1. Abstract Master Thesis: Perple_X derived PT-conditions for garnet peridotites from Gurskebotn,
Western Gneiss Region, SW Norway
The Western Gneiss Region (WGR) in SW Norway which is part of the Scandinavian Caledonides, is a
tectonic window exposing the lowest tectonic unit which is mainly composed of Proterozoic
basement rocks of the Baltic shield (Brueckner and van Roermund, 2004). The peridotite bodies
found in the WGR are considered as one of the few potentials for the study of mantle processes
(Spengler, 2006). Samples of garnet peridotites found at Gurskebotn in the Western Gneiss Region
were provided in order to study. Based on mineral composition data gathered with the use of
optical- and electron microscopy combined with bulk rock composition obtained with XRF whole rock
analyses, pressure- and temperature conditions of the M1- and M2 assemblages found in these
garnet peridotites were determined with both the use of the thermodynamic software Perple_X of
Connolly (2009) and geothermobarometry. Perple_X estimates higher temperatures (±100 °C), but
lower pressures (±1 GPa) than obtained by geothermobarometry for the M1-assemblages, whereas it
estimated lower temperatures (±100 °C) and higher pressures (±2 GPa) for the M2-assemblages
compared with geothermobarometry. The formation of the M1-assemblage is associated with the
pre-Caledonian residence in the subcontinental mantle (Lapen, 2009) at 925 – 1025 °C and 3.3 – 3.8
GPa. This was followed by exhumation of the garnet peridotite bodies due to the opening of the
Iapetus Ocean between 1.15 Ga and 450 Ma and resulted in the formation of symplectite rims
around the porphyroclastic M1-garnets. The subsequent continent collision of Laurentia and Baltica,
resulting in the subduction of Baltica, are associated with the formation of the M2-assemblage at 550
– 625 °C and 5.5 – 6.5 GPa.