Abstract

U–Pb isotope analysis of four zircon separates from a rhyolite porphyry that is part of the bimodal Narakay Volcanic Complex (Dease Arm, Great Bear Lake) yields an age of 1663 ± 8 Ma. East-trending feldspar-phyric dykes that cut the complex yield zircons that are, within analytical error, the same age as the porphyry. The Narakay Volcanic Complex occurs in the upper part of the Hornby Bay Group, the lowermost group of the Coppermine Homocline, and the stratigraphic position of the dated rocks indicates that 1663 Ma is the depositional age of the upper Hornby Bay Group.This is the first U–Pb zircon age determined from rocks of the Coppermine Homocline and has important implications for previous geochronology and extrabasinal correlation. Earlier Rb–Sr dates from the conformably overlying Coppermine River Group basalts of ca, 1200 Ma, when taken together with the new data, suggest that the Dismal Lakes Group, which conformably overlies the Hornby Bay Group, was deposited during a period of more than 400 Ma. Given that the Dismal Lakes Group is a relatively thin, transgressive shallow platformal succession, this amount of time for deposition is interpreted to be excessive. If this interpretation is correct, then it follows that either there are many unrecognized diastems in the Dismal Lakes Group, or presently available dates for rocks of the Mackenzie magmatic event in the Coppermine Homocline region may be too young by several hundred million years.

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