REE, RbSr and SmNd studies of Norwegian eclogites

William L. Griffin*, Hannes K. Brueckner

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

135 Citations (Scopus)

Abstract

REE analyses and isotopic studies have been carried out on Norwegian eclogites and garnet peridotites to determine the age of metamorphism and the origin and history of the protoliths. Whole-rock REE patterns of eclogites in gneiss vary greatly, even within single outcrops. Many show positive Eu anomalies, indicating plagioclase fractionation and thus gabbroic protoliths. The irregular REE patterns and the common depletion of La and Ce relative to the middle REE, may reflect metasomatic effects during high-P metamorphism. The distribution of REE between clinopyroxene (cpx) and garnet (gnt) in the garnet peridotites is consistent with equilibration at lower T than the corresponding phases of gnt-lherzolite nodules in kimberlites. If the Norwegian gnt-peridotites are fragments of subcontinental mantle, they have undergone post-emplacement equilibration. Most eclogites that have high TMET (> 700°C) also show equilibrium distribution of REE between cpx and gnt, with large spreads in Sm/Nd. Gnt-cpx pairs from five such eclogites give SmNd ages from 407-447 Ma. In lower-T eclogites (∼ 600°C) the REE have not equilibrated between gnt and cpx; no spread in Sm-Nd is observed, and no dates have been obtained. ISr varies from 0.7025 to 0.7245 in 21 samples of eclogites in gneisses. Most orthopyroxene (opx)-free eclogites fall in the range 0.7025-0.7060, while 7 or 8 opx-bearing ones have ISr > 0.715. While most opx-free eclogites may be derived from gabbroic protoliths, the opx-eclogites must either be derived from old Rb-rich protoliths, or have interacted extensively with the surrounding gneisses. The Sr and Nd isotopic data suggest that the rocks studied here all had complex crustal histories prior to the eclogite-forming metamorphism. Mica-cpx or mica-whole rock (WR) RbSr "ages" on Norwegian eclogites range from 1200 to < 400 Ma. Many samples show marked isotopic disequilibrium and the detailed significance of these "ages" is debatable. However, a five-point mineral isochron on one kyanite eclogite gives an isochron age of 398±1 Ma, with ISr = 0.7034. The eclogite-forming event in western Norway was apparently part of theCaledonian continent-continent collision, in which the Western Norwegian Gneiss terrane was overridden by the Greenland plate. The Wenlockian SmNd mineral ages may refer to the beginning of uplift after this collision, while the RbSr mineral ages probably reflect a later stage of the uplift-cooling cycle.

Original languageEnglish
Pages (from-to)249-271
Number of pages23
JournalChemical Geology: Isotope Geoscience Section
Volume52
Issue number2
DOIs
Publication statusPublished - 25 Jul 1985
Externally publishedYes

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