MICROPROBE MONAZITE GEOCHRONOLOGY OF FOOTWALL ROCKS BENEATH THE SUDBURY IGNEOUS COMPLEX

This work reports the results of U-Th-Pb dating of monazites from beneath the Sudbury igneous complex (SIC), Canada. The SIC is currently exposed as a 2.5 km thick, elliptical shaped body (60 x 30 km). The SIC represents the remains of an impact melt sheet formed at 1.85 Ga by the hypervelocity impact of a meteorite or comet. The Sudbury impact structure is estimated to be at least 250 km in diameter and the melt sheet originally 100 km in diameter. Studies of the metamorphic aureole at the northern rim of the SIC reveal an innermost pyroxene hornfels facies zone (200 m wide); a hornblende hornfels facies zone (up to 400 m wide); and an outer zone of plagioclase and quartz recrystallisation that equates to the albite-epidote hornfels facies. The aureole has a total thickness of 1.5 km. In order to attempt to understand the evolution of the metamorphic aureole in the context of pre, syn- and post-impact metamorphism, we have performed detailed microprobe geochronology on samples of footwall rocks beneath the SIC. We have analyzed 25 grains from two polished thin sections, one section (MM-43) from 270 m from the SIC and one (MM-45) at 560 m from the SIC. The results show that individual monazite grains show complex U-Th-Pb distributions and multiple age populations. Compositional domains do not necessarily match age domains, and different grains in the same thin section can show different age populations. However, four broad age populations are recognized: (1) 2.65 Ga, (2) 2.5-2.3 Ga, (3) 2.2 Ga and (4) 1.8 Ga. The younger age is present only in the sample closest to the SIC. The distal sample averages 2.65 Ga. These age populations coincide with known geological events in the region: (1) Kenoran metamorphism and anatexis of granitoid gneisses, with the production of the Cartier granites, which occupy much of the North Range of the SIC; (2) lower Huronian basic igneous activity, including gabbro-anorthosite complex formation (e.g., East Bull Lake) and intrusion of the Creighton and Murray granites in the South Range, all of which may coincide with so-called Blezardian metamorphism and anatexis. The Matachewan dyke swarm was intruded at 2.45 Ga; (3) regional-scale intrusion of the Nipissing dyke-sill system and (4) generation of the Sudbury impact structure.