Chemical re-equilibration and dynamic recrystallization of minerals in the Almklovdalen Peridotite Massif, SW Norway.

Abstract

The Western Gneiss Region (WGR) of SW Norway is characterized by the occurrence of numerous garnet peridotite bodies that were emplaced from the hanging-wall mantle into the subducted country rock gneisses of the Baltic basement by continental collision during the Scandian phase of the Caledonian orogeny (430-390 Ma). The Almklovdalen Peridotite Massif (APM) is one of such garnet peridotite bodies. The main objective of this Msc thesis is to determine the physical conditions (in terms of pressure and temperature) under which mantle rocks of the APM were emplaced into the subducting continental crust, when the coarse grained mantle wedge M1 porphyroclast assemblage recrystallized into a finer grained M2 assemblage. This is done by electron microprobe (EMP) analyses on samples/minerals from the APM and the application of geothermobarometric calculations to the results of the EMP analyses. The EMP analyses on minerals from the M1 porphyroclast assemblage yield homogeneous chemical profiles (linescans) for the most abundant elements interpreted to be a result of chemical re-equilibration by solid state diffusion during Scandian metamorphism. The contents of less mobile elements like Al in orthopyroxene and Cr in garnet are however lower in minerals from the M2 assemblage than in minerals from the M1 assemblage and have not completely re-equilibrated. Cr in garnet allowed the distinction between high-Cr M1 garnet porphyroclasts and low-Cr M2 garnet, which recrystallized together with M2 Cr-spinel during Scandian deformation/metamorphism. Geothermobarometry results yield similar temperature estimates for the M1 and M2 assemblage near 750 °C at 35 kbar when using the geothermometer/ geobarometer combination of T[OpxBK90] and p[NimisTaylor00], but an underestimation in PT conditions occurs for the M1 assemblage when the combination of T[OpxBK90] and p[BKN90] are used due to the high Al content of M1 orthopyroxene. The lower amount of Al in M2 orthopyroxene, together with the higher pressure estimate for the M2 assemblage from geothermobarometery suggest that the Scandian crustal emplacement of the APM may have involved prograde metamorphism.