ADDITIONAL EVIDENCE FOR A MAJOR ARCHEAN TERRANE BOUNDARY NEAR THE GRENVILLE FRONT IN ONTARIO

The presence of a southward-deepening Archean crustal section straddling the Grenville Front east of Sudbury was first proposed by the senior author in 2000. From N to S, it consists of: 1) metasedimentary and felsic plutonic rocks, 2) Grenville Front, 3) metatextites derived from the metasedimentary rocks (Pardo & Red Cedar Lake gneiss). To the west, amphibolite-rich diatextites dominate (Street gneiss), and, 4) migmatitic, tonalite to granodiorite gneiss (Crerar gneiss). The section is similar to that found in the Kapuskasing structural zone. The previous maximum age constraint on the rocks south of the Front was that they are intruded by the ~2475 Ma River Valley mafic intrusion. We report new ages on plutons from the Pardo, Crerar and Street gneiss. An alkali feldspar granite that cuts the Pardo gneiss but is itself cut by the River Valley intrusion yielded 3 discordant zircon analyses, that may in part reflecting Pb loss related to emplacement of the mafic intrusion. A highly precise age is not available presently for the granite, although ~2660 Ma is the best estimate. A titanite age of 966+/-6 Ma indicates that the granite was affected by Grenville metamorphism. Zircon and titanite analyses from a late-tectonic, K-feldspar megacrystic granodiorite in the Crerar gneiss yields an upper intercept age of 2663+/-2 Ma. Concordant titanite dates Grenvillian metamorphism at 974+/-12 Ma. An inferred Geon 24 pluton in the Street gneiss exhibited extreme discordance, possibly indicating 2 or 3 periods of Pb loss (metamorphism?). It may have a) been emplaced at ~2450 Ma and subjected to Pb loss at ~1500 Ma, b) been emplaced during Geon 18-19, or c) affected by shock metamorphism by the 1850 Ma Sudbury impact, in addition to Penokean and Grenvillian metamorphism. The ovoid shape of the pluton, in contrast with elongate and folded shapes of dated Geon 24 plutons in the area is an argument in favour of Geon 18 emplacement. The new U-Pb zircon ages further support the existence of a major Archean terrane boundary near the Grenville Front between Sudbury and Temagami, that separates ~2740-2700 Ma rocks of the Temagami greenstone belt to the north from 2680-2660 Ma metasedimentary and plutonic gneisses to the south. It is possible that the Grenville Front east of Sudbury may have been localized along this Archean terrane boundary.