Abstract

An area of Archean rocks extending from northwestern Saskatchewan into the Northwest Territories is referred to as the Nolan block.Whole rocks and minerals separated from granodiorite surrounding an occurrence of amazonite at Portman Lake in the centre of this block have been studied in some detail using several dating techniques. The following ages result: graphic on abraded zircons from the granodiorite; 2589 ± 4 Ma whole-rock Pb/Pb age of the granodiorite; 2561 ± 6 and 2475 ± 13 Ma internal Pb/Pb isochrons; and 2610 ± 10 Ma Pb/Pb model ages on galena and amazonite.Pb/Pb internal isochrons were obtained by successive leaching of whole-rock samples and apparently indicate episodes of minor resetting of the whole-rock system. The close agreement between zircon ages, whole-rock Pb/Pb isochrons, and model ages on galena with no indication of Hudsonian updating suggest that the area has been essentially undisturbed since the Archean. The internal isochrons seem similar to Rb/Sr dates in the area and may indicate similarities in thermal response of the Rb–Sr and U–Pb systems in rocks.

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