Wra. Baragar et al., LONGITUDINAL PETROCHEMICAL VARIATION IN THE MACKENZIE DYKE SWARM, NORTHWESTERN CANADIAN SHIELD, Journal of Petrology, 37(2), 1996, pp. 317-359
Previous magnetic fabric studies of the giant, radiating, 1.27 Ga Mack
enzie 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 d
ykes. To examine petrochemical implications of the model, compositiona
l variation within the basalt sequence is compared with that of the dy
kes along a 'stream line' of the swarm between 400 km (just beneath th
e lavas) and 2100 km from its focus and in a parallel segment farther
east. Evolution of tholeiitic magmas of the main sampled stream is rec
orded in the upward change of composition in the lava sequence from mg
-numbers of 70 to 35. Underlying (feeder) dykes have a comparable rang
e, but outward along the swarm the range of compositions narrows progr
essively towards its more evolved end, and at 2100 km, dyke compositio
ns match those in upper levels of the lava sequence. REE and other tra
ce element abundances show a similar contraction in range, and a shift
towards more evolved compositions, both upward in the lava sequence a
nd outward along the swarm. Apart from complications owing to crustal
contamination, fundamental attributes of the magma (e.g Zr/Y) change l
ittle with stratigraphic level or distance from the focus. The more ea
sterly stream differs in its high proportion of alkalic compositions,
suggesting the existence of distinctive subswarms. Normative mineral v
ariation plots are consistent with fractionation in high- (main stream
) to low-level (eastern stream) crustal magma chambers. Mackenzie magm
atism is compatible with the plume model. Domal uplift related to plum
e activity initiated central graben collapse and outward-extending rad
ial 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 magmat
ic head by which fractionating magmas could be dispatched to successiv
ely greater distances. This, and crystal settling in transport, accoun
ts for the increasingly evolved nature of dykes with distance from the
source.