Comparative equilibration temperatures and pressures of garnet lherzolites in Norwegian gneisses and in kimberlite

Abstract

The chemistry of the pyroxenes suggests that the garnet lherzolites enclosed in the Norwegian basal gneisses have equilibrated at depths greater than 70 kilometres along an expected sub-continental geotherm. Such depths are somewhat shallower than the apparent depths of origin of most garnet lherzolite xenoliths in kimberlite pipes. Distribution coefficients for Fe 2+/Mg 2+ and Mn 2+/Mg 2+ between coexisting clinopyroxenes and garnets support the slightly lower equilibration temperatures deduced for the Norwegian garnet lherzolites compared with the xenolithic garnet lherzolites in kimberlites. The pressure-temperature equilibration conditions deduced for the Norwegian garnet lherzolites (800–1020°C at 22–37 kbs) contrast with previous estimates (625 ± 30° at 14 kbs) for basic eclogite masses in the Norwegian gneisses. This suggests a possible dual paragenesis of the Norwegian eclogites, with the garnet lherzolites being tectonic slices of the sub-continental upper mantle and the basic eclogites deep crustal metamorphic rocks.