Journal of Petrology Advance Access originally published online on September 3, 2007
Journal of Petrology 2007 48(10):1843-1873; doi:10.1093/petrology/egm041
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Origins of Xenolithic Eclogites and Pyroxenites from the Central Slave Craton, Canada
1Gemoc arc National Key Centre, Department of Earth and Planetary Sciences, Macquarie University, NSW 2109, Australia
2Lithosphere Services, 4009 Edinburgh Street, Burnaby, BC, Canada V5C 1R4
RECEIVED SEPTEMBER 27, 2006; ACCEPTED JULY 4, 2007
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Abstract |
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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 
REE 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 REE 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 
Nd 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 Nd and Hf 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.
KEY WORDS: eclogites; pyroxenite xenoliths; mantle xenoliths; eclogite trace elements; eclogite Sr isotopes; eclogite Hf isotopes; eclogite Nd isotopes
*Corresponding author. Present address: Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB, T6G 2E3, Canada. Telephone: +61 2 9850 8953. Fax: +61 2 9850 8943. E-mail: saulbach{at}els.mq.edu.au
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