CONTACT-METAMORPHISM SURROUNDING THE ALTA STOCK - FINITE-ELEMENT MODEL SIMULATION OF HEAT-TRANSPORT AND O-18 O-16 MASS-TRANSPORT DURING PROGRADE METAMORPHISM/

Results of calcite-dolomite geothermometry and oxygen isotope studies of marbles in the southern portion of the contact aureole surrounding the Alta stock (Utah) provide evidence for extensive hydrothermal meta morphism in this part of the aureole, Simulation df these two independ ent data sets with two-dimensional, finite element fluid flow and heat transport models constrains the pattern of fluid how, minimum permeab ility, and the permeability structure in this part of the aureole. Mod el results demonstrate that intrusion of the stock into a homogeneous, isotropic permeability medium yields peak metamorphic temperatures si gnificantly lower than those measured in the marbles and significant O -18 depletions both above and below the Alta-Grizzly thrust system, Th e latter contradicts the observations in the south aureole that O-18 d epletions in the marbles are restricted to marbles below the Alta-Griz zly thrust; dolomitic marbles above the thrust retain original sedimen tary values up to the intrusive contact, Models with horizontal permea bility barriers above the Alta-Grizzly thrust and extending over the t op of the Alta stock are capable of reproducing the observed thermal a nd delta(18)O profiles in the southern aureole, The presence of such h orizontal barriers reduces the predominantly vertical fluid flow and h eat transfer that would occur in a homogeneous and isotropic permeabil ity medium, forcing fluid flow and heat transfer laterally away from t he upper flanks of the stock, Such horizontal flow patterns are necess ary to produce significant O-18 depletion in the marbles beneath the A lta-Grizzly thrust, avoid O-18 depletion above the thrust, and to prov ide the necessary lateral heat transfer to duplicate the observed temp erature profile. Best fit model results to the observed thermal and de lta(18)O profiles provide several new insights into the dynamics of fl uid circulation and hydrogeologic characteristics of the southern Alta aureole during prograde metamorphism, Permeability of less than 1 x 1 0(-18) m(2) is required to exclude fluid flow in the horizontal barrie rs, Successful simulation of the observed temperature profile requires a minimum permeability of approx 2 x 10(-16) m(2) for the infiltrated marbles. Even moderately lower permeabilities (<1.0 x 10(-16) m(2)) p revent sufficient advective fluid flux into the marbles, with the resu lting failure to reproduce the observed temperature-distance profile. This minimum estimate of permeability is not particularly sensitive to values of other parameters such as heat capacity, intrusive temperatu re, or basal heat flow. Timescales of about 5000 to 6000 yrs and time- integrated fluid fluxes (TIFF) of approx 3000 m(3)/m(2) are sufficient to reproduce the observed width of the periclase zone, the temperatur es in the inner aureole, and the observed advance of the oxygen isotop e exchange front into the aureole. The model TIFF compares well with t he value of TIFF calculated from measured reaction progress in the per iclase zone. Peak temperatures are obtained only later at the tremolit e isograd (by 10,000 yrs), after the inner aureole is already undergoi ng thermal retrogradation. These timescales for fluid flow and prograd e heating of the southern aureole are significantly shorter than those estimated from generic models of conductive cooling of the stock and reflect the Important role of advective heat transport during the earl y history at this structural level of the aureole. Fluid how must have decreased significantly after 5000 to 10,000 yrs; otherwise the oxyge n isotopic exchange front would have advanced much farther than observ ed into the aureole. This decrease in fluid flow implies a significant decrease in permeability of the marbles in the inner aureole at this time. Significant thermal retrogradation of the periclase zone also be gins in the time period 5000 to 10,000 yrs in the simulations and is m arked in the inner aureole by replacement of periclase by brucite. The retrograde replacement of periclase by brucite, a reaction that invol ves a significant increase in volume, provides a plausible mechanism a t the proper time for the inferred decrease in permeability.