Temperatures based on the composition of calcite coexisting with dolomite range from 475 to 600C for 63 marbles from the Llano uplift of central Texas. In some cases, high temperatures were determined for marbles that contain an isobarically invariant assemblage consisting of calcite + dolomite + tremolite + diopside + forsterite. In contrast, relatively low calcite + dolomite temperatures of 475-480C were obtained for marbles containing the assemblage calcite + dolomite + tremolite + talc. Agreement between the calcite + dolomite temperatures and those inferred from silicate + carbonate equilibria in the marbles indicates that the temperatures generally reflect peak conditions of metamorphism, although some resetting has occurred. The marbles having the highest calcite-dolomite temperatures are generally found close to post-tectonic pluton contacts. Relatively low temperatures are recorded within approximately 2 km of pluton contacts, suggesting a possible thermal aureole. However, relatively high temperatures of 550-600C are recorded in marbles that are not spatially related to post-tectonic granites. These temperatures may be relicts of an earlier metamorphic event. -from Authors
AMERICAN MINERALOGIST
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