Isotopic fingerprinting (Sr-Nd-Hf-Os-C-O) of mantle source regions to kimberlite magmatism beneath the eastern Superior Craton, Canada
Date
2016
Authors
Brand, Natalie Bronwyn
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Abstract
The northern sector of the Archaean Superior Craton has been a significant region for diamond exploration, hosting numerous alkaline intrusions of Proterozoic age. The focus of this study is on two kimberlite fields that are situated in eastern Canada, 400 km apart. These are the diamond-rich Renard pipes and dykes, and the Wemindji field, consisting of barren sheeted dykes. The nine diamondiferous Renard igneous bodies were emplaced between 655-630 Ma in the eastern sector of Laurentia into Archaean metamorphic rocks. Thin, subhorizontal Wemindji kimberlite sills were emplaced into granitic gneiss terrane of the Superior Province near Wemindji, Quebec, at 629 ± 29 Ma, along the inferred extension of the Kapuskasing Structural Zone. These kimberlite fields are grouped with the extensive Late Neoproterozoic magmatism of ultramafic and volatile-rich character, which is said to be associated with the breakup of Rodinia. Despite overall compositional similarity of the studied magmatic kimberlites, the material from Renard has higher concentrations of SiO2, Al2O3, MgO, and K2O, which reflects higher phlogopite abundances. The Wemindji sills show higher CaO concentrations due to high primary carbonate contents. Renard and Wemindji kimberlite incompatible trace element distributions are similar, with differences in Cs, Rb, and Sr corresponding to variable modal mineralogy. The initial 87Sr/86Sr ratios for the Renard kimberlites range between 0.70241 and 0.70765, while the Wemindji kimberlites have values between 0.70361 and 0.70442. Initial εNd values for the Renard kimberlites lie between +1.2 and +4.6, whereas the Wemindji kimberlite sills range between +0.2 and +4.8. Initial εHf values for the Renard kimberlites lie between +1.7 and +6.3, whereas the Wemindji kimberlite sills yielded values between +1.1 and +6.5. The overlapping Sr-Nd-Hf isotope compositions of these kimberlite suites indicate melt derivation from moderately depleted mantle sources. Osmium isotope compositions fall at the unradiogenic end for global kimberlites, with initial 187Os/188Os ratios ranging between 0.11539 and 0.12620 for Renard kimberlites, and between 0.11078 and 0.11729 for Wemindji kimberlites, with Os concentrations all below 1.3 ppb. These Os values suggest that an additional input from the CLM (i.e., ancient refractory cratonic peridotite), which is not reflected in the Hf and Nd radiogenic isotopes, is
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required. Both kimberlite suites depict mantle δ13C values (ca. -6 to –4 ‰), with evidence of hydrothermal alteration in the δ18O values (between 10 and 20 ‰ relative to SMOW). Production of an isotopically depleted melt occurred during the breakaway of Laurentia from Rodinia. Wemindji sits on the inferred extension of the Kapuskasing Structural Zone, which is suggested to have been a short-lived reactivated translithospheric rift-like feature, promoting CO2-rich melting conditions during the Late Neoproterozoic. The data from this study suggest that this ascending sublithospheric depleted melt component (more CO2-rich beneath Wemindji) interacted with a maximum input volume of 5% of the MARID-enriched CLM beneath the eastern Superior craton, and between 2% and 30% of ancient refractory cratonic peridotite. The lack of significant diamond in the Wemindji kimberlite dykes could be due to the resorption of the potential diamond in the CO2-rich kimberlite melt.
Description
A dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of requirements for the degree of Master of Science. Johannesburg, 2016.
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Citation
Brand, Natalie Bronwyn (2016) Isotopic fingerprinting (Sr-Nd-Hf-Os-C-O) of mantle source regions to kimberlite magmatism beneath the eastern Superior Craton, Canada, University of Witwatersrand, Johannesburg, <http://wiredspace.wits.ac.za/handle/10539/21010>