Petrological and in situ SHRIMP geochronological constraints on the tectonometamorphic evolution of the Committee Bay belt, Rae Province, Nunavut

摘要

Petrology and in situ SHRIMP monazite geochronology provide new constraints on the polymetamorphic and polydeformational history of the Committee Bay belt, Rae Province, Nunavut, Canada. Rare ca. 2.58Ga monazite inclusions in garnet porphyroblasts in the eastern part of the belt are interpreted to reflect regional contact metamorphism (M_1) associated with voluminous 2.61-2.58Ga plutonism. Monazite also occurs as ca. 2.35Ga inclusions in andalusite in the central supracrustal belt, but is absent from the east. These data complement previous studies to the west, and suggest that the Committee Bay belt represents a ca. 2.35Ga tilted crustal section that was tectonically thickened±imbricated during the Arrowsmith orogeny (D_1-M_2). Penetrative reworking of the belt occurred during a diachronous D_2-M_3 tectonometamorphic event which involved northwest-vergent thickening to depths of ～5kbar across most of the belt. Although D_2 folds and fabrics across the belt are geometrically correlative, monazite is ～20Myr older in the western migmatite domain (ca. 1.86Ga) than in the western supracrustal belt (ca. 1.84Ga). This temporal difference is attributed to burial of more radiogenic lithologies in the migmatite domain. Tectonic thickening is attributed to ca. 1.87Ga collision of Meta Incognita microcontinent with the southeastern flank of the Rae Province. The eastern supracrustal belt exposes similar amphibolite-facies (M_3) northeast-striking, northwest-vergent D_2 structures that are dated at 1.815Ga. This younger reworking may reflect the influence of a major northward projecting promontory of the Superior Province which continued to affect the softened hinterland of the Rae Province during terminal collision at ca. 1.82Ga. The central Committee Bay belt records a younger, ca. 1.79Ga structural overprinting (D_3-M_4) which involved dextral shearing and differential burial to temperatures above monazite growth in a gentle synformal structure formed during the final stages of amalgamation of Laurentia.