Relationship between Proterozoic dykes and associated volcanic sequences: evidence from the Harp Swarm and Seal Lake Group, Labrador, Canada

Abstract

Abstract Geochemical studies of the Harp dyke swarm and extrusive/intrusive igneous rocks of the Seal Lake Group of Labrador, Canada show that each can be subdivided into three distinct chemical groups. Two of the chemical groups within the Seal Lake Group show strong similarities to those present within the Harp dykes, but with more restricted and generally more primitive compositions. Chemical variation within individual Harp dykes suggests a dominant role for phenocryst differentiation processes, with little or no influence by crustal contamination. However, fractional crystallisation processes cannot account for the variation observed within each of the Harp dyke chemical groups, which instead is likely to be dominated by in-situ crystallisation processes. The greater range of incompatible trace element concentrations within many Harp dykes compared to Seal Lake Group igneous rocks is most likely a result of such mechanisms, rather than by fractional crystallisation or crustal 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 similarities 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 not strictly coeval, and hence most Harp dykes did not act as feeders to Seal Lake Group lavas. A model of heterogeneous lithosphere extension with a progressively increasing pure shear component with time can satisfactorily explain the Harp-Seal Lake magma relationships.