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

2013/07/01 by J. C. Vrijmoed, Johannes C. Vrijmoed, H. Austrheim +9
Earth and Planetary Sciences · #Geological and Geochemical Analysis #earthquake and tectonic studies #High-pressure geophysics and materials

paper · pdf · doi:10.1093/petrology/egt032

Abstract

Garnet-peridotites often contain veins or layers of pyroxenite and eclogite of uncertain origin. We investigate the Svartberget garnetperidotite 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^garnetwebsterite, garnet-websterite, orthopyroxene^phlogopite^garnetwebsterite, coarse-grained phlogopite^garnet-websterite, phlogopite arnet-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 SiO 2 and Al 2 O 3 . Concentrations of fluid-mobile elements increase from the most pristine peridotite towards the garnetite, whereas Ni and Cr decrease from 700 to 10 ppm and 2600 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 Na 2 O concentrations in amphibole, and most notably in clinopyroxene, increase from 02 to 30 and from 02 to 8 wt %, respectively. The trivalent ions Cr and Al display complex intragranular 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 3972 AE 12 Ma, formation of leucosomes in host-rock gneiss at 3912 AE 08 Ma, and amphibole-pegmatite in the core of a garnetite vein at 3901 AE 09 Ma. Initial 87 Sr/ 86 Sr values calculated at 397 Ma are elevated (0723) in the most pristine peridotites and increase to 0743 in the most metasomatized samples. The initial 87 Sr/ 86 Sr values of both the host gneiss and its leucosomes are also elevated (0734⁰776), 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

Citations

• Geothermobarometry in Four-phase Lherzolites II. New Thermobarometers, and Practical Assessment of Existing Thermobarometers
• U–Pb dating of monazite and its application to geological problems
• Origin of Pyroxenite-Peridotite Veined Mantle by Refertilization Reactions: Evidence from the Ronda Peridotite (Southern Spain)

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