2-D NUMERICAL SIMULATION OF REACTION-PROGRESS IN CALC-SILICATE CONTACT AUREOLES

The progress of calc-silicate reactions in contact aureoles is influenced by the heat and fluid-flow fields, which change over duration of metamorphism The final mineral assemblages reflect the integrated flow fields. We have modeled the progress of calc-silicate reactions in evolving heat and H₂O-CO₂ fluid flow fields using a 2-D, finite-element approach. Sedimentary, metamorphic, and magmatic fluid sources were considered. The simulations allow us to track the temporal evolution of P-T-X(CO₂) of metamorphic reactions. The results low to middle-grade, phlogopite to diopside-forming reactions are mainly driven by heat as the CO₂ concentration and fluid pressure increase with reaction progress. The reactions reach completion in several kyears after magma intrusion within a CO₂-rich environment. CO₂-poor fluids may infiltrate the low-grade rocks only after the reactions have gone to completion. In the inner aureole, after initial infiltration of magmatic water, the fluids become CO₂-rich with progress of early reactions. The low density of the fluids enhances upward fluid flow near the pluton. The fluids become CO₂-poor once the early reactions go to completion. The production of wollastonite is mainly driven by infiltration of the CO₂-poor fluids as the hydrostatic pressure in the inner aureole drops from ~1300 bar to 700 bar. In fact, most of the wollastonite is produced during cooling by 15 kyears. The CO₂-poor fluids can be either sedimentary or magmatic, although infiltration of the latter significantly enhances the production of wollastonite. These results show that low-grade metamorphic rocks likely record the presence of a CO₂-rich fluid that exists in the outer aureole during early metamorphic stages, whereas high-grade rocks likely reflect the presence of a CO₂-poor fluid that drives the wollastonite-forming reaction. The distribution of mineral assemblages predicted by our simulations matches well the observed distribution in the Notch Peak aureole, Utah. The results provide new insights into the hydrologic and metamorphic evolution of the Notch Peak and other contact aureoles.