Undeformed and unmetamorphosed diabase dykes up to 5 m wide occur in an area of ∼100 km2 about 50 km northwest of Montreal near the village of Sainte-Sophie. The dykes are subvertical and oriented east–west. The most common phenocryst phase is plagioclase (1–2 mm, exceptionally 10 mm), followed by olivine, and in one dyke, pyroxene. Most dykes have vesicles 0.1–10 mm in diameter and fine-grained margins. Alteration is extensive: plagioclase is partly sericitized, olivine is serpentinized, and zeolites and carbonate are developed in the vesicles and matrix. Ar–Ar dating was challenging, but the weighted mean of two plateau ages indicates intrusion and cooling at 591.6 ± 2.5 Ma. The overall characteristics of these dykes suggest that they may be a facies of the much larger Grenville dyke swarm to the west. However, there are distinct chemical differences that suggest they were fed from a different, generally more enriched mantle source. This zone may be related to the one that produced the alkaline Monteregian intrusions some 470 million years later in the same region. If the Sainte-Sophie dykes are part of the Grenville dyke event, then the use of chemical fingerprints to identify components of this swarm must be used with caution. Another difference between the Sainte-Sophie and main Grenville dykes is the presence of vesicles and zeolites in the former, which suggests the dykes were emplaced close to the surface; hence, this crustal block has seen little erosion during the last 600 million years.
Higgins, M.D., Hankard, F., Ganerød, M., Van der Voo, R., 2017. The vesicular Sainte-Sophie dykes: a chemically distinct, near-surface facies of the Grenville Dyke Swarm?, Canadian Journal of Earth Sciences: 55(3): 241-251 https://doi.org/10.1139/cjes-2017-0168
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Paleogeographic history of Laurentia during the Ediacaran (630-542 Ma) still remains a puzzling controversy. The existing paleomagnetic data for this period suggest that Laurentia was located at low latitudes at 615 Ma, drifted toward high southerly latitudes from 590-575 Ma, back to lower latitudes by 573 Ma, then back to higher latitudes by 561 Ma, and finally returned to an equatorial position after about 550 Ma. These multiple migrations imply extremely high velocities sometimes exceeding 45cm/yr. This strongly conflicts with an estimated 20 - 30 cm/yr speed limit for plate movements. Possible alternative causes of these rapid latitudinal motions of Laurentia are: (1) unreliable paleomagnetic data, (2) very rapid true polar wander; (3) a particular and unusual geometry of the magnetic field during this period; (4) or inaccurate age dates. In order to unravel this complex Ediacaran position of Laurentia, we have undertaken paleomagnetic and geochronological studies of diabase dykes collected from Sainte-Sophie, NE of Saint-Jerome, Quebec, Canada. Our results could be critical in understanding this conundrum that divides paleomagnetists.
Hankard, F Higgins, M Van Der Voo, R Verdel, C, 2010 The puzzling late Precambrian paleoposition of Laurentia: new insights from preliminary paleomagnetism of the Sainte-Sophie diabase dyke swarm, Quebec. fall AGU abstract