Nature of assean lake ancient crust, Manitoba: a combined SHRIMP–ID-TIMS U–Pb geochronology and Sm–Nd isotope study

The Assean Lake Crustal Complex (ALCC) located at the northwest periphery of the Superior Province in northern Manitoba, Canada, hosts an ancient assembly of rocks that record a complex and prolonged crustal history spanning more than two billion years from ∼3.9 to 1.8 Ga. A supracrustal assemblage consisting of amphibolite-grade greywacke, quartz arenite, arkose, mafic to intermediate volcanics, and to a lesser extent iron formation dominates the ALCC and likely formed in a shallow marine arc/back-arc setting. The supracrustal rocks are intruded by amphibolite-grade, calc-alkaline tonalite to granite orthogneiss. Nd model ages for felsic para- and orthogneisses range from ∼3.5 to 4.2 Ga and suggest that Paleoarchean basement may be preserved at Assean Lake. SHRIMP U–Pb zircon ages and ID-TIMS conventional U–Pb zircon and monazite ages show that felsic magmatism occurred at ∼3.1–3.2 Ga whereas a record extending back to at least 3.9 Ga is preserved in detritus in paragneiss that may have a depositional age of ∼3.2 Ga. Orthogneiss was metamorphosed at ∼2.68 Ga whereas local migmatisation of paragneiss occurred at ∼2.61 Ga. Monazite growth events at ∼2.6, 2.45, and 1.8 Ga might represent episodes of accretion of Archean crustal segments including the ALCC and possibly the Sask craton (Trans-Hudson basement) along the northwest Superior Province margin. At present, matching segments of the exotic ALCC are unknown but remnants may exist in parts of the Sask craton.