P-T-X CONDITIONS OF CALC-SILICATE FORMATION - EVIDENCE FROM FLUID INCLUSIONS AND PHASE-EQUILIBRIA - LLANO UPLIFT, CENTRAL TEXAS, USA
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Abstract The Llano Uplift in central Texas is a Grenville aged (c. 1.1 Ga) metamorphic terrane consisting predominantly of amphibolite facies mineral assemblages. The formation of these assemblages has been attributed to the emplacement of relatively late granite plutons throughout the area. Two types of granitic intrusion have previously been recognized: (1) Town Mountain Granites, which occur as relatively large, circularshaped bodies of coarsegrained granite, and (2) Younger Granites which are present as smaller and more irregular bodies of finergrained granite. In the central part of the uplift, wollastonitebearing calcsilicate rocks occur within the Valley Spring Gneiss. The development of these calcsilicate rocks has been linked to infiltrating fluids presumably derived from spatially associated Younger Granites. The stability of coexisting quartz, calcite, wollastonite, grossular and anorthite and coexisting quartz, calcite, wollastonite, andradite and hedenbergite shows that the calcsilicate rocks equilibrated under H2Orich conditions with CO2 <0.10. Fluid inclusions present within the calcsilicate minerals are H2Orich with salinities of <17 wt% equivalent NaCl. The absence of any detectable CO2 in the fluid inclusions may indicate entrapment of the inclusions at lower pressures and more H2Orich conditions compared to the stability of the peak metamorphic mineral assemblage. Homogenization temperatures, measured for texturally primary inclusions, range from 360 to 368 C corresponding to a density range from 0.53 to 0.82 g/cm3. Isochores for these fluid inclusions, when combined with the stability of the solidsolid equilibria Grs + Qtz = Wo + An, yield formation conditions of 500550 C at 12 kbar. This indicates that the granitic intrusions involved in the formation of the Blount Mountain calcsilicates were emplaced at a pressure of at least 12 kbar.
