Constraining High Oxide Mineral Concentrations in High-Grade Metamorphic Rocks: Insights from Multidisciplinary Analysis

Oxide mineral phases within high-grade metamorphic rocks are often largely ignored compared to silicate minerals, except for when constraining the redox state of a sample. It is becoming increasingly apparent that unusual concentrations of oxide phases (e.g. magnetite, ilmenite and spinel) are more common in granulite facies metamorphic rocks that previously thought. However, the mechanism of their formation remains poorly constrained. For example, it is currently unclear what process or combination of processes result in high (over 50% oxide concentration in a sample in some cases) concentrations. There is an ongoing debate if a single process can be applied across all protoliths, with the goal that these assemblages could be used to pinpoint particular crustal process(es). A number of mechanisms have been suggested to form such extreme concentrations of oxides within metamorphic rocks. These include melt fluxing in a deformation zone (Ghatak et al., 2022), partial melt loss (Morrissey et al., 2016), deformation related metamorphic reactions and protolith composition or a combination thereof. Within a collection of high grade metapelites from Rogaland, SW Norway, we see variations in mineralogy, including changes in orthopyroxene and cordierite content with oxide concentrations, variations in grain size, variable layering as well as variable signature of the amount of deformation. Using a combination of microstructures, EBSD, EDS, XCT and other data we will assess and illustrate the processes behind the generation of high oxide concentrations within metapelites and what this could mean for crustal processes during high-grade metamorphism. Ghatak, H., Gardner, R. L., Daczko, N. R., Piazolo, S., & Milan, L. (2022). Oxide enrichment by syntectonic melt-rock interaction. Lithos, 414–415, 106617. https://doi.org/10.1016/J.LITHOS.2022.106617Morrissey, L. J., Hand, M., Lane, K., Kelsey, D. E., & Dutch, R. A. (2016). Upgrading iron-ore deposits by melt loss during granulite facies metamorphism. Ore Geology Reviews, 74, 101–121. https://doi.org/http://doi.org/10.1016/j.oregeorev.2015.11.012