Three dense swarms of Archean mafic dykes, which intrude the Beaulieu River and Cameron River volcanic belts and adjacent granitoid terranes of the Sleepy Dragon Complex and Meander Lake complex in the southern Slave Province, all contain clusters of multiple, parallel-sided "sheeted" dykes. None of the swarms can be convincingly modelled as being part of an ophiolite assemblage. The 3 km by 20+ km Step'nduck dyke complex, comprising about 350 mainly metabasaltic to meta-gabbroic dykes separated by screens of granitoid gneiss (>2750 Ma), represents profuse mafic magmatism in the Meander Lake complex. Dense dyke swarms within the Cameron River belt fed the lavas and are an integral part of the volcanic stratigraphy.All basaltic magmas represented by the volcanic belts, and the dyke sets, are predominantly mantle-derived, fractionated, tholeiites. Nd-isotope geochemistry indicates that they were all contaminated by Archean sialic material. Isotopic systematics are satisfied by mixing of depleted mantle and varying proportions of Sleepy Dragon-type crust.The dyke swarms in the Sleepy Dragon Complex and the Step'nduck swarm record extensional events in Archean continental crust. Possibly they are subsurface equivalents of now-eroded flood basalts that signalled the activity of mantle plumes during Archean times. This hypothesis is consistent with magmatic underplating of continental lithosphere and with extensive involvement of continental material.