Petrology of the Grenville swarm of gabbro dikes, Canadian Precambrian Shield

Abstract

The Grenville tholeiitic dikes of Late Proterozoic or early Paleozoic age cut marbles and gneisses of the Grenville Province within and adjacent to the Ottawa rift structure. Where traversed the swarm consists of about 40 large dikes (up to 100 m wide) representing a crustal extension of 1 km. The principal minerals are olivine, augite, pigeonite, plagioclase, magnetite, ilmenite, K-feldspar, and quartz. Crystals of pigeonite around olivine and complex augite–pigeonite composite grains suggest that the reactions olivine + melt → pigeonite and pigeonite + melt → augite have taken place. Conspicuous zoning and grain to grain variation in the composition of augite are consistent with fractional crystallization, but crystal – melt equilibrium during crystal growth is indicated by a restricted range in pyroxene paleotemperatures of 1180–1060 °C. Rock textures (subophitic, combined ophitic–subophitic, and equigranular) in dike centres are related to dike width and are determined principally by the influence of cooling rate on the nucleation of augite. The K2O content of the gabbro (centres of 15 dikes, 3–100 mm wide) ranges from 0.2 to 1.2% and is closely correlated with other elements, positively with Ti, Na, P, Rb, Sr, Y, Zr, and Ba, and negatively with Mg, Ca, and Cr. Relatively minor within-dike variation exists in the form of local (centimetre-scale) inhomogeneity, slight enrichment in K and Na, and depletion in Ca and Mg in the centre of one large dike and complex rhythmic variation in K, Na, Ca, and Fe across one small dike. Centre–margin comparisons in several large dikes indicate minor residence con tamination (Ca, Mg) in the margins of some dikes cutting marble. Of the various possible causes for variation in composition, those favoured at present are Fractional crystallization at depth (separation of augite and plagioclase), the partial preservation of compositional heterogeneity during intrusion, and the gravitational sinking of early formed crystals (with the upward displacement of K-enriched melt) in the central regions of large dikes.

A paleomagnetic and U–Pb geochronology study of the western end of the Grenville dyke swarm: Rapid changes in paleomagnetic field direction at ca. 585Ma related to polarity reversals?

“…Thinner dykes are known, some only a few cm thick but always occur in the vicinity of thicker parent dykes, so are just offshoots or apophyses. To the east dykes are more numerous with a greater range in thickness (from 3 to 100 m with a mean of 30 m from 7 dykes, Kretz et al, 1985), which may support the idea that thicker master dykes travel farther from the plume source than thin ones, leading to a skewed distribution towards thicker dykes with increasing distance travelled.…”

“…We do not know if this pattern exists along the entire length of the Grenville swarm; both high and low TiO 2 diabase dykes are known in more easterly locations (e.g. Kretz et al, 1985;Kumarapeli et al, 1990;Seymour and Kumarapeli, 1995) but a central concentration of the former cannot be tested from the information available.…”

“…U-Pb baddeleyite ages obtained from the dykes range from 580 to 598 Ma, with most lying between 584 and 589 Ma, showing that dyke injection may have lasted over a relatively short time of about 5-10 Myr (Kamo et al, 1989(Kamo et al, , 1995Davidson et al, 2009;Halls et al, 2011;Halls and Hamilton, 2012). Petrological studies of the dyke swarm, notably those by Kretz et al (1985) and taken mainly from the eastern half of the swarm, show that the dykes are composed of tholeiitic basalt, with subophitic to equigranular texture, containing generally fresh augite and plagioclase. Olivine, fresh to completely serpentinized also accounts for about 3% by volume.…”

Grenville metagabbro complexes of the Otter Lake area, Quebec

“…%) of element oxide or parts (1983) expressed precision in terms of k, defined as Introduction Gabbroic rocks are widespread in the southern Grenville Province of the Canadian Shield and have recently attracted much attention. Their distribution was shown in a compilation by Baer et al (1977), and detailed studies have been carried out by Buddington (1939), Whitney and McLelland (1973), Grieve and Gittins (1975), Nadeau (1984), Kretz et al (1985, Davidson and Grant (1986), Grant (1988), and others. The present study deals with certain metamorphosed gabbroic and associated ultramafic rocks (metagabbro complexes) that lie northwest of Ottawa ( Fig.…”

DENDRITIC MAGNETITE AND ILMENITE IN 590 Ma GRENVILLE DIKES NEAR OTTER LAKE, QUEBEC, CANADA

“…In a previous study of the Grenville swarm of gabbro dikes (Kretz et al 1985), data were reported on the microstructure of the gabbro, especially on the shape, size, and spatial distribution of crystals of olivine, calcic pyroxene, pigeonite, and plagioclase. The focus at present is on the shape of Ti-bearing magnetite, especially on crystals found in a relatively narrow (2 m wide) dike.…”