New trace-element and Nd isotope analyses were carried out on the Coppermine River basalts, a suite of 1.27 Ga old
continental flood basalts in the Northwest Territories of Canada. Although all the samples have tholeiitic basaltic compositions, there are progressive changes upwards through the sequence. The lowermost unit has relatively high contents of SiO2 and incompatible trace elements, high ratios of incompatible to compatible elements (e.g. La/Sm), high Gd/Yb, negative Nd
anomalies, positive Pb anomalies and low eNd(T). Samples from higher in the sequence have less pronounced enrichment and fractionation of incompatible elements coupled with an absence of Nb and Pb anomalies and positive eNd(T) values. Samples from the uppermost unit have mantle-normalized U and Th concentrations markedly lower than those of elements with similar compatibility (Ba, Nb, La).
These results are interpreted to indicate that the lavas lowest in the sequence were produced by melting in the garnet stability field, at a depth greater than 90 km and probably in a mantle plume beneath the continental lithosphere. These magmas passed through magma chambers in the lower and upper crust where they became contaminated with crustal rocks. During the course of the eruption the lavas progressively lost the chemical signature of residual garnet, and the extent of crustal contamination became minimal. The youngest lavas formed by higher percentage melting in the absence of garnet and were free of contamination. The site of mantle melting apparently became shallower, perhaps because of lithosphere thinning. The low contents of Th and U in the upper lavas are thought to reflect strong depletion of incompatible trace elements in their mantle source. (The lack of depletion of Cs, Rb and Ba is explained by addition of these mobile elemetns during alteration). Taken together with positive eNd(T) in these lavas, these results indicate that at least part of the plume source was chemically and isotopically depleted.