Longitudinal Petrochemical Variation in the Mackenzie Dyke Swarm, Northwestern Canadian Shield

Previous magnetic fabric studies of the giant, radiating, 1.27 Ga Mackenzie dyke swarm concluded that flow patterns within the dykes support the concept of a mantle plume that is centred beneath the swarm focus and supplies magma to overlying flood basalts and developing radial dykes. To examine petrochemical implications of the model, compositional variation within the basalt sequence is compared with that of the dykes along a ‘stream line’ of the swarm between 400 km (just beneath the lavas) and 2100 km from its focus and in a parallel segment farther east. Evolution of tholeiitic magmas of the main sampled stream is recorded in the upward change of composition in the lava sequence from mg-numbers of 70 to 35. Underlying (feeder) dykes have a comparable range, but outward along the swarm the range of compositions narrows progressively towards its more evolved end, and at 2100 km, dyke compositions match those in upper levels of the lava sequence. REE and other trace element abundances show a similar contraction in range, and a shift towards more evolved compositions, both upward in the lava sequence and outward along the swarm. Apart from complications owing to crustal contamination, fundamental attributes of the magma (e.g. Zr/Y) change little with stratigraphic level or distance from the focus. The more easterly stream differs in its high proportion of alkalic compositions, suggesting the existence of distinctive subswarms. Normative mineral variation plots are consistent with fractionation in high- (main stream) to low-level (eastern stream) crustal magma chambers. Mackenzie magmatism is compatible with the plume model. Domal uplift related to plume activity initiated central graben collapse and outward-extending radial fractures, thus providing access to plume-derived magmas and loci for magma chamber and dyke swarm development. Multiple magma chambers, forming around the apex, each fed relatively independent subswarms of dykes. Uplift, accompanying fractionation, provided increasing magmatic head by which fractionating magmas could be dispatched to successively greater distances. This, and crystal settling in transport, accounts for the increasingly evolved nature of dykes with distance from the source.