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Abstract: Evidence for both vertical and horizontal movement is well preserved in the Cross Lake greenstone belt in the northwestern Superior Province. The vertical movement components are concentrated in high-strain [...]

New U-Pb geochronological and Sm-Nd isotopic results across the northwestern margin of the Superior Province in Manitoba, Canada, require a major reinterpretation of the nature, structure, and location of the Superior--Trans-Hudson boundary in the area. A significant finding of this study is that an area northwest of Assean Lake, previously interpreted to be dominantly Early Proterozoic, contains ancient crust. Detrital zircon U-Pb results from two metagraywacke samples in this ancient crustal segment reveal [is greater than or equal to] 3.75 Ga source material, consistent with crustal residence Nd model ages ([T.sub.CR]) of 3.85 and 3.76 Ga, and the U-Pb zircon age for one tonalite gneiss sample is ca. 3.54 Ga ([T.sub.CR] of 3.63 Ga). This newly discovered pre-3.5 Ga crust provides the first evidence for the possible preservation of some very old rocks (pre-3.8 Ga) in this region, older than any known Superior Province crust. Keywords: Superior Province, ancient crust, Early Archean, Manitoba, U-Pb zircon, Sm-Nd.

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. [Copyright &y& Elsevier]