U-PB GEOCHRONOLOGY AND GEOCHEMICAL VARIATION WITHIN 2 PROTEROZOIC MAFIC DYKE SWARMS, LABRADOR

An Early Proterozoic Kikkertavak mafic dyke intruding the Archaean Hop edale block, Labrador, gives an age of 2235 +/-2 Ma using U - Pb techn iques on baddeleyite. A Harp mafic dyke in the same area gives an age of 1273 +/- 1 Ma using U - Pb techniques on baddeleyite and zircon. Th e latter age is almost identical to that of the giant Mackenzie swarm and to the age of the BD0 dykes in South Greenland, and points to a ma jor pulse of mafic magmatism over much of the North Atlantic craton at this time. The former age is a little older than available Rb - Sr ag es for the extensive MD swarm in West Greenland, but there are possibl e correlatives. Geochemical data are presented to ascertain whether th ere are significant compositional differences between the Harp and Kik kertavak dyke swarms. In fact. two distinct chemical subgroups can be recognized within the Kikkertavak dykes, and three others are recogniz ed within the Harp suite. These differences apply more to trace elemen t patterns rather than major element abundances, but although there ar e compositional differences between the average Harp and average Kikke rtavak dyke, it is unlikely that geochemistry could be used unequivoca lly to separate the two. The compositional differences probably reflec t evolutionary processes in the lithosphere. The range of composition exemplified by the subgroups is most easily interpreted in terms of pr oportion of asthenosphere and lithosphere components, and does not nec essarily imply that either dyke swarm was emplaced over a long period. The presence of subgroups within both swarms urges some caution in as suming all dykes correspond to one or other age.