Nd isotope delineation of Archean and Paleoproterozoic crustal terranes: Cryptic suturing and ensialic arc formation in the Parent-Clova region of south-central Quebec

Abstract

<p>p.p1 {margin: 0.0px 0.0px 0.0px 0.0px; font: 11.5px Times; color: #2b2b2b} span.s1 {color: #3f3f3f}</p> <p>One of the key components in the crustal evolution of the Canadian Shield was the -1.0 Ga Grenville orogeny, which resulted in major northwest directed thrusting of pre-Grenvillian crustal allochthons against the Laurentian foreland. This orogenic episode resulted in extensive metamorphism, terminal collisional uplift and subsequent erosion, such that current surface geology expresses mid- to deep- crustal components.</p> <p>Originally distinguished from adjacent provinces in the Canadian Shield based on isotopic, structural and metamorphic signatures; the first-order crustal terranes of the Grenville Province in Quebec are primarily comprised of the relatively stationary (Archean - Paleoproterozoic) Parautochthonous Belt and the more laterally transported (Paleoproterozoic - Mesoproterozoic) Allochthonous Belts. Although a first-order crustal formation map for most of the Grenville has already been compiled, 50,000 km² of southcentral Quebec has remained unmapped, with weak constraints on boundary locations as</p> <p>well as an overall lack of understanding of crustal mixing processes within the region.</p> <p>Over 90 new Nd-isotope analyses provide crustal formation age constraints for granitoid orthogneisses from south central Quebec, and are used to delineate three principal crustal terranes, interpreted respectively as reworked Archean, a mixed ensialic arc, and juvenile Mesoproterozoic crust. The resistance of the Sm-Nd isotope system to metamorphic resetting has provided a reliable method to investigate the ages of these pre-Grenvillian terranes, as age of initial extraction from the mantle provides the best possible protolith indication.</p> <p>Nd-isotope and major element analysis has indicated the presence of reworked Archean crust in the north, bounded by the ABT, and a juvenile Mesoproterozoic terrane to the south (Quebecia). A block of mixed ages, of possible ensialic arc origin, constitutes the central section and may overlie a diffuse cryptic Archean-Proterozoic boundary. The tectonic evolution of the region can be explained through a subduction flip model, in which a 1.5 Ga oceanic arc terrane became obliquely sutured onto the continental margin, resulting in a flip to north-dipping subduction under the new composite margin. This subduction flip caused ensialic arc formation in a 50 km wide zone subparallel to the ABT, which reworked Quebecia rocks through continental arc magmatism.</p> <p>To the south of this region, back arc spreading within the ensialic arc led to the development of a rift zone that was filled by clastic and carbonate sedimentation, forming the Central Metasedimentary Belt. Some of these rocks were later thrust northwards over older crust during the Grenville Orogeny, forming the Cabonga Nappe complex. The extent of this terrane has also been further constrained by Nd isotope mapping in this study.</p>