RECENT CALCITE-GRAPHITE, TI-IN-BIOTITE, GARNET-BIOTITE, AND TWO FELDSPAR THERMOMETRY OF THE ADIRONDACK LOWLANDS, NY, AND SOUTHERN FRONTENAC TERRANE, ONTARIO

The Proterozoic Grenville Province of southern Ontario and northeast New York includes the granulite facies Frontenac terrane of Ontario and the amphibolite facies Adirondack Lowlands of upstate New York, separated by the St. Lawrence River. To investigate a potential tectonic break near the Black Lake Shear Zone (BLSZ), about 10 km southeast of the St. Lawrence, we collected marbles for calcite-graphite thermometry, and pelitic schists and gneisses for Ti-in-biotite, two feldspar, and garnet-biotite thermometry in a NW-SE traverse from Gouverneur, NY, to just over the St. Lawrence into Ontario. Previous work in the region has constrained metamorphic temperatures to 700-750°C in the Frontenac terrane, and 650-700°C in the Adirondack Lowlands (Streepy et al., 1997). Calcite-graphite thermometry by Kitchen and Valley (1995) in the Lowlands found a trough of low temperature (640±30°C) trending NNE-SSW and centered just west of Gouverneur. Temperatures to the west and east of this thermal trough, including the zone adjacent to the St. Lawrence in New York, were 30-40°C greater.

We found remarkable temperature discrepancies between different thermometers for the region. The Ti-in-biotite thermometer of Henry et al. (2006), gives results of 737±15°C west of Gouverneur increasing to an unreasonable 811°C just west of the St. Lawrence . The garnet-biotite thermometer of Perchuk and Lavrent'eva (1983) gives a range of 580±50°C west of Gouverneur increasing to 756°C in the Frontenac terrane. The two feldspar calibration of Stormer (1975) gives a range of 503±20°C west of Gouverneur to around 600°C in the Frontenac terrane. Temperatures obtained from calcite-graphite thermometry generally agree with those found by Kitchen and Valley (1995) with the greater temperatures found closer to the BLSZ. All four thermometers show the same general NE-SW trend from the Adirondack Lowlands into the Frontenac terrane, despite the temperature differences. The possibility of a terrane boundary at the BLSZ is still open to interpretation and the area is in need of more investigation . We recommend using the garnet-biotite and calcite-graphite geothermometers for future work in this region as they seem to give the most reasonable temperatures in relation to mineral assemblages.