The Paleoproterozoic granite factory: Voluminous post-collisional, ferroan, A-type granites and implications for crust formation and metallogenic tenor, Labrador, Canada

Abstract

Arc magmatism along convergent plate boundaries has been a major contributor to continental crustal growth. During the late Paleoproterozoic−Mesoproterozoic assembly of the supercontinent Nuna, following arc-backarc-continent collisions, the Makkovik Orogen, along the southern margin of the Archean North Atlantic Craton, experienced a pulse of continental arc magmatism at ca. 1800 Ma. Lithogeochemistry indicates compositions typical of ferroan, alkali to alkali-calcic, A-type granites. The granitic rocks have εNd(t) values varying from −5.3 to +2.0 and model ages [T(DM)] from 2750 to 1880 Ma. The very high εNd(t) and young model ages imply a substantial juvenile mantle component in their formation. Oxygen fugacity (ƒO2) ratios, estimated by the proxy ratio of FeOT/(FeOT + MgO), range from 0.58 to 0.99, indicating the parental magmas formed in both reducing and oxidizing conditions. The granites also preserve an alkali-calcic alteration that together with inferred ƒO2 makes them more prospective for a variety of intrusion-related styles of mineralization including iron-oxide-copper-gold and uranium. Based on the petrology, lithogeochemistry, and isotopic compositions, the ca. 1800 Ma magmatic event is interpreted to have been developed post-collisionally following the docking of the Cape Harrison Arc/micro-continent with the North Atlantic Craton during the assembly of the supercontinent Nuna. This collision was followed by slab rollback (possibly breakoff), extensional collapse, and mantle upwelling resulting in the generation of the abundant ca. 1800 Ma felsic magmatism. The timing of the ca. 1818−1799 Ma plutonism in the Makkovik Orogen is coeval with significant felsic plutonism preserved in the Julianehåb Igneous Complex (Ketilidian Orogen) of southern Greenland.