Origin and isotopic evolution of the Muskox Intrusion, Nunavut

The 1.27 Ga Muskox Intrusion, Nunavut, is a layered mafic/ultramafic intrusion associated with Mackenzie Igneous Events. Eight samples from the intrusion and associated Keel Dyke were investigated for whole rock Hf, Nd, and Sr isotopes and in situ laser ablation MC-ICP-MS Pb and Sr isotopes in plagioclase. The in situ initial 87 Sr/86Sr ratios vary between 0.70366 and 0.70692 and are consistently more primitive than Sr whole rock values for the same samples. In situ Pb isotope compositions define similar linear arrays with 206Pb/204Pb and 207Pb/204Pb ratios that vary between 15.79-16.41 and 14.73-15.63, respectively. The epsilon Nd (1270 Ma) values vary between -3.2 and -0.2. Whole rock Hf isotopic data display a narrow range in epsilonHf (1270 Ma) between -0.3 and +3.2 with five of the samples tightly clustered at epsilonHf= +1.4+/-0.1. In general the Nd and Hf whole rock isotopic data indicate derivation from a chondritic-like mantle reservoir and do not display isotopic signatures of a depleted mantle source, such as those reported previously for the Coppermine River flood basalts.