Layered mantle lithosphere in the Lac de Gras area, Slave Craton: Composition, structure and origin

Heavy-mineral concentrates (garnets, chromites) and xenoliths from 21 Creta ceous-Tertiary kimberlite intrusions have been wed to map the lithospheric mantle beneath the Lac de Gras area in the central part of the Slave Provin ce. Analyses of Nickel Temperature (T-Ni) and Zinc Temperature (T-Zn) have been used to place garnet and chromite xenocrysts, respectively, in depth c ontext. Paleogeotherms derived from both xenoliths and concentrates lie nea r a 35 mW/m(2) conductive model at T less than or equal to 900 degrees C, a nd near a 38 mW/m(2) model at higher T, implying a marked change in conduct ivity and/or a thermal transient. Plots of garnet composition m T-Ni also s how a sharp discontinuity in mantle composition at 900 degrees C. Garnets f rom <145 km depth are ultradepleted in Y, Zr, Ti and Ga, whereas those from greater depths (to greater than or equal to 200 km) an similar to garnets from Archean mantle world-wide. Relative abundances of garnet types indicat e that the shallow layer consists of similar to 60% (clinopyroxene-free) ha rzburgite and 40% lherzolite, whereas the deeper layer contains 15-20% harz burgite and 80-85% lherzolite. T estimates on eclogite xenoliths show that all were derived from the deeper layer Xenolith data and garnet composition s indicate that the shallow layer is more magnesian (Fo(92-94)) than the de eper layer (Fo(91-92)), and both layers are more olivine rich than South Af rican or Siberian Archean peridotite xenoliths. the composition and sharply defined structure of the Lac de Gras lithosphere are unique within our cur rent knowledge of archean mantle sections. The shallow layer of this lithos phere section is similar to peridotites from some highly depleted ophiolite s from convergent-margin settings, and may have formed in a similar situati on during the accretion of the Hackett and Contwoyto terranes (magmatic arc and accretionary prism, respectively) to the ancient continental Anton ter rane 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 assembla ge (ferropericlase-perovskite) in the diamond population. This event could have provided heat for generation of the widespread 2.6 Ga post-tectonic gr anites. Proterozoic subduction from east and west may have modified the cra tonic loot, mainly by introduction of eclogites near its base.