CONSTRAINING THE GENERATION OF HIGH OXIDE MINERAL CONCENTRATIONS IN HIGH GRADE METAMORPHIC ROCKS

Oxide phases within high grade metamorphic rocks are largely ignored compared to silicate phases except for when assessing the redox state and being noted as a part of a mineral paragenesis. It is becoming increasingly apparent that unusual concentrations of oxide phases (e.g. magnetite, ilmenite and/or spinel) can occur within such high grade metamorphic rocks. However, the nature of their formation remains to be poorly constrained. It is for example unclear, what process or combination of processes allow high (over 50% oxide concentration in a sample in some cases) concentrations. In particular there is an ongoing debate if a single process can explain their formation across all protoliths so that these enrichments could be used to identify particular crustal process(es). A number of processes have been suggested to generate such extreme concentrations of oxides within metamorphic rocks. These include partial melt loss (Morrissey et al., 2016), melt fluxing in a deformation zone (Ghatak et al., 2022), deformation related metamorphic reactions and pre-existing compositional bulk composition variations or a combination thereof.

Within a suite of high grade metapelites from the Rogaland area, Norway, we see variations in mineralogy, such as changes in orthopyroxene and cordierite content with oxide content, variations in grain size, variable layering as well as variable signature of the amount of deformation. Using microstructures, EBSD, EDS and other in situ data we will assess the processes behind the generation of high oxide concentrations within metapelites and what this could mean for crustal processes during high grade metamorphism.