Abstract

Geological development of the Archean Abitibi and Wawa greenstone–granite subprovinces of the Superior Province occurred in three main stages: (1) a volcanic–sedimentary depositional stage ~2750–2696 Ma ago, (2) a metamorphic–tectonic event ~2700–2685 Ma ago, and (3) a late plutonic event at ~2685–2668 Ma. The area between Wawa in the west and Ivanhoe Lake in the east represents an oblique section through some 20 km of the crust, where geochronological relationships can be examined at different structural levels. Much of the eastern Wawa Subprovince is characterized by tonalite–granodiorite gneiss containing abundant mafic xenoliths and represents a crustal level intermediate between high-level metavolcanic rocks near Lake Superior and the deep-seated gneisses of the Kapuskasing structural zone. A tonalitic phase of the complex gneiss of the Wawa Subprovince has a minimum U–Pb zircon date of 2707 Ma but the fine zircons are partly reset, possibly in response to lit-par-lit injections of gneissic to foliated granodiorite, dated at graphic. The tonalitic gneiss, intrusive into metavolcanics, is probably synvolcanic in age; heat transfer into the upper crust during its emplacement would have caused metamorphism of volcanic rocks and may have initiated regional tectonism. The bodies of gneissic granodiorite crystallized at mesozonal levels at the same time that the post-tectonic Ivanhoe Lake pluton graphic was emplaced into metavolcanic rocks of the Abitibi Subprovince.The Kapuskasing stuctural zone, separating the Abitibi and Wawa Subprovinces, comprises northeast-striking belts of paragneiss, mafic gneiss, dioritic and tonalitic rocks, and the Shawmere anorthosite complex, all metamorphosed to upper amphibolite and granulite facies. A minimum lead–lead age of 2765 Ma was obtained from garnetiferous tonalite associated with the Shawmere anorthosite but the zircons have been strongly affected by subsequent metamorphism. Concordant dates on metamorphic zircon in mafic gneiss and on zircon from leucosome in paragneiss are 2650 ± 2 and graphic, respectively. These dates are at least 30 Ma younger than the age of the metamorphic–tectonic event inferred for the greenstone–granite subprovinces and indicate that the rocks of the Kapuskasing zone were at an elevated temperature 2650–2627 Ma ago. Two alternate models are possible. (1) A second metamorphic event, restricted to the Kapuskasing structural zone, produced prograde zircon in mafic gneiss 2650 ± 2 Ma ago and caused anatexis in paragneiss graphic ago. This would require a history of rapid cooling from > 750 °C in both rocks shortly after zircon formation in order to prevent high-temperature lead loss. (2) Metamorphic zircons grew during the pre-2680 Ma event, and the 2650 and 2627 Ma dates result from differential lead retention in the interval between the metamorphic peak and final cooling sometime after 2627 Ma ago. Both models require that the rocks now exposed in the Kapuskasing zone were hot long after the emplacement of post-tectonic plutons in the adjacent Abitibi and Wawa Subprovinces.

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