Comparison of garnet-biotite, calcite-graphite, and calcite-dolomite thermometry in the Grenville Orogen; Ontario, Canada

The Elzevir Terrane of the Grenville Orogen in southern Ontario contains me tapelites and abundant graphitic marbles that were regionally metamorphosed from the upper greenschist to upper amphibolite facies. Comparative thermo metry was undertaken with widely used calibrations for the systems garnet-b iotite, calcite-dolomite, and calcite-graphite. Temperatures that are obtai ned from matrix biotites paired with prograde garnet near-rim analyses are usually consistent with those determined using calcite-graphite thermometry . However, calcite-graphite thermometry occasionally yields low temperature s due to lack of equilibration of anomalously light graphite. Application o f calcite-graphite and garnet-biotite systems may yield temperatures up to 70 degrees C higher than calcite-dolomite in amphibolite facies rocks. Calc ite-dolomite temperatures most closely approach those from calcite-graphite and garnet-biotite when the samples contain a single generation of dolomit e and calcite grains contain no visible dolomite exsolution lamellae. Howev er, some of these samples yield temperatures considerably lower than temper atures calculated from calcite-graphite and garnet-biotite thermometry, ind icating that the calcite-dolomite thermometer may have been partially reset during retrogression. Estimated peak metamorphic temperatures of regional metamorphism between Madoc (upper greenschist facies) and Bancroft (upper a mphibolite facies) range from 500 to 650 degrees C. These results place the chlorite-staurolite isograd at 540 degrees C, the kyanite-sillimanite isog rad at 590 degrees C, and the sillimanite-K-feldspar isograd at 650 degrees C. Although each thermometer may have an absolute uncertainty of as much a s +/- 50 degrees C, the 50 to 60 degrees C temperature differences between the isograds are probably accurate to 10 to 20 degrees C. An incomplete pic ture of the thermal gradients can result from the application of only one t hermometer in a given area. Simultaneous application of several systems all ows one to recognize and overcome the inherent limitations of each thermome ter.