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Temperatures based on the composition of calcite coexisting with dolomite (calcite + dolomite thermometry) range from 475 to 600 °C for 63 marbles from the Llano uplift of central Texas. The highest temperatures, ∼600 °C, were obtained by carefully reintegrating calcite containing exsolved lamellae of dolomite. In some cases, these high temperatures were determined for marbles that contain an isobarically invariant assemblage consisting of calcite + dolomite + tremolite + diopside + forsterite. At a pressure of 3 kbar, these five minerals are stable at 630 °C and X CO2  = 0.62. In contrast, relatively low calcite + dolomite temperatures of 475-480 °C were obtained for marbles containing the assemblage calcite + dolomite + tremolite + talc. This talc-bearing assemblage is stable at ≤475 °C, depending on fluid composition, at a pressure of 3 kbar. Additional isobarically univariant equilibria are stable at intermediate temperatures (generally between 535 and 630 °C), and these are also generally consistent with results obtained from calcite + dolomite thermometry. 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, as well as those containing the high- temperature isobarically invariant assemblage, are generally found close to post-tectonic pluton contacts, indicating that some of the amphibolite-facies assemblages are related to the emplacement of the granitic intrusions. Relatively low temperatures are recorded with­in approximately 2 km of pluton contacts, suggesting a possible thermal aureole. However, relatively high temperatures of 550-600 °C are recorded in marbles that are not spatially related to post-tectonic granites (&gt;2 km from pluton contacts). These temperatures may be relicts of an earlier metamorphic event, although they could be related to granites that were emplaced above or below the current erosional level.

american mineralogist

Comparison of calcite + dolomite thermometry and carbonate + silicate equilibria: Constraints on the conditions of metamorphism of the Llano uplift, central Texas, U.S.A.