EVIDENCE FOR A MAJOR MESOPROTEROZOIC MAGMATIC PROVINCE WITHIN THE WESTERN GRENVILLE PROVINCE NEAR NORTH BAY, ONTARIO

Mafic intrusions near Temagami provide evidence for a bimodal magmatic province in northwestern Grenville Orogen. The intrusions occur in Tomiko domain, near the Grenville Front. The Fall Lake intrusion consists of little metamorphosed gabbro and leucotroctolite. The Fanny Lake intrusion consists of olivinite and troctolite, preserves igneous texture, and has metamorphic coronas around olivine & clinopyroxene. Geochemistry indicates that both bodies are slightly alkalic, compositionally similar to the Sudbury diabase dike swarm (1238 ± 4 Ma), and have affinities to within-plate basalts. The Fall Lake intrusion yielded pristine baddeleyite, with 3 concordant or just slightly discordant grains giving an average 207Pb/206Pb age of 1235 ± 2 Ma - the best estimate of its age. The Fanny Lake sample yielded baddeleyite, with some grains having thin zircon overgrowths, consistent with the presence of coronas in the body. Two concordant grains without overgrowths gave an average 207Pb/206Pb age of 1238 ± 2 Ma. This result suggests a small but distinct age difference, although the ages overlap within uncertainty. The slightly older age of the Fanny Lake body is consistent with field evidence that it is more metamorphosed. Both intrusions are spatially associated with the A-type Mulock granite, dated previously at 1244+4/-3 Ma. Intrusions of similar age include the Sudbury dikes, Mercer anorthosite, and West Bay, Powassan and Pine Chute granitoids. The new data provides further evidence for a bimodal magmatic province active from 1270-1235 Ma within the Laurentian margin of the Grenville Orogen. The tectonic setting is interpreted as an extensional rift formed inboard of a continental arc active along the southern margin of North America from 1450-1300 Ma. This setting resembles that of the Cenozoic Columbia River Basalt Group. Tectonic linkage, if any, between Tomiko magmatism and bimodal anorogenic magmatism at ~1245 Ma in the Central Metasedimentary Belt, Grenville Orogen, is unresolved.