Fluid evolution and transport during metamorphism: evidence from the Llano Uplift,Texas

A field, petrologic and stable isotopic investigation of the marbles and calc-silicates of the 1.15 b.y. Valley Spring Gneiss documents the dilution of internally evolved CO₂-rich fluids by externally derived aqueous fluids introduced along channelways. Reaction textures within calcsilicates record the evolution through time of initially CO₂-rich fluids toward increasingly more aqueous compositions. Assemblage zonations within calc-silicates require equilibration within local gradients of the mole fraction of CO₂ in the fluid, and suggest that the infiltration of aqueous fluids was largely channelized along more permeable lithologies. Localized depletions in ¹³C and ¹⁸O corroborate petrologic evidence for channelized infiltration. Isotopic compositions reflect both devolatilization and the introduction of low-¹⁸O fluids; estimated minimum oxygen-equivalent fluid-to-rock ratios are near unity. Both mineralogical and stable isotopic systematics document the essential role of infiltration in driving decarbonation reactions during calc-silicate formation. The calc-silicate assemblages which equilibrated with fluids of the lowest mole fraction of CO₂ record isotopic exchange equilibrium with fluids of ¹⁸O typical of those derived from normal granites, as do the granitic aplites and pegmatites which transect most calcsilicate occurrences. Thus the infiltrating fluids are believed to be genetically linked to the intrusion of a suite of granitic plutons emplaced after the peak of regional metamorphism.