Comparison of calcite + dolomite thermometry and carbonate + silicate equilibria; constraints on the conditions of metamorphism of the Llano Uplift, central Texas, U.S.A.

Abstract

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 carefullyreintegrating 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 OCand Xco, = 0.62. In contrast, relatively low calcite + dolomite temperatures of 475-480 °Cwere obtained for marbles containing the assemblage calcite + dolomite + tremolite + talc. This talc-bearing assemblage is stable at ~475 OC, depending on fluid composition, at a pressure of 3 kbar. Additional isobarically univariant equilibria are stable at intermediate temperatures (generallybetween 535 and 630 0q, and these are also generallyconsistent 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 hightemperature isobarically invariant assemblage, are generally found close to post-tectonic pluton contacts, indicating that some of the amphibolite-facies assemblagesare related to the emplacement ofthe granitic intrusions. Relatively low temperatures are recorded within approximately 2 km of pluton contacts, suggestinga possible thermal aureole. However, relatively high temperatures of 550-600 °C are recorded in marbles that are not spatially related to post-tectonic granites (>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.