Archaean cratonization and deformation in the northern Superior Province, Canada: an evaluation of plate tectonic versus vertical tectonic models

Abstract

The Archaean Minto Block, northeastern Superior Province, is dominated by tonalite–trondhjemite, enderbite (pyroxene tonalite), granodiorite and granite, with subordinate mafic rocks and supracrustal belts. The plutons have been interpreted as the batholithic roots of Andean-type plate margins and intra-oceanic arcs. Existing horizontal-tectonic models propose that penetrative recrystallization and transposition of older fabrics during terrane assembly at ∼2.77 and ∼2.69 Ga produced a N-NW tectonic grain. In the Douglas Harbour domain (northeastern Minto Block), tonalite and trondhjemite dominate the Faribault–Thury complex (2.87–2.73 Ga), and enderbite constitutes 50–100 km-scale ovoid massifs (Troie and Qimussinguat complexes, 2.74–2.73 Ga). Magmatic muscovite and epidote in tonalite–trondhjemite have corroded edges against quartz+plagioclase, suggesting resorption during ascent of crystal-charged magma. Foliation maps and air photo interpretation show the common development of 2–10 km-scale ovoid structures throughout the Douglas Harbour domain. Outcrop and thin-section scale structures imply that many plutons experienced a phase of syn-magmatic deformation, typically followed by high temperature sub-magmatic overprints. Thermobarometric data for plutons indicate near-solidus recrystallization at 4–6 kbar pressures. The common preservation of syn-magmatic fabrics in plutons of different ages seems incompatible with the origin of these fabrics through superimposed regional orogenesis. The broad uniformity of intrusion ages and lithologies throughout the Minto Block, and the rarity of shallowly-dipping planar fabrics, also seem inconsistent with accretion of disparate older terranes, each of which should preserve distinct histories. A possible alternative explanation for these features is provided by vertical tectonic models, whereby buoyant felsic magmas ascended as crystal slurries, while dense supracrustal rocks (and solidified felsic intrusions emplaced into them) subsided as cold fingers (10–20 km-scale instabilities). Shear between upwelling and downwelling limbs would have concentrated in the weak intrusions, generating steeply-plunging syn-magmatic fabrics, and producing ductile overprints in solidified rocks.