Metasomatism in the Ultrahigh-pressure Svartberget Garnet-peridotite (Western Gneiss Region, Norway): Implications for the Transport of Crust-derived Fluids within the Mantle

J. C. Vrijmoed*, H. Austrheim, T. John, R. C. Hin, F. Corfu, G. R. Davies

*Corresponding author for this work

Research output: Contribution to journalArticle (Academic Journal)peer-review

46 Citations (Scopus)


Garnet-peridotites often contain veins or layers of pyroxenite and eclogite of uncertain origin. We investigate the Svartberget garnet-peridotite from the northernmost ultrahigh-pressure domain in the Western Gneiss Region (WGR) in Norway and show that the observed layering represents a sequence of metasomatic reaction zones developed along a fracture system. From the garnet-peridotite wall-rock to the fractures the following sequential reaction zones are recognized: clinohumite bearing garnet-peridotite, olivine-garnet-websterite, garnet-websterite, orthopyroxene-phlogopite-garnet-websterite, coarse-grained phlogopite-garnet-websterite, phlogopite-garnet-websterite, phlogopite-free garnet-websterite, inclusion-rich garnetite, garnetite, eclogite, retrograde omphacitite and felsic amphibole-pegmatite. The MgO, FeO and CaO contents generally decrease from the pristine peridotite towards the most metasomatized samples, with an associated increase in SiO2 and Al2O3. Concentrations of fluid-mobile elements increase from the most pristine peridotite towards the garnetite, whereas Ni and Cr decrease from similar to 700 to similar to 10 ppm and similar to 2600 to similar to 25 ppm, respectively. Changes in mineral mode are accompanied by changes in mineral chemistry. All minerals display decreasing Mg# and Cr content with degree of metasomatism, whereas Na2O concentrations in amphibole, and most notably in clinopyroxene, increase from 0 center dot 2 to 3 center dot 0 and from 0 center dot 2 to 8 wt %, respectively. The trivalent ions Cr and Al display complex intra-granular vein-like or patchy zoning in garnet and pyroxenes that may be characteristic of metasomatized peridotites. Dating by the U-Pb method suggests metamorphic growth of zircon in the garnetite at 397 center dot 2 +/- 1 center dot 2 Ma, formation of leucosomes in host-rock gneiss at 391 center dot 2 +/- 0 center dot 8 Ma, and amphibole-pegmatite in the core of a garnetite vein at 390 center dot 1 +/- 0 center dot 9 Ma. Initial Sr-87/Sr-86 values calculated at 397 Ma are elevated (similar to 0 center dot 723) in the most pristine peridotites and increase to similar to 0 center dot 743 in the most metasomatized samples. The initial Sr-87/Sr-86 values of both the host gneiss and its leucosomes are also elevated (0 center dot 734-0 center dot 776), which suggests that the leucosomes found in the gneisses are the most likely, now solidified, remnants of the reactive agent that metasomatized the Svartberget peridotite. A scenario is envisaged in which material derived from the country rock gneiss was the source of the metasomatic addition of elements to the peridotites and the gneisses acted as the host for all elements removed from the peridotite. The Svartberget peridotite may provide an important analogue of how felsic, slab-derived material interacts with the overlying mantle wedge peridotite in regions of arc magma generation.

Original languageEnglish
Pages (from-to)1815-1848
Number of pages34
JournalJournal of Petrology
Issue number9
Publication statusPublished - Sep 2013


  • garnet-peridotite
  • mantle wedge metasomatism
  • ultrahigh-pressure metamorphism
  • metasomatic eclogite
  • Western Gneiss Region
  • FE-TI
  • FLOW
  • PB


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