Abstract

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 are tholeiitic basalts, their chemical and isotopic compositions change upwards in the sequence. The lowermost unit has relatively high contents of SiO2 and incompatible trace elements, high ratios of elements with different compatibilities (e.g., Th/Nb, La/Sm), high Gd/Yb, negative Nb anomalies, and low εNd. Samples at the top of the sequence have less pronounced enrichment and fractionation of incompatible elements coupled with an absence of Nb anomalies and positive εNd values. These results are interpreted to indicate that the lavas lowest in the sequence were produced by melting in the garnet stability field, at depths 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 of the entire volcanic sequence the extent of crustal contamination became minimal and the lavas lost the chemical signature of residual garnet. The youngest lavas formed by melting in the spinel field and were free of crustal contamination. The site of mantle melting apparently became shallower, perhaps because of lithosphere thinning.

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