New depositional age constraints for the Murmac Bay group of the southern Rae craton, Canada

The Murmac Bay group is one of several early supracrustal successions that were deposited on the southern Rae craton, Churchill Province, western Canadian Shield, and subsequently affected by Paleoproterozoic tectono-metamorphism. Where best understood, the succession comprises basal quartzite and/or psammite with intercalated dolostone, patchy iron formation, mafic volcanic flows, and overlying psammopelite–pelite. Previous attempts to date the Murmac Bay group have resulted in both Archean and Paleoproterozoic estimates. Re-interpretation of critical field relationships together with acquisition of new geochronological data have greatly improved estimates for its age of deposition, which in turn yield new constraints on the tectonic evolution of the region. Circa 2.33–2.32 Ga detrital zircon from three of four dated samples, including a rare basal conglomerate, establishes a maximum age for the initiation of deposition. A 2322 ± 5 Ma quartz–feldspar porphyry dyke intruding mafic volcanic rocks near the base of the succession indicates that deposition of the Murmac Bay group was initiated ca. 2.33 Ga, coeval with the earliest members of a 2.33–2.29 Ga granitic suite. These post-collisional granites were emplaced following a 2.37 Ga thermal culmination attributed to the Arrowsmith orogeny. Deposition of the Murmac Bay group continued until at least 2.17 Ga, the youngest detrital zircon age analyzed from the upper part of the succession. Deposition was over by ca. 1.93 Ga, when the Murmac Bay group underwent regional metamorphism and was intruded by the Donaldson Lake pluton, for which a revised ca. 1.93 Ga crystallization age has been obtained.According to our new age constraints, the upper Murmac Bay group may be temporally correlative with the Waugh Lake group, exposed about 100 km to the west-northwest at the eastern margin of the Taltson Magmatic Zone, and with rocks of the Rutledge River Basin, located 150 km to the northwest. A stronger case can be made for lithostratigraphic correlation of the Murmac Bay group with the lower three of four assemblages in the Amer and Ketyet River groups, 750 km to the northeast.The abundance of ca. 2.17 Ga detrital zircon in the Murmac Bay group, together with the presence of 2.2–2.07 Ga detritus in the Amer and Ketyet River groups, Rutledge River Basin, and in younger sedimentary successions in the region are thought to record an early accretionary event on the western margin of the Rae craton.

► New age data indicate initial deposition of Murmac Bay group ca. 2.33 Ga.
► Upper psammopelite deposited post-2.17 Ga; possibly in foreland basin.
► Abundant detritus may indicate terrane accretion to Rae craton at 2.17 Ga.
► Murmac Bay group broadly correlative with Amer and Ketyet River groups.
► Broad correlation of Waugh Lake group and Rutledge River Basin probable.