Origins of xenolithic eclogites and pyroxenites from the central slave craton, Canada

Sonja Aulbach, Norman J. Pearson, Suzanne Y. O'reilly, Buddy J. Doyle

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    77 Citations (Scopus)

    Abstract

    Major- and trace-element and Sr-Nd-Hf isotopic compositions of garnet and clinopyroxene in kimberlite-borne eclogite and pyroxenite 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 eclogites, 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 accumulation, could indicate a subduction origin from a broadly gabbroic protolith. In this scenario, the low ΣREE and small positive Eu anomalies of the high-Mg eclogites suggest more primitive, plagioclase-rich protoliths, whereas the high-Ca eclogites are proposed to have more evolved protoliths with higher (normative) clinopyroxene/plagioclase ratios plus trapped melt, consistent with their lower Mg-numbers, higher ΣREE and absence of Eu anomalies. In contrast, 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 εNd for high-Mg eclogites than for low-Mg eclogites. Within the low-Mg group, samples that show evidence for metasomatic enrichment in LREE and HFSE have lower εNd and εHf than a sample that was apparently not enriched, pointing to long-term evolution at their respective parent - daughter ratios. Garnet and clinopyroxene in pyroxenites show different major-element relationships from those in eclogites, such as an opposite CaO-Na2O trend and the presence of a CaO-Cr2O3 trend, independent of whether or not opx is part of the assemblage. Therefore, these two rock types are probably not related by fractionation processes. The presence of opx in about half of the samples precludes direct crystallization from eclogite-derived melts. They probably formed from hybridized melts that reacted with the peridotitic mantle.

    Original languageEnglish
    Pages (from-to)1843-1873
    Number of pages31
    JournalJournal of Petrology
    Volume48
    Issue number10
    DOIs
    Publication statusPublished - Oct 2007

    Bibliographical note

    Erratum can be found in Journal of Petrology, Volume 48(11), 2233-2234, http://dx.doi.org/10.1093/petrology/egm063

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