Fluid-rock interaction between syenites and marbles at Stephen Cross Quarry, Québec, Canada: petrological and stable isotope data

Abstract

Grenville dolomitic marbles and calc-silicates at Stephen Cross Quarry, Québec, underwent contact metamorphism and metasomatism associated with the intrusion of the Wakefield syenite at ambient pressures of ∼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−CO₂ system suggest that fluid infiltration occurred close to the thermal peak of contact metamorphism (715–815°C) and that the fluids hadXCO₂≤0.15. In the metasediments, δ¹⁸O values of calcite (Cc) are as low as 8.6‰, suggesting that the fluids were in isotopic equilibrium with the syenites (δ¹⁸O =8.8–10.2‰). Marble δ¹³C(Cc) values are-0.1 to-3.2‰; the lack of correlation between δ¹³C(Cc) and δ¹⁸O(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 (4×10⁶ mol/m²), corresponding to actual fluxes of 3×10^-11 to 3×10^-12 m³/m²-s and intrinsic permeabilities of 10^-18 to 10^-20 m² for fluid flow lasting 0.1-1Ma. Marble δ¹⁸O(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 δ¹⁸O(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°C), fluid flow formed serpentine (Serp) and brucite (Br) from periclase and forsterite. δ¹⁸O(Br) and δ¹⁸O(Scrp) values correlate with δ¹⁸O(Cc), suggesting that retrogression involved only limited volumes of fluid. The observation that Δ¹⁸O(Cc-Br) and Δ¹⁸O(Cc-Serp) values are higher in marbles that have lower Δ¹⁸O(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 Δ¹⁸O(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.