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Homogenization temperatures of two-phase fluid inclusions in cements, recrystallized allochems, overgrowths, and vein and cavity fillings show that Paleozoic carbonates of the Llano uplift have experienced at least three phases of temperature-related, pervasive crystallization-recrystallization. These phases have modes at 70 to 80°C, 110 to 120°C, and 200 to 210°C, with ranges of 60°C, 90°C, and 60°C, respectively. These phases may represent distinct heating-cooling events, or they may be the results of a single, thermally complex event.

The lack of any discernible regional trends in the data and the fact that Cretaceous rocks locally have been heated to at least 110°C suggest that heating was not related to regional metamorphism during the Ouachita orogeny. Yet, these temperatures seem too high to result merely from heating due to depth of burial associated with any reasonable geothermal gradient.

We postulate that northeast-trending faults, which cut basement and overlying Paleozoic rocks in the region, provided avenues for pervasive heating by contemporaneous fluids from the lower crust. These faults, like those of the Balcones-Luling and Talco-Mexia zones to the east, may be genetically related to late Paleozoic-early Mesozoic opening of the Gulf of Mexico. Waning geothermal activity associated with this rifting would explain the localized heating of Cretaceous rocks.

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