Mackenzie Dyke Swarm Research Articles

Paragenesis and U-Pb systematics of baddeleyite (ZrO 2)

The mineral baddeleyite (ZrO 2) occurs in many terrestrial and lunar samples and some achondrite meteorites as a trace constituent that most often crystallizes in the late-stage, most chemically fractionated portions of mafic magmas together with apatite, ilmenite, ± zircon, ± zirconolite. In terrestrial mafic rocks these interstitial regions commonly contain K-feldspar, Cl-rich mica, amphibole and a small amount of quartz. In addition, baddeleyite occurs in tektites as a dissociation product of zircon during meteorite impact and as rims on mantle zircon megacrysts recovered from kimberlites. Baddeleyite is a major carrier of Hf, Ti, Fe and U and strongly fractionates Hf from Zr as it can have very high Zr/Hf (> 50). The baddeleyite rims on mantle zircon xenocrysts have a unique chemical composition with significantly higher TiO 2 and lower FeO contents. Baddeleyite is an ideal mineral for U-Pb dating because it has abundant U (up to 3000 ppm), negligible initial common Pb, rarely occurs as xenocrysts, and, in unmetamorphosed samples, experiences negligible Pb loss. As an example of the reproducibility of conventional U-Pb-baddeleyite dating, several individual fragments from a large crystal obtained from the Phalaborwa carbonatite, South Africa, yielded remarkable consistency with a mean 207 Pb 206 Pb age of 2059.8 ± 0.8 Ma (2σ), interpreted as the age of carbonatite emplacement. Perhaps the best example of the geological reproducibility of U-Pb-baddeleyite dating is the example of the Mackenzie dyke swarm in North America where eight dyke and sill samples, collected over distances of > 2000 km, all have consistent 207 Pb 206 Pb ages of 1267 and 1268 Ma. The ability to obtain very precise (± 1−2 Ma) and accurate U-Pb-baddeleyite ages for mafic magmatic outbursts on Earth, such as the Mackenzie event, is of paramount importance in attempting to make supercontinent reconstructions and in the global correlation of ancient mafic magmatic activity worldwide. In metamorphosed mafic rocks, baddeleyite can also survive very high grades of metamorphism but often develops a rim of polycrystalline metamorphic zircon. The width of the zircon rims and the grain size of the zircon aggregate increase when baddeleyite is subjected to progressively higher grades of metamorphism. A particularly interesting example is the variably metamorphosed Hurwitz gabbro sill, Northwest Territories, Canada. In a relatively unmetamorphosed sample, the mean 207 Pb 206 Pb age of 2111.2 ± 0.6 Ma (2σ) for four baddeleyite fractions with negligible zircon growth preserves the age of gabbro crystallization while baddeleyite from a middle to upper greenschist-facies sample collected along strike, and likely from within the same sill, contains significant metamorphic zircon rim development with corresponding discordance of between 12% and 31% depending on the size of the zircon rims. It is also noteworthy that both the primary age of crystallization and the age of metamorphism can be obtained from some metamorphosed gabbroic rocks. In an upper amphibolite- to granulite-facies coronitic gabbro from the Grenville Province, Ontario, Canada, magmatic baddeleyite and zircon retain a primary crystallization age of 1152 ± 2 Ma while polycrystalline metamorphic zircon pseudomorphs after baddeleyite record the age of metamorphism at ∼ 1050 Ma.

Evidence from magnetic fabric for the flow pattern of magma in the Mackenzie giant radiating dyke swarm

CONTINENTAL flood basalts, and their associated swarms of mafic dykes, are thought to result from melting in mantle plumes1–3, but the processes by which the resulting magma makes its way from mantle to crust remain obscure. Here we use magnetic anisotropy measurements to reconstruct the pattern of magma flow in the Proterozoic Mackenzie dyke swarm. Our results indicate that magma was injected vertically within 500 km of the focal point of the dyke swarm, and then travelled horizontally at all further distances out to at least 2,100 km. The sharp transition from vertical to horizontal flow at between 500 and 600 km may correspond to the outer boundary of melt generation in the mantle plume believed to be responsible for the Mackenzie igneous events.

Mackenzie igneous events, Canada: Middle Proterozoic hotspot magmatism associated with ocean opening

U Pb ages have been obtained for the Muskox intrusion and the Mackenzie dyke swarm. The age of a pyroxenite from the layered series of the intrusion is 1270 ± 4Ma. Baddeleyite fractions from four widely-spaced Mackenzie diabases define a single discordia line with an upper intercept age of 1267 ± 2Ma. The dyke age of 1267 Ma provides a precise time-marker for much of the northwestern Canadian Shield. Mackenzie intrusive events were coeval with eruption of Coppermine River flood basalts in the Coppermine homocline. The short time-span, large volume and specific focus of Mackenzie igneous events suggest that magmatism occurred above a large hotspot caused by the presence of a mantle plume. We infer that magmatism was initiated when rifting breached a large domal uplift supported by the plume-generated hotspot. The uplift-subsidence record in the Coppermine homocline is predicted by numerical models for rifting above hot asthenospheric mantle leading to ocean opening. An array of five large gravity anomalies north of the homocline may outline a region of stretched continental crust extensively intruded by Mackenzie mafic igneous bodies.

Geochronologic interpretations of Pb isotope ratios in nickel sulfides of the Thompson Belt, Manitoba

Lead isotopes in nickel sulfides from seven deposits in the Thompson Belt appear to record events of four distinct ages. The earliest and most prominent of these may represent the emplacement of the nickel ores and their ultramafic hosts.Typical massive and disseminated low-lead nickeliferous sulfides were selected for analysis, and several partitioning techniques were used in order to obtain information about the range of isotope ratios in different phases and fractions of the samples. The data were interpreted in terms of the isotopic systematics discussed by Gale and Mussett.Data from four deposits produce two well defined, parallel "primary" isochrons whose calculated mean slope corresponds to an age of 2320 ± 30 Ma. Interpretation of this date as the time of emplacement of the nickel deposits and their ultramafic hosts is consistent with geological evidence that points to a post-Archean, pre-Hudsonian age. The 2320 Ma date may be consistent, within probable error limits, with the age inferred from Rb/Sr data (2100 Ma) for deposition of metasedimentary rocks of the Thompson Belt, into which the ultramafic lenses were intruded (Brooks and Theyer).The other isotopically recorded events occurred at 2015 ± 15 Ma (possibly early folding of the Thompson Belt supracrustal rocks), 1620 ± 25 Ma (probably a late retrograde stage of the Hudsonian Orogeny), and 1125 ± 60 Ma (possibly a thermal event associated with emplacement of the Mackenzie dyke swarm).

Paleomagnetic evidence for the extent of Mackenzie igneous events

North-northwesterly striking Mackenzie diabase dikes of middle-Proterozoic (Helikian) age are profuse in the western part of the Canadian Shield. Published paleomagnetic data on dikes of this trend in Mackenzie District, on the Muskox Intrusion, the Coppermine River volcanic rocks, and the Sudbury dikes suggest that they are all products of closely related igneous events. This paper presents paleomagnetic data that suggest that the intrusion of extensive diabase sheets in the East Arm of Great Slave Lake, and of dikes as far to the northeast as Melville Peninsula and as far to the southeast as Manitoba, were also parts of these events. The mean paleomagnetic pole position for the Mackenzie dikes and for related intrusive and extrusive rocks is [Formula: see text], 171 °W with [Formula: see text]. Radioactive age determinations, some of which are unpublished, indicate an age of about 1200 m.y. for the formation of these rocks. It is suggested that for convenience all of these apparently related intrusive and extrusive igneous episodes be referred to as Mackenzie igneous events.