AR-40 AR-39 PHLOGOPITE AND U-PB PEROVSKITE DATING OF LAMPROPHYRE DYKES FROM THE EASTERN LAKE-SUPERIOR REGION - EVIDENCE FOR A 1.14 GA MAGMATIC PRECURSOR TO MIDCONTINENT RIFT VOLCANISM/

Thin lamprophyre dykes are abundant in the vicinity of the ca. 1.1 Ga Midcontinent Rift System and the late Archean - Early Proterozoic Kapu skasing Uplift in the Canadian Shield northeast of Lake Superior. Howe ver. the relationship between these dykes and spatially associated alk alic-carbonatite complexes has been unclear. To ascertain the temporal relationship between the two, we have determined, by Ar-40/Ar-39 and U-Pb dating, precise emplacement ages for six lamprophyre dykes from t he region, The 1144(-4)(+6) Ma U-Pb date for perovskite from the weste rn Marathon and Wawa areas is indistinguishable from the 1144 +/- 7 Ma Ar-40/Ar-39 date for phlogopite from the eastern Kapuskasing Uplift, and it is concluded that these dykes are part of a regional 1.14 Ga sw arm that extends over 300 km. These dykes were therefore emplaced some 35 Ma prior to the 1109 Ma commencement of Midcontinent Rift volcanis m and coeval alkaline-complex activity, hut essentially simultaneously with emplacement of the 1141 +/- 2 Ma Abitibi diabase dyke swarm. It is suggested that these widespread 1144 Ma lamprophyre dykes and the A bitibi diabase dykes represent the first magmatic response of initiati on of the Midcontinent rifting event, and were precursors to the main rift magmatism at 1109 Ma.