THERMOBAROMETRY AND PRESSURE TEMPERATURE PATHS IN THE GRENVILLE PROVINCE OF ONTARIO

To constrain the tectonic and metamorphic history of the Grenville Province of southern Ontario we have quantitatively evaluated changes in peak metamorphic pressures and temperatures in the region. Pressures increase northwest from the Frontenac Axis towards the Grenville Front, and they increase from 4-6 kb near Madoc to 10-11 kb south of North Bay. Further to the north pressures decrease to 8-9 kb in the Grenville Front Tectonic Zone north of the French and Mattawa Rivers. Temperatures form a broad high, reaching 800°C northeast of Parry Sound, and decreasing to 400-500°C in the Hastings Low near Madoc, 600-650°C east towards the Ottawa River, and 650-700°C near Sudbury. This regional P-T distribution is in good agreement with constraints available from the distribution of aluminosilicate polymorphs.Comparison of thermobarometric results with regional tectonic features shows a sharp discontinuity across the Mattawa and French Rivers, with a 1-2-kb pressure drop to the north. This implies that the major movement along this zone since the Grenville event was 'south-side-up' rather than 'north-side-up' as suggested by Lumbers (1971). Large P-T discontinuities are not apparent across the domain boundaries mapped by Davidson and co-workers east of Parry Sound, but small discontinuities may exist. Sparse data may indicate that the Central Metasedimentary Belt equilibrated at 1-2 kb lower pressures than the Central Gneiss Terrane.Zoning profiles in garnet-pyroxene pairs have been used to place constraints on the metamorphic pressure-temperature-time path in the Parry Sound, Port Severn, Bancroft, and Mattawa areas of the Grenville Province, Ontario. A nonlinear fitting routine was used to obtain best-fit core and rim analyses for garnets and pyroxenes. These results were combined with plagioclase core/rim analyses to obtain estimates of peak and retrograde conditions. The resultant retrograde P-Tpath has a slope of 7 ± 10 b/°C, and involves pressure changes of 0·6-2·1 kb for temperature changes of 60-130°C. © 1990 Oxford University Press.