Layered Mantle Lithosphere in the Lac de Gras Area, Slave Craton: Composition, Structure and Origin.

Heavy-mineral concentrates (garnets, chromites) and xenoliths from 21 Cretaceous-Tertiary kimberlite intrusions have been used to map the lithospheric mantle beneath the Lac de Gras area in the central part of the Slave Province. Analyses of Nickel Temperature (TNi) and Zinc Temperature (TZn) have been used to place garnet and chromite xenocrysts, respectively, in depth context. Paleogeotherms derived from both xenoliths and concentrates lie near a 35 mW/m2 conductive model at T <= 900°C, and near a 38 mW/m2 model at higher T, implying a marked change in conductivity and/or a thermal transient. Plots of garnet composition vs TNi also show a sharp discontinuity in mantle composition at 900°C. Garnets from <145 km depth are ultradepleted in Y, Zr, Ti and Ga, whereas those from greater depths (to >= 200 km) are similar to garnets from Archean mantle world-wide. Relative abundances of garnet types indicate that the shallow layer consists of ~60% (clinopyroxene-free) harzburgite and 40% lherzolite, whereas the deeper layer contains 15-20% harzburgite and 80-85% lherzolite. T estimates on eclogite xenoliths show that all were derived from the deeper layer. Xenolith data and garnet compositions indicate that the shallow layer is more magnesian (Fo92-94) than the deeper layer (Fo91-92), and both layers are more olivine rich than South African or Siberian Archean peridotite xenoliths. The composition and sharply defined structure of the Lac de Gras lithosphere are unique within our current knowledge of Archean mantle sections. The shallow layer of this lithosphere section is similar to peridotites from some highly depleted ophiolites from convergent-margin settings, and may have formed in a similar situation during the accretion of the Hackett and Contwoyto terranes (magmatic arc and accretionary prism, respectively) to the ancient continental Anton terrane at 2·6-2·7 Ga. The deeper layer is interpreted as a plume head, which rose from the lower mantle and underplated the existing lithosphere at 2·6 Ga; evidence includes a high proportion of the superdeep inclusion assemblage (ferropericlase-perovskite) in the diamond population. This event could have provided heat for generation of the widespread 2·6 Ga post-tectonic granites. Proterozoic subduction from east and west may have modified the cratonic root, mainly by introduction of eclogites near its base. [ABSTRACT FROM PUBLISHER]