Lithosphere formation in the central Slave Craton (Canada): plume subcretion or lithosphere accretion?

Byline: Sonja Aulbach (1,2), William L. Griffin (1,3), Norman J. Pearson (1), Suzanne Y. O'Reilly (1), Buddy J. Doyle (4) Keywords: Mantle peridotite origins; Mantle melting; Plume mantle; Subcontinental lithosphere formation; Lac de Gras lithospheric mantle Abstract: Major-element compositions of minerals in peridotite xenoliths from the Lac de Gras kimberlites provide constraints on the mode of lithosphere formation beneath the central Slave Craton, Canada. Magnesia contents of reconstructed whole rocks correlate positively with NiO and negatively with CaO contents, consistent with variable partial melt extraction. Alumina and [Cr.sub.2]O.sub.3 contents are broadly positively correlated, suggestive of melt depletion in the absence of a Cr--Al phase. Garnet modes are high at a given [Al.sub.2]O.sub.3 content (a proxy for melt depletion), falling about a 7 GPa melt depletion model. These observations, combined with high olivine Mg and major-element relationships of FeO-poor peridotites (3 GPa (residual FeO content being a sensitive indicator of melt extraction pressure), and similar high pressures of last equilibration ([proportional to]4.2 to 5.8 GPa), provide multiple lines of evidence that the mantle beneath the central Slave Craton has originated as a residue from high-pressure melting, possibly during plume subcretion. Apparent low melt depletion pressures for high-FeO peridotites (>7.5 wt%) could suggest formation in an oceanic setting, followed by subduction to their depth of entrainment. However, these rocks, which are characterised by low SiO.sub.2 contents ( Author Affiliation: (1) GEMOC ARC National Key Centre, Department of Earth and Planetary Sciences, Macquarie University, NSW, 2109, North Ryde, Australia (2) Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB, Canada, T6G 2E3 (3) CSIRO Exploration and Mining, N. Ryde, NSW, 2113, Australia (4) Lithosphere Services, 4009 Edinburgh Street, Burnaby, BC, Canada, V5C 1R4 Article History: Registration Date: 03/04/2007 Received Date: 27/09/2006 Accepted Date: 02/04/2007 Online Date: 08/05/2007 Article note: Communicated by T.L. Grove.