Regional Geobarometry in the Contact Aureoles of the Anorthositic Nain Complex, Labrador

The 1.4 Gyr anorthositic Nain complex was emplaced by multipleintrusion into Archean and Aphebian rocks whose metamorphicgrades range regionally from greenschist to granulite and whosetemperature at the time of emplacement was probably no higherthan 200–300 °C. Using experimental data on Ga-Cdand Hy-Ol-Qz equilibria, compositional studies allow the determinationof contact metamorphic pressures and temperatures for samplesover an area > 10,000 km². The results indicate that forthese specimens temperatures ranged from 645° to 915 °Cand pressures ranged from about 3.7 to 6.6 kbar. Locally, originalgarnet porphyroblasts have partially reacted to produce symplectitesof cordierite+hypersthene, resulting in a corona-like texture.These textures and associated mineral zoning indicate that someof the specimens have experienced a polymetamorphic history.This poly metamorphism presumably resulted from the multipleintrusions of magmas of the Nain complex. The regional distributionof pressures appears systematic and suggests an arch in theancient pressure surfaces (paleo-isobars). This postulated arch,which trends WNW-ESE, roughly coincides with a proposed grabenstructure in the Conrad discontinuity based on gravity studies(Stephenson, 1974). The association of these two features canbe rationalized with a mechanism of incipient continental rifting.The emplacement of the unmetamorphosed northern Labrador anorthositemassifs in a non-orogenic environment of low temperature andmoderate depth suggests that the metamorphosed anorthositessouth of the Grenville Front may have been accidental participantsin a post-anorthosite Grenville orogeny and metamorphism.