PROGRESSIVE PROTEROZOIC GROWTH OF SOUTHERN LAURENTIA BY MAGMATIC STABILIZATION OF LITHOSPHERE

From ~1.9 to 1.0 Ga, Laurentia grew by the successive addition of oceanic terranes and magmatic arcs to a long-lived “southern” compressive/transpressive plate margin. A new compilation of the southern Laurentian portion of Rodinia for the IGCP 440 project enables a sequential look at the growth of Laurentia, from which we examine the processes that transform initially thin crust and lithospheric mantle of juvenile terranes into stable continental lithosphere. The Archean cratons of Canada and the northwestern U.S. had stabilized as continental lithosphere prior to 2.5 Ga, were rifted about 2.1 Ga, then assembled into a large continental mass during 2.0-1.8 Ga Trans-Hudson orogenesis. Juvenile terrane accretion to southern Laurentia initiated with the Penokean orogeny at ~1.88-1.83 Ga, in which Archean basement and Paleoproterozoic supracrustal rocks were deformed and metamorphosed during collisions with oceanic arc terranes. This was followed by the successive accretion of NE-trending juvenile terranes, including the 1.78-1.68 Ga Mojave province (which incorporates reworked Archean and Paleoproterozoic basement), the 1.8-1.7 Ga Yavapai province and the 1.67-1.65 Mazatzal province. An underappreciated event was the addition of 1.5-1.3 Ga juvenile crust that now underlies much of the mid-continental U.S. from Texas through eastern Canada. This was likely a magmatic arc linked to ~1.45-1.35 Ga A-type magmatism that intruded and helped stabilize the older Proterozoic provinces. Accretion of juvenile crust to southern Laurentia culminated with the 1.3-1.0 Ga Grenville orogeny: the final stage in the growth of Rodinia prior to collapse and breakup at ~0.8-0.7 Ga. A principal focus of the new southern Laurentia map is the extent and ages of granitoid magmatism within the progressively assembled Proterozoic orogens. With each addition of juvenile crust, granitoid magmatism outlasted shortening deformation and invaded across province boundaries (defined by Nd data) to stitch young juvenile crust with older basement. This process is hypothesized to enhance cratonization by mechanical strengthening of the middle crust via pluton emplacement and reduction of anisotropy, strengthening of the lower crust via differentiation and development of a mafic residue, and stabilization of lithospheric mantle via thickening and de-densification due to basalt extraction.