Trace element and Sm-Nd 'age' zoning in garnets from peridotites of the Caledonian and Variscan Mountains and tectonic implications

H. K. Brueckner*, J. Blusztajn, N. Bakun-Czubarow

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

84 Citations (Scopus)

Abstract

Ion probe traverses across garnets from peridotites of the Caledonides of Norway and the Variscides of Poland show zoning patterns for Y, V, Zr, Cr, Ti and the REE. The complexly zoned patterns of garnets from the Bystrzyca Górna peridotite, Poland, are interpreted in terms of a changing P-T history (isobaric cooling followed by decompression and cooling). Weak rimward gradients in REE concentrions in garnets from the Almklovdalen and Sandvika peridotites, Norway, may be relicts of the original growth history of the garnets, but the nearly flat Y, V, Zr, Cr and Ti profiles from the same garnets imply a later period of near-homogenization at uniform P-T. Crushed garnet separates from each body were separated into three or more fractions on the assumption that density and magnetic susceptibility vary with Fe/Mg ratio, and Fe/Mg ratios change from garnet core to rim. Sm-Nd garnet-clinopyroxene 'ages' were determined for each fraction to determine whether they are also zoned. Four garnet fractions from the Góry Sowie peridotite give nearly the same ages (397-412 Ma) that are believed to span the interval of garnet growth. Garnet fractions from the Norwegian peridotites define scattered ages (816-1350 Ma) that are suspect, but hint at a Sveconorwegian equilibration event. The data indicate the Variscan and Norwegian peridotites had different histories, despite superficial mineralogical and tectonic similarities. Norwegian garnet peridotites had a long pre-Caledonian history and were extracted from a relatively cold mantle whereas the Variscan garnet peridotites had a comparatively short pre- or Eo-Variscan history and were extracted from a hot mantle.

Original languageEnglish
Pages (from-to)61-73
Number of pages13
JournalJournal of Metamorphic Geology
Volume14
Issue number1
Publication statusPublished - Jan 1996
Externally publishedYes

Keywords

  • Garnet peridotites
  • Ion probe
  • Sm-Nd chronology
  • Trace elements
  • Zoned garnets

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