Contrasting fluid/rock interaction between the Notch Peak granitic intrusion and argillites and limestones in western Utah: evidence from stable isotopes and phase assemblages

Abstract

The Jurassic Notch Peak granitic stock, western Utah, discordantly intrudes Cambrian interbedded pure limestones and calcareous argillites. Contact metamorphosed argillite and limestone samples, collected along traverses away from the intrusion, were analyzed for δ18O, δ13C, and δD. The δ13C and δ18O values for the limestones remain constant at about 0.5 (PDB) and 20 (SMOW), respectively, with increasing metamorphic grade. The whole rock δ18O values of the argillites systematically decrease from 19 to as low as 8.1, and the δ13C values of the carbonate fraction from 0.5 to −11.8. The change in δ13C values can be explained by Rayleigh decarbonation during calcsilicate reactions, where calculated \(\Delta ^{13} {\text{C}}_{\left( {{\text{CO}}_{\text{2}} - {\text{cc}}} \right)}\) is about 4.5 permil for the high-grade samples and less for medium and low-grade samples suggesting a range in temperatures at which most decarbonation occurred. However, the amount of CO2 released was not anough to decrease the whole rock δ18O to the values observed in the argillites. The low δ18O values close to the intrusion suggest interaction with magmatic water that had a δ18O value of 8.5. The extreme lowering of δ13C by fractional devolatilization and the lowering of δ18O in argillites close to the intrusion indicates oxgen-equivalent fluid/rock ratios in excess of 1.0 and X(CO2)F of the fluid less than 0.2. Mineral assemblages in conjunction with the isotopic data indicate a strong influence of water infiltration on the reaction relations in the argillites and separate fluid and thermal fronts moving thru the argillites. The different stable isotope relations in limestones and argillites attest to the importance of decarbonation in the enhancement of permeability. The flow of fluids was confined to the argillite beds (argillite aquifers) whereas the limestones prevented vertical fluid flow and convective cooling of the stock.