Journal of Petrology Volume 41 Number 6 Pages 739-757 2000
© Oxford University Press 2000
Mineralogical Evidence for Fluid–Rock Interaction Accompanying Prograde Contact Metamorphism of Siliceous Dolomites: Alta Stock Aureole, Utah, USA
DEPARTMENT OF GEOLOGY AND GEOPHYSICS, THE UNIVERSITY OF UTAH, SALT LAKE CITY, UT 84112, USA
Contact metamorphism of siliceous dolomite in the southern part of the metamorphic 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 general prograde T–X(CO2) path of decreasing X(CO2) with rising temperature for the dolomite. High-variance assemblages typify the aureole. Per + Cc and Fo + Cc + Do characterize the inner aureole (Per and Fo zones), and Tr + Do + Cc and Tc + Do + Cc are widespread in the outer aureole (Tr and Tc zones). 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 progress of isograd reactions, and the prograde T–X(CO2) path all 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 is also consistent with the results of Cc–Do geothermometry and patterns of 18O depletion in this area. The close spatial association of reacted and unreacted chert nodules in both the tremolite and talc zones 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 was heterogeneous. The occurrence of dolomite-rich and periclase (brucite)-absent, high-
18O marble layers within the periclase zone indicates that fluid flow in the innermost aureole was also heterogeneous. Estimates of the average time-integrated fluid flux (qmTIFF) experienced by the periclase, forsterite, and tremolite zones are 4·2 x 107, 6·65 x 105, and 2·0 x 105 mol fluid/m2, respectively. The average value of qmTIFF for the periclase zone agrees well with the qmTIFF (3·4 x 107mol/m2) determined by numerical simulation of the temperature and 18O depletion profiles preserved in the southern aureole. The estimates of qmTIFF 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 geometry indicated by a variety of petrologic, geochemical, and geothermometry evidence presented here and elsewhere, this decrease implies that fluid has leaked from the flow system between the periclase and tremolite zones.
KEY WORDS: Alta; fluid flow; infiltration; marble; mineralogical
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