PROTEROZOIC CRUSTAL GROWTH OF LAURENTIA: INSIGHTS FROM LITHOPROBE

Most of Laurentia was assembled as a result of Proterozoic accretionary and collisional tectonics around Archean nuclei, which began with the Slave-Rae plate interaction at about 2.02 Ga and ended with the final assembly of Rodinia about 1.0 Ga ago. A simplified reconstruction of the accretion and collision history suggests that tectonic assembly occurred in three phases characterized by: i) amalgamation of a “Slave-Rae-Hearne-Wyoming-Sask-Meta Incognita” microcontinent during the interval 2.2-1.83 Ga, ii) collision the Superior and Nain cratons with the above collage at about 1.83-1.78 Ga, and iii) migration of convergence to the southeastern margin of Laurentia in a Cordilleran-type framework during the interval 1.78-1.20 Ga, followed by Grenvillian (1.20 – 1.00 Ga) reactivation of part of that margin. Tectonic assembly during this protracted period involved accretion of both evolved and juvenile crust and mantle, as well as magmatic addition. Advances made by the multidisciplinary LITHOPROBE program, especially in the 3D characterization of the bounding Proterozoic orogenic belts, suggest that although a substantial amount of juvenile material was either tectonically or magmatically added during the Paleo- and Mesoproterozoic, a greater proportion than previously recognized is actually represented by recycled Archean crust. Orogen-scale cross sections along the volumetrically dominant Trans-Hudson Orogen, constrained by deep seismic, magnetotelluric, geochronologic and tracer isotope data, suggest that in general, the middle and lower crust is dominated by Archean age rocks whereas juvenile Proterozoic crust is more commonly (with exceptions) preserved in upper crustal levels. This has important implications for quantitative models of continental growth through time and suggests that production of perhaps thicker and more buoyant lithosphere during the Archean may have favored preservation of Archean versus younger crust. This is especially the case for Proterozoic and younger accretionary orogens that were succeeded by continent-continent collision.