Fluid Flow During Contact Metamorphism at Mary Kathleen, Queensland, Australia |
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Authors: | CARTWRIGHT, IAN OLIVER, NICHOLAS H. S. |
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Affiliation: | Victorian Institute of Earth and Planetary Sciences, Department of Earth Sciences, Monash University Clayton, Vic. 3168, Australia |
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Abstract: | Corella marbles in the Mary Kathleen Fold Belt were infiltratedby fluids during low-pressure (200-MPa) contact metamorphismassociated with the intrusion of the Burstall granite at 17301740Ma. Fluids emanating from the granite [whole-rock (WR) 18O=8.18.6%]produced Fe-rich massive and banded garnetclinopyroxeneskarns [18O(WR)=9.111.9%]. Outside the skarn zones, marblemineralogies define an increase in temperature (500 to >575C) and XCO2 (0.05 to >0.12) towards the granite, andmost marbles contain isobarically univariant or invariant assemblagesin the end-member CaOMgOAl2O3SiO2H2OCO2system. Marbles have calcite (Cc) 18O and 13C values of 12.324.6%and 1.0 to 3.9%, respectively. A lack of down-temperaturemineral reactions in the marbles suggests that pervasive fluidinfiltration did not continue after the thermal peak of contactmetamorphism. The timing of fluid flow probably correspondsto a period of high fluid production and high intrinsic permeabilitiesduring prograde contact metamorphism. The petrology and stableisotope geochemistry of the marbles suggest that these rockswere infiltrated by water-rich fluids. If fluid flow occurredup to the peak of contact metamorphism, the mineralogical andisotopic resetting is best explained by fluids flowing up-temperaturetoward the Burstall granite. However, if fluid flow ceased beforthe peak of regional metamorphism, the fluid flow directioncannot be unambiguously determined. At individual outcrops,marble 18O(Cc) values vary by several permil over a few squaremetres, suggesting that fluid fluxes varied by at least an orderof magnitude on the metre to tens-of-metre scale. Fluids werefocused across lithological layering; however, mesoscopic fracturesare not recognized. The focusing of fluids was possibly viamicrofractures, and the variation in the degree of resettingmay reflect variations in microcrack density and fracture permeability.The marbleskarn contacts represent a sharp discontinuityin both major element geochemistry and 18O values, suggestingthat, at least locally, little fluid flow occurred across thesecontacts. |
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