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Geologische Rundschau

, Volume 85, Issue 4, pp 822–831 | Cite as

Early Cambrian oceanic plagiogranite in the Silvretta Nappe, eastern Alps: geochemical, zircon U-Pb and Rb-Sr data from garnet-hornblende-plagioclase gneisses

  • Bernhard Müller
  • Urs Schaltegger
  • Urs Klötzli
  • Markus Flisch
Original paper

Abstract

Garnet-hornblende-plagioclase gneisses rich in incompatible elements occur in the crystalline basement of the Austro-Alpine Silvretta nappe and are associated with clinopyroxene norites and harzburgite cumulates. It is proposed here that the gneisses were formerly oceanic plagiogranites. An εNd(530) value of +5.6 for the gneisses as well as initial87Sr/86Sr values of 0.7036–0.7037 for the gabbroic rocks and 0.7026–0.7027 for the ultramafic rocks suggest a mantle source for this rock association. The geochemical characteristics of the garnet-hornblende-plagioclase gneisses indicate that their precursors were derived by fractional crystallization from a basaltic parent magma, by the same process which produced the adjacent clinopyroxene norites and ultramafic cumulates as well. The combined U-Pb upper intercept ages of zircons from two gneiss samples yield an igneous crystallization age of 532 ± 30 Ma, similar to previously dated (mostly calcalkaline) orthogneisses in the same area. High-quality transparent zircons showed the least degree of discordance, but contain extremely low U and Pb levels. The rock suite, including this plagiogranite, was emplaced within oceanic crust which formed in the latest Precambrian-early Palaeozoic off the northern continental margin of Gondwana.

Key words

Austro-Alpine Eastern Alps Oceanic rocks Rb-Sr Zircon U-Pb Magmatism 

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Copyright information

© Springer-Verlag 1996

Authors and Affiliations

  • Bernhard Müller
    • 1
  • Urs Schaltegger
    • 1
  • Urs Klötzli
    • 2
  • Markus Flisch
    • 3
  1. 1.Department of Earth SciencesInstitute of Isotope Geology and Mineral ResourcesZurichSwitzerland
  2. 2.Department of Geology, Laboratory for Geochronology and Isotope GeologyUniversity of ViennaViennaAustria
  3. 3.BernSwitzerland

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