Thin lamprophyre dykes are abundant in the vicinity of the ca. 1.1 Ga Midcontinent Rift System and the late Archean – Early Proterozoic Kapuskasing Uplift in the Canadian Shield northeast of Lake Superior. However, the relationship between these dykes and spatially associated alkalic-carbonatite complexes has been unclear. To ascertain the temporal relationship between the two, we have determined, by 40Ar/39Ar and U–Pb dating, precise emplacement ages for six lamprophyre dykes from the region. The [Formula: see text] Ma U–Pb date for perovskite from the western Marathon and Wawa areas is indistinguishable from the 1144 ± 7 Ma 40Ar/39Ar date for phlogopite from the eastern Kapuskasing Uplift, and it is concluded that these dykes are part of a regional 1.14 Ga swarm that extends over 300 km. These dykes were therefore emplaced some 35 Ma prior to the 1109 Ma commencement of Midcontinent Rift volcanism and coeval alkaline-complex activity, but 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 Abitibi diabase dykes represent the first magmatic response of initiation of the Midcontinent rifting event, and were precursors to the main rift magmatism at 1109 Ma.

The Kapuskasing uplift (KU) in the Superior Province of the Canadian Shield has been interpreted as an oblique cross section through the Archean mid-crust. However, the time of juxtaposition of the granulites of the KU against the lower grade rocks of the Abitibi greenstone belt (AGB) along the Ivanhoe Lake fault zone is problematic. To constrain the postmetamorphic tectonothermal history of the KU, we have conducted 57 40Ar/39Ar step-heating analyses on mineral and rock samples collected in a transect across the southern KU and adjacent AGB. The age spectra record a complex thermal history. Amphiboles from the AGB in the footwall of the Ivanhoe Lake fault zone have ca. 2.66 Ga dates, similar to closure ages for amphiboles from farther east in the AGB. Amphibole dates of 2.46–2.52 Ga from the deepest structural levels of the KU place an upper limit on the time of major uplift of the granulites and their juxtaposition with the AGB. Biotite and muscovite dates from the transect cluster into three age groups. The presence in the deepest structural levels of the KU of biotite with 2.40–2.45 Ga dates indicates that significant uplift (15–20 km or more) of the granulites had occurred by this time. Micas with dates in the 2.25–2.30 Ga range are close to fault zones; these dates may indicate a ca. 2.30 Ga episode of fault reactivation. Feldspar, fault-related whole rocks, and some micas record events post 2.1 Ga. These correspond to the emplacement of mafic and lamprophyric dykes and fault reactivation.