Evidence for Archean ocean opening and closing in the Southern Slave Province

Archean mafic volcanic and plutonic rocks of the Cameron and Beaulieu River greenstone belts outcrop in the southern Slave Structural Province of the Canadian Shield. Lower portions of the greenstone belts have ophiolitic characteristics, but young felsic units may represent superimposed arc magmatism. The greenstone belts are separated from older (circa 2.7–3.2 Ga) quartzo‐feldspathic gneisses of the Sleepy Dragon Complex by a regional décollement zone (Patterson Lake Structural Complex) consisting of deformed mafic volcanic rocks, thin slivers of gneiss, ultramafic rocks, and banded iron formation mixed in a highly‐deformed phyllonitic and phacoidally‐cleaved metapelitic matrix. High synmetamorphic strains are indicated by abundant intrafolial folds, transposed layering, rare sheath folds and by the parallelism of elongated primary features and metamorphic mineral lineations. The Patterson Lake Structural Complex is similar to mélanges of Phanerozoic mountain belts. Adjacent quartzo‐feldspathic gneisses are strongly mylonitized. The mélange and deeper mylonites formed when the greenstone belts were emplaced as thrust sheets. Wildflysch shed from tips of emergent thrusts was in part incorporated into the basal mélange as the allochthon overrode its own detritus. Continued compression involved the basement in the deformation, uplifting the Sleepy Dragon Complex in a structural culmination mantled by the imbricated greenstone stack. Late granodioritic to granitic intrusions formed regional thermal aureoles which overprint earlier metamorphic fabrics.