THE CORE OF LAURENTIA: A SYNOPSIS

The core of Laurentia formed during the growth of the supercontinent Nuna (or Columbia) between ca. 1.98 and 1.79 Ga ago. It consists of large Archean cratons that remained relatively intact (Slave, Superior, North Atlantic) and a collage of other “cratons”, micro-continents, as well as smaller Archean to earliest-Paleoproterozoic age crustal blocks which form a mosaic of accreted blocks that underwent variable degrees of tectono-thermal reactivation. Historically, the latter have collectively been referred to as the Churchill Province. During this continental convergence event, a potentially large amount of oceanic crust was recycled in the mantle through subduction, in the process generating a large volume of oceanic and continental arc magmas, as well as oceanic arcs, plateaus and back-arc basins. Crust was also added through magmatism along rifted continental margins, or preserved in intra-continental basins, passive margins, flysch and molasses basins. On a large scale, accretion of both Juvenile Proterozoic and Archean crust began on the Slave-Rae-Hearne side of the intervening ocean (Manikewan) and grew progressively southeastwards (present-day coordinates). Terminal collision occurred at ca. 1.83-1.79 Ga when the Manikewan Ocean eventually closed and the Superior Craton eventually collided with the pre-accreted crustal collage.

One of the remarkable features resulting from this accretion and collision event, as observed in part through 3D sections provided by Lithoprobe seismic images, is the highly skewed proportion of Archean relative to Paleoproterozoic-age crust preserved in the geological record. Commonly, ancient sutures where Proterozoic crust supposedly rooted are now flanked uniquely by Archean crust, the former having been removed either by subduction or obduction. Seismic and isotopic data also suggest that where present, Paleoproterozoic crust represents mostly an upper- to middle-crust veneer sitting on Archean mid- to lower-crust. Collectively, these observations suggest that thick, more competent and buoyant Archean-age sub-continental lithospheric mantle, relative to Paleoproterozoic-age SCLM, may have set the stage for dominance of Archean-age crust in the Laurentian core.