ORIGINS OF XENOLITHIC ECLOGITES AND PYROXENITES FROM THE CENTRAL SLAVE CRATON, CANADA

Abstract

Major- and trace-element and Sr^Nd^Hf isotopic compositions of garnet and clinopyroxene in kimberlite-borne eclogite and pyroxe- nite xenoliths were used to establish their origins and evolution in the subcontinental lithospheric mantle beneath the central Slave Craton, Canada. The majority of eclogites can be assigned to three groups (high-Mg, high-Ca or low-Mg eclogites) that have distinct trace-element patterns. Although post-formation metasomatism involving high field strength element (HFSE) and light rare earth element (LREE) addition has partially obscured the primary compositional features of the high-Mg and high-Ca eclo- gites, trace-element features, such as unfractionated middle REE (MREE) to heavy REE (HREE) patterns suggestive of garnet-free residues and low Zr/Sm consistent with plagioclase accu- mulation, could indicate a subduction origin from a broadly gabbroic protolith. In this scenario, the low P REE and small positive Eu anomalies of the high-Mg eclogites suggest more primitive, plagi- oclase-rich protoliths, whereas the high-Ca eclogites are proposed to have more evolved protoliths with higher (normative) clinopyrox- ene/plagioclase ratios plus trapped melt, consistent with their lower Mg-numbers, higher P REE and absence of Eu anomalies. In con- trast, the subchondritic Zr/Hf and positive slope in the HREE of the low-Mg eclogites are similar to Archaean second-stage melts and point to a previously depleted source for their precursors. Low ratios of fluid-mobile to less fluid-mobile elements and of LREE to HREE are consistent with dehydration and partial melt loss for some eclogites. The trace-element characteristics of the different eclogite types translate into lower eNd for high-Mg eclogites than for low-Mg eclogites.