CALCITE-GRAPHITE GEOTHERMOMETRY AND MINERAL EQUILIBRIA IN AMPHIBOLITE FACIES MARBLES NEAR BANCROFT, ONTARIO

The Elzevir terrane of the southern Grenville Province (Central Metasedimentary Belt) contains metasedimentary rocks with abundant marble. These are interlayered with metavolcanic rocks and intruded by a wide variety of igneous plutons. Fifty marble samples from southern Faraday Township have been collected for geothermometry and phase equilibria study. Results of petrography indicate that assemblages of dolomite + quartz (Dol+Qtz), tremolite + calcite + quartz + K-feldspar (Tr+Cc+Qtz+Kfs), and diopside + phlogopite (Di+Phl) occur throughout a roughly 100 km2 area. Relatively high-grade assemblages (Di+Phl) occur in close proximity to low-grade assemblages (Dol+Qtz) throughout this area. This most likely reflects differences in mineral compositions and fluid compositions, rather than temperature variations. Univariant assemblages of Di + Dol + Qtz are common suggesting that fluid buffering was dominant during metamorphism, as opposed to open-system fluid behavior.

Metamorphic pressures of approximately 5 kbars are indicated by the regional distribution of aluminosilicates and garnet-sillimanite-quartz-plagioclase barometry near Bancroft. End-member phase equilibria for the reaction Dol + 2Qtz=Di + 2CO2 requires a temperature of ~610°C. Preliminary calcite-graphite geothermometry for six samples across the area range from 450° to 620°C. Clinopyroxene solid solution in natural samples ought to shift the Dol + Qtz=Di + CO2 reaction to lower temperatures, consistent with the T range provided by the thermometry. Mineral compositions will be obtained and used to quantify this shift. When combined with geothermometry, we will be able to constrain metamorphic fluid compositions.