Rhenium-Osmium Isotope and Platinum-Group Element Constraints on the Origin and Evolution of the 1{middle dot}27 Ga Muskox Layered Intrusion

Platinum-group element (PGE: Os, Ir, Ru, Pt, Pd) and Re–Os isotope systematics determined for the entire preserved stratigraphy of the 1·27 Ga Muskox intrusion provide an exceptional view of magma chamber processes and mineralization in the main plutonic system of the Mackenzie large igneous provinc...

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Bibliographic Details
Published in:Journal of Petrology
Main Authors: Day, James M. D., Pearson, D. Graham, Hulbert, Larry J.
Format: Text
Language:English
Published: Oxford University Press 2008
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Online Access:http://petrology.oxfordjournals.org/cgi/content/short/49/7/1255
https://doi.org/10.1093/petrology/egn024
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Summary:Platinum-group element (PGE: Os, Ir, Ru, Pt, Pd) and Re–Os isotope systematics determined for the entire preserved stratigraphy of the 1·27 Ga Muskox intrusion provide an exceptional view of magma chamber processes and mineralization in the main plutonic system of the Mackenzie large igneous province (LIP). We present new Re–Os isotope data for the intrusion, together with PGE and trace element abundances, and oxygen and Sm–Nd isotope data on samples that include local crustal materials, layered series peridotites, stratiform chromitites, marginal and roof zone rocks, and the Muskox Keel feeder dyke. Intrusive rocks span wide ranges in initial isotopic compositions (γ Os i = + 1·0 to + 87·6; ε Nd i = −0·4 to −6·6; δ18O Ol = + 5·5 to + 6·9‰) and highly siderophile element abundances (HSE: PGE and Re; Re = 0·02–105 ppb; Pt = 0·23–115 ppb; Os = 0·02 to > 200 ppb). HSE and fluid-immobile trace element abundance variations are consistent with relative compatibilities expected for cumulate rocks. The most radiogenic Os and unradiogenic Nd isotope compositions occur in the Muskox marginal and roof zones. Negative γ Os i values in these rocks and their non-isochronous relations result from mobilization of Re in the intrusion through post-magmatic hydrothermal processes. The most significant process causing Os and Nd isotope variations in the layered series of the intrusion is crustal contamination of mantle-derived magma batches feeding individual cyclic units. This process may be directly responsible for formation of chromitite horizons within the intrusion. Accounting for crustal assimilation, the Muskox intrusion parental magma has γ Os i = + 1·2 ± 0·3, ε Nd i > –1·0 ± 0·4, δ18O ∼ +5·5‰ and HSE abundances similar to those expected from ≥ 15% partial melting of the Mackenzie LIP mantle source. This composition is similar to that calculated for 1·27 Ga primitive upper mantle. Parental magmas were probably derived from a mantle source unaffected by long-term, large-scale melt depletion, with no appreciable input ...