Multiphase solid inclusions in Svartberget peridotite body, Western Gneiss Region (Norway): Implications for crust-mantle interaction and mantle metasomatism

Ultramafic bodies hosted within subducted continental crust can provide unique opportunities to investigate crust-mantle interaction. Crustal-derived melts and fluids can interact with the surrounding mantle and be trapped as primary inclusions in phases crystalized during the process. The study of these inclusions provides direct constraints on crust-mantle interaction during the subduction of the continental crust. In Svartberget (Western Gneiss Region, Norway), a garnet-peridotite body hosted by migmatitic gneiss exhibits a complex network of crosscutting veins with a composition ranging from olivine-garnet-websterite to phlogopite-garnet-websterite and garnetite. Previous studies ascribed the presence of all these different generations of veins to a metasomatic interaction between a crustal-derived fluid (from country rock) and the ultramafic body (Vrijmoed et al., 2013). In order to better investigate this process from the fluid perspective, we present a study of multiphase solid inclusions (MSI) trapped in garnets and clinopyroxenes of phlogopite-garnet-websterite and phlogopite-garnetite veins.

The phlogopite-garnet-websterite is coarse-grained and the major mineral phases are clinopyroxene, orthopyroxene, garnet, phlogopite and amphibole. Two types of MSI, ~ 30 µm in diameter, occur either isolated or in clusters in poikilitic clinopyroxene and skeletal garnet. They frequently exhibit negative crystal shape. These were investigated with micro-Raman spectroscopy to determine the main mineral assemblages. Type I inclusions contain quartz and feldspar polymorphs, i.e., kumdykolite and kochetavite. Type II consists of quartz, cristobalite, pyrophyllite, carbonates, corundum, and CO₂.

Phlogopite-garnetites are dominated by poikilitic garnet and phlogopite with minor clinopyroxene and amphibole. Garnets host primary MSI (~ 30 µm) consisting of quartz, plagioclase, feldspar polymorphs (kumdykolite and bonaccorsite), biotite, muscovite, osumilite, and CO₂ bubbles. Often in these inclusions, zircons occur as accidentally trapped phases.

Type I MSI in phlogopite-garnet-websterite and MSI in the garnetites have a mineral assemblage suggesting a granitoid composition, and thus they represent a trapped granitic melt potentially similar to the original melt that metasomatized the peridotite body. Type II inclusions in phlogopite-garnet-websterite might represent a COH fluid. Further analysis on the geochemistry of these inclusions will help to better constrain crust-mantle interaction, fluid evolution during metasomatism, and incompatible trace elements and volatiles mobility during crustal subduction.

Vrijmoed et al. (2013) Metasomatism in the Ultrahigh-pressure Svartberget Garnet-peridotite (Western Gneiss Region, Norway): Implications for the Transport of Crust-derived Fluids within the Mantle. J Petrol, 54(9), 1815-1848.