Origin of eclogite and pyroxenite xenoliths from the Victor kimberlite, Canada, and implications for Superior craton formation

A suite of 30 eclogite and pyroxenite xenoliths recovered from the Jurassic Victor kimberlite in the western Superior Province are investigated to determine their formation and emplacement in the sub-continental lithospheric mantle (SCLM). The samples have a wide compositional range, including low-Mg and high-Mg varieties. The low-Mg eclogites have a shallow origin as plagioclase-bearing protoliths that were subsequently subducted and emplaced into the SCLM. This is supported by their generally flat MREE to HREE compositions, the presence of kyanite and a positive Eu anomaly in the kyanite-bearing sample, as well as δ18O in three low-Mg eclogites that are higher than the pristine mantle value. LREE depletion in the low-Mg eclogites, along with unradiogenic 87Sr/86Sr indicate that they were not affected by widespread metasomatism after emplacement in the SCLM. The high-Mg eclogites and pyroxenites have compositional characteristics that require a distinct origin to the low-Mg eclogites. Their bulk compositions, LREEN-enriched trace element patterns and in particular, occurrence of unradiogenic 187Os/188Os in pyroxenite, is consistent with formation by reaction of broadly siliceous melts (generated from the melting of low-Mg eclogites) with depleted peridotite. A subduction origin of the eclogites studied here is consistent with seismic and field-based studies that have reported terrane accretion by successive subduction of the west-east orientated terranes in the western Superior Province. Although the timing of eclogite and pyroxenite formation could not be constrained, radiogenic 187Os/188Os require long-term isolation from the convecting mantle and supports a Neorchaean age for their formation.