Abstract
The Llano Uplift in Central Texas is a Grenville-aged metamorphic terrane consisting predominantly of amphibolite facies mineral assemblages. Temperatures for the amphibolite facies metamorphism are constrained at 475-650°C. Pressures of 1-4 and 6 -8 kbar are inferred for the southeast and northwest uplift, respectively. In the central uplift, calc-silicates contain quartz-calcite-wollastonite-grossular-anorthite. The stability of this assemblage shows that the calc-silicates equilibrated under H2O-rich conditions. Fluid inclusions occurring within the calc-silicate minerals are generally H2O-rich. Homogenization temperatures of 360-368°C, measured for texturally primary inclusions, correspond to densities of 0.53-0.82 g/cm^3. Isochores for these fluid inclusions, when combined with the equilibrium: grossular + quartz = wollastonite + anorthite, yield formation conditions of 500-550°C at 1-2 kbar. Calcite-dolomite temperatures of ~475-600°C were determined for 63 marbles. The highest temperatures of ~600°C were obtained from marbles containing calcite, dolomite, tremolite, diopside, and forsterite. At P = 3 kbar, these five minerals are stable at ~630°C. In contrast, lower calcite-dolomite temperatures of 475-480°C were obtained for marbles containing calcite, dolomite, tremolite, and talc. This talc-bearing assemblage is stable at [less than or equal to] 475°C, depending on fluid composition, at P = 3 kbar. Agreement between the calcite-dolomite temperatures and those inferred from silicate-carbonate equilibria indicate that the temperatures reflect peak conditions of metamorphism. The marbles having the highest calcite-dolomite temperatures are generally found close to post-tectonic plutons, providing evidence that some of the amphibolite facies assemblages are related to granite emplacement. Calcite-dolomite temperatures of ~600°C, determined for some samples away from pluton contacts, may indicate effects of an earlier metamorphism. Temperatures of ~590-650°C are obtained from garnet K[D] thermometry. Pressures determined using garnet-rutile-ilmenite-plagioclase-quartz, garnet-plagioclase-biotite-quartz, and garnet-aluminosilicate-quartz-plagioclase barometers are ~3-4 kbar in the southeast and ~6 - 8 kbar in the northwest...
Letargo, Maria Rosario R. (1993). Geothermobarometry and the metamorphic evolution of the Llano Uplift, Central Texas. Texas A&M University. Texas A&M University. Libraries. Available electronically from
https : / /hdl .handle .net /1969 .1 /DISSERTATIONS -1520219.