Fluid Flow During Contact Metamorphism at Mary Kathleen, Queensland, Australia

Abstract

Corella marbles in the Mary Kathleen Fold Belt were infiltrated by fluids during low-pressure (∼200-MPa) contact metamorphism associated with the intrusion of the Burstall granite at 1730–1740 Ma. Fluids emanating from the granite [whole-rock (WR) δ18O=8.1–8.6%] produced Fe-rich massive and banded garnet—clinopyroxene skarns [δ18O(WR)=9.1–11.9%]. Outside the skarn zones, marble mineralogies define an increase in temperature (500 to > 575°C) and XCO2 (∼0.05 to >0.12) towards the granite, and most marbles contain isobarically univariant or invariant assemblages in the end-member CaO−MgO−Al2O3−SiO2−H2O−CO2 system. Marbles have calcite (Cc) δ18O and δ13C values of 12.3–24.6% and −1.0 to −3.9%, respectively. A lack of down-temperature mineral reactions in the marbles suggests that pervasive fluid infiltration did not continue after the thermal peak of contact metamorphism. The timing of fluid flow probably corresponds to a period of high fluid production and high intrinsic permeabilities during prograde contact metamorphism. The petrology and stable isotope geochemistry of the marbles suggest that these rocks were infiltrated by water-rich fluids. If fluid flow occurred up to the peak of contact metamorphism, the mineralogical and isotopic resetting is best explained by fluids flowing up-temperature toward the Burstall granite. However, if fluid flow ceased befor the peak of regional metamorphism, the fluid flow direction cannot be unambiguously determined. At individual outcrops, marble δ18O(Cc) values vary by several permil over a few square metres, suggesting that fluid fluxes varied by at least an order of magnitude on the metre to tens-of-metre scale. Fluids were focused across lithological layering; however, mesoscopic fractures are not recognized. The focusing of fluids was possibly via microfractures, and the variation in the degree of resetting may reflect variations in microcrack density and fracture permeability. The marble—skarn contacts represent a sharp discontinuity in both major element geochemistry and δ18O values, suggesting that, at least locally, little fluid flow occurred across these contacts.