Geodynamic setting of Proterozoic Massif-type Anorthosites in the Eastern Canadian Shield

Abstract

Massif-type anorthosites occur worldwide and were predominantly formed during the Paleo- to Mesoproterozoic Era, with an exceptionally large number emplaced in southeastern Laurentia, both within the Grenville Province and in the foreland of the Southeastern Churchill Province and North Atlantic Craton (Nain Province). Their secular nature infers that physical and chemical conditions necessary for their formation were optimized during that time period. A review of geochronological and ambient tectonic regimes that were operating during their emplacement suggests that they formed during four periods, ca. 1.65 Ga, ca. 1.45-1.30 Ga, ca. 1.16-1.14 Ga, and 1.08-1.02 Ga. The first two pulses overlap with continent-ocean convergence and subduction beneath the SE Laurentian margin. The last two pulses correlate with continent-continent collisional tectonics. We argue that higher mantle temperatures during the Proterozoic, as inferred in published models, were not sufficient, by themselves, to produce the necessary volume of basaltic underplate – a key factor in the generation of massif-type anorthosites – and that mantle re-fertilization through metasomatism and/or lateral accretion of oceanic lithosphere might have played a key role. High geothermal gradients acquired during the 2.0-1.2 Ga accretionary and continental arc phase may have helped sustain abnormally high temperatures during the 1.2-1.0 Ga collisional phase of the Grenville Province.