Perple_X derived PT-conditions for garnet peridotites from Gurskebotn, Western Gneiss Region, SW Norway

Abstract

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), and higher 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.