Nd and Sr isotopic crustal contamination patterns in an Archaean meta-basic dyke from northern Labrador

Samples from the core to the margin of a 20 cm wide meta-dolerite dyke are sequentially enriched in K, Rb, Sr, and the light REE's. Rb-Sr and Sm-Nd compositional and isotopic profiles in the dyke are interpreted to be the result of selective contamination with components of country rock derivation, rather than the result of simple bulk mixing. 87Rb/86Sr ratios are higher at the edge of the dyke than at its centre, although they are somewhat irregular, due probably to the effects of subsequent alteration. This profile and one shown by unsupported 87Sr are both consistent with contamination of the dyke by a fluid phase derived by the breakdown of biotite. Common Sr shows a parallel, albeit weaker, contamination profile which is interpreted to reflect the contribution of a Sr-bearing phase such as plagioclase. 147Sm/144Nd ratios and 144Nd concentrations increase and decrease respectively from the margin to the core of the dyke. In addition, the margin of the dyke is significantly less radiogenic than the interior. This contrasts with the relatively radiogenic character of an adjacent pegmatite vein. (author)