Trace element and Sm–Nd ‘age’ zoning in garnets from peridotites of the Caledonian and Variscan Mountains and tectonic implications

Journal of Metamorphic Geology - Tập 14 Số 1 - Trang 61-73 - 1996
Hannes K. Brueckner1, J. Blusztajn2, N. Bakun-Czubarow3
1Queens College of the City University of New York, Flushing, NY 11367, and Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964, USA (email: [email protected])
2Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA
3Institute of Geological Sciences, Polish Academy of Sciences, Warsaw, Poland

Tóm tắt

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 concentrations 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.

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Journal of Metamorphic Geology

Tập 14 Số 1

61-73

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