Age and Nd-Hf isotopic constraints on the origin of marginal rocks from the Muskox layered intrusion (Nunavut, Canada) and implications for the evolution of the 1.27 Ga Mackenzie large igneous province

The Muskox mafic-ultramafic layered intrusion, Nunavut, is part of the widespread 1.27 Ga Mackenzie large igneous province in northern Canada. The age and Nd-Hf isotope geochemistry of samples from the marginal zone of the Muskox intrusion and adjacent gneisses allow for an assessment of contamination mechanisms along the floor and walls of the intrusion, relationships between the marginal zone and overlying layered series, and genetic links with the Coppermine River flood basalts and Mackenzie dikes. The marginal zone, intersected in two diamond drill holes at stratigraphically low (West Pyrrhotite Lake) and high (Far West Margin) positions along the western margin of the intrusion, comprises a thick upper subzone (similar to 100-150 m) of olivine and olivine-chromite cumulates and a relatively thin (<10 m) lower subzone containing granophyre-bearing gabbronoritic rocks. U-Pb baddeleyite geochronology yields a crystallization age of the marginal zone of ca. 1269 Ma based on dating of three samples (1268.8 +/- 1.7 Ma, chromite-rich peridotite; 1268.7 +/- 2.9 Ma, gabbronorite; 1271.5 +/- 3.4 Ma, peridotite), which is synchronous with published ages from the layered series and Mackenzie dikes. The peridotites from both sections mostly span a restricted range of isotopic compositions (initial epsilon(Nd) = -2.8 to +0.5; initial epsilon(Hf) = -3.9 to +1.5), which is similar to that observed in the overlying layered series and consistent with the lack of significant chamber-wide crustal contamination of the Muskox parent magmas during differentiation and consolidation. In contrast, the gabbronorites within 10 m of the margin of the intrusion (initial epsilon(Nd) = -13 to -5.5; initial epsilon(Hf) = -16 to -5.9) were strongly contaminated by melting and exchange with their respective host rocks; the extent of contamination is greater adjacent to paragneiss wallrocks than adjacent to orthogneiss. Crystallization of this thin, contaminated zone along the margin of the Muskox intrusion likely sealed the contacts with the host gneisses and prevented magmas within the interior of the chamber from interacting with the surrounding crustal rocks. Comparison of the Nd isotopic compositions from the marginal zone with published results from the layered series and the Coppermine River flood basalts indicates that only the lowermost portion of the volcanic succession (lower member of the Copper Creek Formation and lowermost part of the middle member) could represent residual magmas that resided and fractionated in the Muskox magma chamber prior to eruption. Construction of the Muskox layered intrusion was thus restricted to the early stages of emplacement of the Mackenzie large igneous province. (C) 2009 Elsevier B.V. All rights reserved.