Paleoproterozoic submarine intrabasinal rifting, Baffin Island, Nunavut, Canada: volcanic structure and geochemistry of the Bravo Lake Formation

Abstract

Rocks of the Bravo Lake Formation on Turtle Back Island and Pillow Island off the western coast of central Baffin Island, Nunavut, represent a well-exposed example of the mafic volcanic and intrusive sequences commonly preserved near the base of Paleoproterozoic intracratonic basins in the Archean Rae and Hearne provinces of the Canadian Shield. The Bravo Lake Formation is composed of relatively undeformed amygdaloidal pillowed flows, radial columnar and tortoise-shell jointed pillows, hydroclastic breccia, extensional partially sheeted dyke swarms, laminated mafic sediments, massive and fragmental flows, and layered and megacrystic intrusions. Volcanic structures and textures imply emplacement in a low-energy shallow (<2 km depth) submarine environment. High-temperature submarine hydrothermal alteration of the Bravo Lake Formation mobilized Ba and Rb and was subsequently followed by dry closed system prograde amphibolite-facies regional metamorphism. Selected immobile trace element and rare-earth element (REE) compositions characterize these rocks as alkali basalt, minor tholeiitic basalt, and minor fractionated intermediate rocks that display significant light REE enrichment and high field-strength element concentrations suggestive of within-plate basalts. The stratigraphic setting, volcanic and intrusive structures, and chemical compositions suggest formation during local strike-slip-related rifting, rather than a mantle plume environment. The Bravo Lake Formation and its correlatives in other Paleoproterozoic intracratonic basins indicate that crustal thinning and the resultant local rifting of continental crust was widespread throughout the Rae and Hearne provinces of the Canadian Shield during the Paleoproterozoic.