr 97

    Part of the OUP Journals World Wide Web service.

    Copyright Oxford University Press, 1997