An investigation of the metamorphic evolution of the Kvalvika garnet peridotite body, Flemsøya, Nordøyane, WGR, SW Norway

Abstract

Quantitative analyses of pressure and temperature conditions are combined with microstructural and mineral compositional analyses to reconstruct the PT path of the Kvalvika garnet peridotite (KGP) lens situated in the Northern (U)HP domain of the Wester Gneiss Region (WGR), Flemsøya island, SW Norway. The KGP lens is an orogenic garnet-spinel peridotite with different modal proportions of olivine ±garnet ±clinopyroxene ±orthopyroxene ±spinel defining compositional banding expressed as garnet clinopyroxenite and garnet websterite bands. Structural observations, using naked eye, light-optical and electron-optical microscopic techniques, together with mineral compositional analyses (EMP) of garnet phenocryst, revealed the presence of 3 stable mineral assemblages M1-M2-M¬3 seemingly equivalent to previously reported studies of garnet peridotite bodies at the (U)HP domain of the WGR. Specifically, 2 generations of garnets, called M1 and M2 were identified with microstructural evidence indicating that M2 garnet together with Cr-rich spinel are the recrystallized product of the M1 garnet. Thermodynamic modeling, using the software Perple_X and the available upgraded solid solution data set for Cr-rich phases, yield pressure and temperature conditions for the M1 garnet of T=1500°C and P=3,4GPa and for the M2 garnet, of T=1000°C and P=2,6GPa. Supplementary, classic/empirical geothermobarometric methods were also applied to the M3a assemblage of the garnet peridotite and to the garnet pyroxenite rocks. The results indicate P=2,8GPa and T=700-720°C The resulted PT conditions allowed the reconstruction of the PT path of the KGP which includes sub-isobaric cooling from asthenospheric temperatures around 1500°C to 1000°C and pressures around 3,4GPa to 2,6GPa, Finally, the PT path of the KGP lens is comparable to the early tectonometamorphic evolution of garnet peridotite lenses situated in Otrøy and Bardane from the (U) HP domain of the WGR implying similar geodynamic evolution.