The paleomagnetic significance of new U-Pb age data from the Molson dyke swarm, Cauchon Lake area, Manitoba

Abstract

U-Pb geochronology, paleomagnetism, and petrography indicate that the Molson dyke swarm, along the western margin of the Superior craton, is a composite of at least two ages of intrusion. The more extensive younger dyke set, the Molson swarm sensu stricto, generally has a 030° trend, is mainly pyroxenitic to noritic with subordinate diabase, and has been related to rifting in a back-arc environment during closure of the Manikewan ocean at about 1920-1800 Ma which culminated in the Trans-Hudson Orogen. A U-Pb zircon age from one of these dykes, located at Cauchon Lake, indicates emplacement at 1877+7&#150 4, similar to two previous U-Pb age determinations on Molson dykes. Another dyke from Cauchon Lake yields a baddeleyite-zircon U-Pb date of 2091 ± 2 Ma and appears to be part of an older, mainly diabasic suite of east-northeast-trending dykes that may represent a continental rifting episode that preceded the opening of the Manikewan ocean. The new U-Pb age data require a revision to the interpretation of the A, B, and C paleomagnetic poles previously reported from Molson dykes. The A pole (16.1°N, 96.5°W), initially assigned an age of 1883 Ma, is now considered to be younger and derived from a Paleoproterozoic overprint associated with the Trans-Hudson Orogen at about 1700-1800 Ma. Pole B (27.1°N, 140.8°W) from the Molson swarm sensu stricto is now regarded as primary, and dated at 1880 Ma. Pole C can be subdivided into two poles, one virtually the same as B but of opposite polarity (and therefore about 1880 Ma old) and a new pole (53°N, 180°W) derived from a primary remanence and dated at 2091 Ma. The new paleomagnetic interpretations may have important consequences for tectonic models of the Trans-Hudson Orogen and for Paleoproterozoic continental reconstructions.