Thermobarometry and Pressure-Temperature Paths in the Grenville Province of Ontario

To constrain the tectonic and metamorphic history of the GrenvilleProvince of southern Ontario we have quantitatively evaluatedchanges in peak metamorphic pressures and temperatures in theregion. Pressures increase northwest from the Frontenac Axistowards the Grenville Front, and they increase from 4–6kb near Madoc to 10–11 kb south of North Bay. Furtherto the north pressures decrease to 8–9 kb in the GrenvilleFront Tectonic Zone north of the French and Mattawa Rivers.Temperatures form a broad high, reaching 800?C northeast ofParry Sound, and decreasing to 400–500?C in the HastingsLow near Madoc, 600–650?C east towards the Ottawa River,and 650–700?C near Sudbury. This regional P-T distributionis in good agreement with constraints available from the distributionof aluminosilicate polymorphs. Comparison of thermobarometric results with regional tectonicfeatures shows a sharp discontinuity across the Mattawa andFrench Rivers, with a 1–2-kb pressure drop to the north.This implies that the major movement along this zone since theGrenville event was ‘south-side-up’ rather than‘north-side-up’ as suggested by Lumbers (1971).Large P-T discontinuities are not apparent across the domainboundaries mapped by Davidson and co-workers east of Parry Sound,but small discontinuities may exist. Sparse data may indicatethat the Central Metasedimentary Belt equilibrated at 1–2kb lower pressures than the Central Gneiss Terrane. Zoning profiles in garnet-pyroxene pairs have been used to placeconstraints on the metamorphic pressure-temperature-time pathin the Parry Sound, Port Severn, Bancroft, and Mattawa areasof the Grenville Province, Ontario. A nonlinear fitting routinewas used to obtain best-fit core and rim analyses for garnetsand pyroxenes. These results were combined with plagioclasecore/rim analyses to obtain estimates of peak and retrogradeconditions. The resultant retrograde P-Tpath has a slope of7 ? 10 b/?C, and involves pressure changes of 0?6–2?1kb for temperature changes of 60–130?C. Present address: Department of Geosciences, University of Arizona, Tucson, Arizona 85721