FLUID-ROCK INTERACTION BETWEEN SYENITES AND MARBLES AT STEPHEN-CROSS QUARRY, QUEBEC, CANADA - PETROLOGICAL AND STABLE ISOTOPE DATA

Grenville dolomitic marbles and calc-silicates at Stephen Cross Quarry, Quebec, underwent contact metamorphism and metasomatism associated with the intrusion of the Wakefield syenite at ambient pressures of approximately 0.4GPa at 1090-1070Ma. Fluid infiltration produced exoskarns, calcite + periclase + forsterite +/- diopside +/- orthoclase assemblages in the marbles, and quartz + calcite +/- wollastonite +/- diopside +/- anorthite assemblages in the calc-silicates. Phase-equilibria in the CaO-MgO-Al2O3-SiO2-H2O-RCO2 system suggest that fluid infiltration occurred close to the thermal peak of contact metamorphism (715-815-degrees-C) and that the fluids had XCO2 less-than-or-equal-to 0.15. In the metasediments, deltaO-18 values of calcite ( Cc) are as low as 8.6 parts per thousand, suggesting that the fluids were in isotopic equilibrium with the syenites (deltaO-18 =8.8-10.2 parts per thousand). Marble deltaC-13(Cc) values are -0.1 to -3.2 parts per thousand; the lack of correlation between deltaC-13(Cc) and deltaO-18(Cc) is consistent with the infiltration of water-rich fluids. The resetting of stable isotopes and the mineralogical changes can be explained by time-integrated fluid fluxes of up to 110 m3/m2 (4 x 10(6) mol/m2), corresponding to actual fluxes of 3 x 10(-11) to 3 x 10(-12) m3/m2-s and intrinsic permeabilities of 10(-18) to 10(-20) m2 for fluid flow lasting 0.1-1Ma. Marble deltaO-18(Cc) values do not correlate well with distance from the syenite, and fluids were probably channelled across lithological layering. The correlation between the degree of resetting of marble deltaO-18(Cc) values with the abundance of submillimetre-wide veins, suggests that fluid focussing may have resulted from variations in fracture density. Late, lower temperature (< 500-degrees-C), fluid flow formed serpentine (Serp) and brucite (Br) from periclase and forsterite. deltaO-18(Br) and deltaO-18(Serp) values correlate with deltaO-18(Cc), suggesting that retrogression involved only limited volumes of fluid. The observation that DELTAO-18(Cc-Br) and DELTAO-18(Cc-Serp) values are higher in marbLes that have lower DELTAO-18(Cc) values is interpreted as indicating that fluid flow persisted to lower temperatures in those rocks due to higher intrinsic permeabilities. Calcite in the syenite was also formed by the influx of fluids during cooling. Syenite deltaO-18(Cc) values are approximately in isotopic equilibrium with the high-temperature silicate minerals, suggesting that again only minor volumes of fluid were involved. In detail fluid flow was prolonged and complex, creating problems for the application of quantitative fluid flow models.