PLUME-LITHOSPHERE INTERACTION AND CRUSTAL CONTAMINATION DURING FORMATION OF COPPERMINE RIVER BASALTS, NORTHWEST-TERRITORIES, CANADA

New trace-element and Nd-isotope analyses were carried out on the Copp ermine River basalts, a suite of 1.77 Ga old continental flood basalts in the Northwest Territories of Canada. Although all the samples are tholeiitic basalts, their chemical and isotopic compositions change up wards 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 epsilon(Nd). Samples at the top of the sequence have less pronounced enrichment and fractionation of incompatible elem ents coupled with an absence of Nb anomalies and positive epsilon(Nd) values. These results are interpreted to indicate that the lavas lowes t in the sequence were produced by melting in the garnet stability fie ld, al depths greater than 90 km, and probably in a mantle plume benea th the continental lithosphere. These magmas passed through magma cham bers in the lower and upper crust where they became contaminated with crustal rocks. During the course of the eruption of the entire volcani c sequence the extent of crustal contamination became minimal and the lavas lost the chemical signature of residual garnet. The youngest lav as formed by melting in the spinel field and were free of crustal cont amination. The site of mantle melting apparently became shallower, per haps because of lithosphere thinning.