The Influence of Retrograde Cation Exchange on Granulite P-T Estimates and a Convergence Technique for the Recovery of Peak Metamorphic Conditions

Chemical relationships in garnet-orthopyroxene-plagioclase-quartzrocks are governed principally by three equilibria: the Fe-Mgexchange reaction between garnet and orthopyroxene, the solubilityof alumina in orthopyroxene coexisting with garnet, and thereaction of garnet and quartz to form orthopyroxene and plagioclase.Various thermobarometric calibrations of these equilibria havebeen applied to granulite-facies gneisses from two areas ofthe Proterozoic Complex of East Antarctica, and a wide rangeof P-T estimates is obtained for each area. Some of this P-Tvariation reflects the different thermodynamic data and mineralmixing models used by each calibration, but other differencesare attributed to the effects of retrograde Fe-Mg exchange.An inter-specimen spread of temperatures in each area, obtainedfor mineral core compositions with a single calibration of thegarnet-orthopyroxene exchange reaction, is attributed to a variableextent of Fe-Mg exchange on cooling from peak metamorphic conditions.A similar spread of pressures from the garnet-orthopyroxenealumina solubility barometer indicates that this calibrationis also reset by retrograde Fe-Mg exchange. In contrast, pressuresfrom the garnet-orthopyroxene-plagioclase-quartz barometer formineral cores show little variation between specimens from thesame area, indicating that this equilibrium is relatively insensitiveto changes in the Fe-Mg distribution coefficient and that derivedpressures are more likely to reflect peak metamorphic conditionsthan those from the alumina solubility barometer. Temperaturescan be corrected for Fe-Mg exchange using the Fe-Mg distributioncoefficient required to bring pressures from the exchange-sensitivealumina solubility barometer into agreement with reference pressurescalculated from the exchange-insensitive garnet-orthopyroxene-plagioclase-quartzbarometer. These corrected temperatures are closure temperaturesfor Al diffusion, which in many cases are likely to be goodestimates for the peak metamorphic temperature. The extent oftemperature correction in these specimens is 0–140C,and can be qualitatively related to textural features such asgrain size and mutual proximity of garnet and orthopyroxenegrains. Retrograde Fe-Mg exchange has clearly been significantin these rocks, with major consequences for thermobarometry.It is likely that Fe-Mg exchange during cooling is more widespreadthan currently recognized, and that the suggested convergencemethod for retrieving peak metamorphic conditions is applicableto other granulite terrains.