38 Undergraduate Research in the Geological Sciences II (Posters)

Saturday, 29 March 2003
8:00 AM - 4:30 PM

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

Paper number: 38-43
Program: THEME SESSIONS
Time: 8:00 AM – 4:30 PM

Authors

Abstract

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.