The stable isotope signature of kilometre‐scale fracturedominated metamorphic fluid pathways, Mary Kathleen, Australia

Abstract Large calcite veins and pods in the Proterozoic Corella Formation of the Mount Isa Inlier provide evidence for kilometre‐scale fluid transport during amphibolite facies metamorphism. These 10‐ to 100‐m‐scale podiform veins and their surrounding alteration zones have similar oxygen and carbon isotopic ratios throughout the 200 × 10‐km Mary Kathleen Fold Belt, despite the isotopic heterogeneity of the surrounding wallrocks. The fluids that formed the pods and veins were not in isotopic equilibrium with the immediately adjacent rocks. The pods have δC values of –2 to –7% and δO values of 10.5 to 12.5%. Away from the pods, metadolerite wallrocks have δO values of 3.5 to 7%. and unaltered banded calc‐silicate and marble wallrocks have δC of –1.6 to –0.6%, and δO of 18 to 21%. In the alteration zones adjacent to the pods, the δO values of both metadolerite and calc‐silicate rocks approach those of the pods. Large calcite pods hosted entirely in calc‐silicates show little difference in isotopic composition from pods hosted entirely in metadolerite. Thus, 100‐ to 500‐m‐scale isotopic exchange with the surrounding metadolerites and calc‐silicates does not explain the observation that the δO values of the pods are intermediate between these two rock types. Pods hosted in felsic metavolcanics and metasiltstones are also isotopically indistinguishable from those hosted in the dominant metadolerites and calc‐silicates. These data suggest the veins are the product of infiltration of isotopically homogeneous fluids that were not derived from within the Corella Formation at the presently exposed crustal level, although some of the spread in the data may be due to a relatively small contribution from devolatilization reactions in the calc‐silicates, or thermal fluctuations attending deformation and metamorphism. The overall L‐shaped trend of the data on plots of δC vs. δO is most consistent with mixing of large volumes of externally derived fluids with small volumes of locally derived fluid produced by devolatilization of calc‐silicate rocks. Localization of the vein systems in dilatant sites around metadolerite/calc‐silicate boundaries indicates a strong structural control on fluid flow, and the stable isotope data suggest fluid migration must have occurred at scales greater than at least 1 km. The ultimate source for the external fluid is uncertain, but is probably fluid released from crystallizing melts derived from the lower crust or upper mantle. Intrusion of magmas below the exposed crustal level would also explain the high geothermal gradient calculated for the regional metamorphism