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

Abstract

from some highly depleted ophiolites from convergent-margin settings, Heavy-mineral concentrates (garnets, chromites) and xenoliths from and may have formed in a similar situation during the accretion of 21 Cretaceous–Tertiary kimberlite intrusions have been used to map the Hackett and Contwoyto terranes (magmatic arc and accretionary the lithospheric mantle beneath the Lac de Gras area in the central prism, respectively) to the ancient continental Anton terrane at part of the Slave Province. Analyses of Nickel Temperature ( TNi) 2·6–2·7 Ga. The deeper layer is interpreted as a plume head, and Zinc Temperature ( TZn) have been used to place garnet and which rose from the lower mantle and underplated the existing chromite xenocrysts, respectively, in depth context. Paleogeotherms lithosphere at 2·6 Ga; evidence includes a high proportion of the derived from both xenoliths and concentrates lie near a 35 mW/ superdeep inclusion assemblage (ferropericlase–perovskite) in the m conductive model at T Ζ 900°C, and near a 38 mW/m diamond population. This event could have provided heat for model at higher T, implying a marked change in conductivity and/ generation of the widespread 2·6 Ga post-tectonic granites. Proteroor a thermal transient. Plots of garnet composition vs TNi also zoic subduction from east and west may have modified the cratonic show a sharp discontinuity in mantle composition at 900°C. root, mainly by introduction of eclogites near its base. 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