Mineralogical evidence for fluid-rock interaction accompanying prograde contact metamorphism of siliceous dolomites: Alta Stock Aureole, Utah, USA

Contact metamorphism of siliceous dolomite in the southern part of the meta morphic aureole of the Alta stock (Utah, USA) produced the prograde isograd sequence: talc (Tc), tremolite (Tr), forsterite (Fo), and periclase (Per). Calcite (Cc)-dolomite (Do) geothermometry and phase equilibria define a ge neral prograde T X(CO2) path of decreasing X(CO2) with rising temperature f or the dolomite. High-variance assemblages typify the aureole. Per + Cc and Fo + Cc + Do characterize the inner aureole (Per and Fo zones), and Tr + D o + Cc and Tc + Do + Cc are widespread in the outer aureole (Tr and Tc zone s). Low-variance assemblages are rare and the thickness of reaction zones ( coexisting reactant and product minerals) at the isogradic reaction fronts are narrow (tens of metres or less). The mineral assemblages, calculated pr ogress of isograd reactions, and the prograde T X(CO2) path all indicate th at massive dolomite was infiltrated by significant fluxes of water-rich flu ids during prograde metamorphism, and that the fluid flow was down-temperat ure and laterally away from the igneous contact. Fluid infiltration continu ed through at least the initial retrograde cooling of the periclase zone. D own-T fluid flow is also consistent with the results of Cc-Do geothermometr y and patterns of O-18 depletion in this area. The close spatial associatio n of reacted and unreacted chert nodules in both the tremolite and talc zon es plus the formation of tremolite by two reactions indicate that the outer aureole varied in X(CO2), and imply that fluid flow in the outer aureole w as heterogeneous. The occurrence of dolomite-rich and periclase (brucite)-a bsent, high delta(18)O marble layers within the periclase zone indicates th at fluid flow in the innermost aureole was also heterogeneous. Estimates of the average time-integrated fluid flux (q(TIFF)(m)) experienced by the per iclase, forsterite, and tremolite zones are 4.2 x 10(7), 6.65 x 10(3), and 2.0 x 10(5) mol fluid/m(2), respectively. The average value of q(TIFF)(m) f or the periclase zone agrees well with the q(TIFF)(m) (3.4 x 10(7) mol/m(2) ) determined by numerical simulation of the temperature and O-18 depletion profiles preserved in the southern aureole. The estimates of q(TIFF)(m) for the forsterite and tremolite zones have much greater uncertainty, but may indicate that fluid flux was considerably lower in these zones than in the periclase zone. Given the outward (down-temperature), subhorizontal flow ge ometry indicated by a variety of petrologic, geochemical, and geothermometr y evidence presented here and elsewhere, this decrease implies that fluid h as leaked from the flow system between the periclase and tremolite zones.