Mineralogical Evidence for Fluid–Rock Interaction Accompanying Prograde Contact Metamorphism of Siliceous Dolomites: Alta Stock Aureole, Utah, USA

Abstract

Contact metamorphism of siliceous dolomite in the southern part of agrees well with the q m TIFF (3·4 × 10 7 mol/m 2 ) determined by the metamorphic aureole of the Alta stock (Utah, USA) produced numerical simulation of the temperature and 18 O depletion profiles the prograde isograd sequence: talc (Tc), tremolite (Tr), forsterite preserved in the southern aureole. The estimates of q m TIFF for the (Fo), and periclase (Per). Calcite (Cc)–dolomite (Do) ge- forsterite and tremolite zones have much greater uncertainty, but othermometry and phase equilibria define a general prograde T– may indicate that fluid flux was considerably lower in these zones X(CO2) path of decreasing X(CO2) with rising temperature for than in the periclase zone. Given the outward (down-temperature), the dolomite. High-variance assemblages typify the aureole. Per + subhorizontal flow geometry indicated by a variety of petrologic, Cc and Fo + Cc + Do characterize the inner aureole (Per and geochemical, and geothermometry evidence presented here and elseFo zones), and Tr + Do + Cc and Tc + Do + Cc are where, this decrease implies that fluid has leaked from the flow widespread in the outer aureole (Tr and Tc zones). Low-variance system between the periclase and tremolite zones. 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 progress of isograd reactions, and the prograde T–X(CO2) path all KEY WORDS: Alta; fluid flow; infiltration; marble; mineralogical indicate that massive dolomite was infiltrated by significant fluxes of water-rich fluids during prograde metamorphism, and that the fluid flow was down-temperature and laterally away from the igneous contact. Fluid infiltration continued through at least the initial retrograde cooling of the periclase zone. Down-T fluid flow INTRODUCTION is also consistent with the results of Cc–Do geothermometry and Both mineral assemblages and stable isotopes have been patterns of 18 O depletion in this area. The close spatial association