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Journal of Petrology Advance Access originally published online on May 25, 2008
Journal of Petrology 2008 49(7):1255-1295; doi:10.1093/petrology/egn024
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© The Author 2008. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org

Rhenium–Osmium Isotope and Platinum-Group Element Constraints on the Origin and Evolution of the 1·27 Ga Muskox Layered Intrusion

James M. D. Day1,2,*, D. Graham Pearson1 and Larry J. Hulbert3

1Department of Earth Sciences, University of Durham, Science Laboratories, South Road, Durham DH1 3LE, UK
2Department of Geology, University of Maryland, College Park, MD 20742, USA
3Geological Survey of Canada, 601 Booth Street, Ottawa, ONT. K1A 0E8, Canada

RECEIVED MAY 22, 2007; ACCEPTED APRIL 18, 2008


  Abstract

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 ({gamma}Osi = + 1·0 to + 87·6; {varepsilon}Ndi = –0·4 to –6·6; {delta}18OOl = + 5·5 to + 6·9{per thousand}) 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 {gamma}Osi 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 {gamma}Osi = + 1·2 ± 0·3, {varepsilon}Ndi > –1·0 ± 0·4, {delta}18O ~ +5·5{per thousand} 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 from recycled crust and lithosphere, or putative core contributions.

KEY WORDS: chromitite seams; crustal contamination; mantle source; Muskox layered intrusion; O; Re–Os; Sm–Nd; isotopes; platinum-group elements (PGE); sulphide

*Corresponding author. Telephone: +1 301-405-2707. E-mail: jamesday{at}geol.umd.edu


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