The Archean Murmac Bay Group: evidence for a giant archean rift in the Rae Province, Canada

The lower amphibolite to-granulite facies Archean Murmac Bay Group of the southwestern Rae Province, Canada, formed during continental extension (rifting) of an underlying granitoid basement complex that includes the 3060 +/- 40 Ma Lodge Bay Granite and the 2999 +/- 7 Ma Cornwall Bay Granite. The contact between the Murmac Bay Group and the basement complex is an unconformity that is locally marked by polymictic conglomerate. Overlying the conglomerate is basal quartzite of the Murmac Bay Group that contains a laterally extensive (>10 km) intraformational oligomictic quartzite-pebble conglomerate that may have formed by uplift during rifting. Distinctive medium- to coarse-grained gabbro dykes and sills that cut both the basement complex and the basal quartzite are interpreted as feeders to stratigraphically overlying tholeiitic basalt flows that have geochemical signatures consistent with formation in a continental rift setting. The quartzite-basalt contact exhibits rare stratigraphic interlayering, and is a horizon along which oxide-and silicate-facies iron formation, dolostone, and pelite locally occur. Ultramatic rocks with komatiitic compositions also intrude these lower stratigraphic units. The upper Murmac Bay Group is dominated by pelitic to psammitic rocks that may represent deepening of the sedimentary basin during continued extension. Ancient approximately 3.62 and >3.7 Ga grains dominate the detrital zircon population of the Murmac Bay Group sediments, but deposition occurred after approximately 2.77 Ga, the youngest detrital zircon in our study. The age of the Murmac Bay Group is thus constrained as younger than approximately 2.77 Ga and older than the crosscutting 2631 +/- 12 Ma Dead Man and Donaldson Lake Granites. The gross lithological and stratigraphic make-up of the Murmac Bay Group closely resembles that of the Prince Albert, Woodburn Lake and Mary River groups of the northern Rae Province, all three of which have been interpreted as approximately 2.7 Ga continental rift successions. We propose that the Murmac Bay Group is correlative with these successions. If correct, the total length of this Neoarchean rift is at least 2000 km, extending from Baffin Island to northwestern Saskatchewan, Canada. The supracrustal rocks of the rift record the break-up of a large Paleo- to early Neoarchean continent (Nunavutia), which experienced magmatic events at approximately 3.0, 3.62, and >3.7 Ga. (C) 2004 Elsevier B.V. All rights reserved.