RELATIONSHIP BETWEEN PROTEROZOIC DYKES AND ASSOCIATED VOLCANIC SEQUENCES - EVIDENCE FROM THE HARP SWARM AND SEAL-LAKE-GROUP, LABRADOR, CANADA

Geochemical studies of the Harp dyke swarm and extrusive/intrusive ign eous rocks of the Seal Lake Group of Labrador, Canada show that each c an be subdivided into three distinct chemical groups. Two of the chemi cal groups within the Seal Lake Group show strong similarities to thos e present within the Harp dykes, but with more restricted and generall y more primitive compositions. Chemical variation within individual Ha rp dykes suggests a dominant role for phenocryst differentiation proce sses, with little or no influence by crustal contamination. However, f ractional crystallisation processes cannot account for the variation o bserved within each of the Harp dyke,chemical groups, which instead is likely to be dominated by in-situ crystallisation processes. The grea ter range of incompatible trace element concentrations within many Har p dykes compared to Seal Lake Group igneous rocks is most likely a res ult of such mechanisms, rather than by fractional crystallisation or c rustal contamination processes. Although dykes that do not chemically correspond with sampled lavas may have fed flows which are now eroded, it is more likely that they were non-emergent. Hence the chemical sim ilarities and differences between the dykes and lavas can be linked to progressive mantle melting processes. This is consistent with spatial and temporal evidence that the Harp and Seal Lake igneous rocks are n ot strictly coeval, and hence most Harp dykes did not act as feeders t o Seal Lake Group lavas. A model of heterogeneous lithosphere extensio n with a progressively increasing pure shear component with time can s atisfactorily explain the Harp-Seal Lake magma relationships.