U–Pb zircon geochronology of the Kapuskasing structural zone and vicinity in the Chapleau–Foleyet area, Ontario

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 [Formula: see text]. 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 [Formula: see text] 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 [Formula: see text], 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 [Formula: see text] 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.

The tectonic evolution of the Abitibi greenstone belt of Canada

“…In addition, a similar intrusion (the Shawmere anorthosite) is present within the Kapuskasing structural zone (Percival & Coe, 1981;Percival & Krogh, 1983) and smaller gabbroanorthosite masses are present throughout the zone between Timmins and Chibougamau. Allard (1976) and Ludden, Francis & Allard (1984) have interpreted these layered intrusions as equivalents of the mafic magmas forming the volcanics of cycle I in the NVZ which have undergone gabbroic fractionation.…”

Age and isotopic geochemical characteristics of Archean carbonatites and alkaline rocks of the Baltic shield

“…These rocks host the Shawmere Anorthosite Complex, one of the largest (~800 km 2 ) among Archaean anorthosites. The age of intrusion is constrained by the time span of 2.68-2.62 Ga, while granulite facies metamorphism occurred between 2.66 and 2.64 Ga [Percival, Krogh, 1983;Krogh, 1993;Ashwal, Myers, 1994]. It is noteworthy that metamorphism of the low-crustal rocks took place soon after formation of the granite-greenstone association at the upper crustal level (2.73-2.67 Ga) [Corfu, 1987;Corfu, Ayres, 1991;Davis, 2002;Moser et al, 2008].…”

Microstructures, Mixed Layering, and Polymorphism of Chlorite and Retrograde Berthierine in the Kidd Creek Massive Sulfide Deposit, Ontario

“…Geology and whole-rock geochemistry of the footwall alteration zone are described by Coad (1985). Post-ore polymetamorphic and deformational history of the area is evidenced by mineral assemblages of the wall rocks and by various deformational features (Walker et al, 1975;Nunes and Pyke, 1981;Percival and Krogh, 1983;Brisbin et al, 1990;Barrie and Davis, 1990). The maximum metamorphic grade is believed to have corresponded with the lower to middle greenschist facies.…”

“…A small increase in diagenetic grade (or burial depth) results in replacement of berthierine by chlorite. The Kidd Creek area has experienced regional polymetamorphism to greenschist facies conditions together with multiple deformational events (Walker et al, 1975;Nunes and Pyke, 1981;Percival and Krogh, 1983;Brisbin et al, 1990;Barrie and Davis, 1990). Observations of deformational features and metamorphic reaction rims in tourmaline by Slack and Coad (1989) are consistent with pervasive metamorphic processes in the deposit.…”