Thermobarometry in granulites: pitfalls and new approaches

Abstract

Computer models of cooling granulite-facies rocks with the assemblage garnet+biotite+sillimanite+K-feldspar±plagioclase+quartz are presented that incorporate simultaneous reaction (net transfer and exchange) and multicomponent diffusion in garnet. In models where cooling is accompanied by exchange reactions plus diffusion only, the calculated garnet core+matrix biotite temperature is always lower than the metamorphic peak by an amount that depends on the cooling rate, the size of the garnet and the volumetric ratio of garnet to biotite. Maximum calculated core temperatures of approximately 700–750°C are to be expected from typical granulite-facies garnet-biotite assemblages and calculated core temperatures can easily be as low as 600°C. In models where both net transfer and exchange reactions are possible, calculated garnet core+matrix biotite temperatures are typically higher than the metamorphic peak temperatures (by more than 150°C) for large garnets ( R > 2 mm) but may be lower than the metamorphic peak for small garnets ( R < 1 mm). The composition of matrix plagioclase in equilibrium with garnet is very sensitive to the amount of modal plagioclase that reacts and hence, calculated P- T trajectories are very sensitive to the petrologic interpretation of this recrystallization history. For models in which all the modal plagioclase reacts, the core and rim pressures may be faithfully recovered, provided that the temperature estimate for the core is reliable. For models in which only a small percentage of the modal plagioclase reacts, very small modal amounts of plagioclase may be in equilibrium with garnet rim, which, if incorrectly interpreted, may lead to grossly erroneous P- T path interpretations. A method to test an inferred P- T path model is presented that involves attempting to match the observed garnet and plagioclase zoning information with a forward model that involves simultaneous reaction and diffusion. An example from western New Hampshire, USA, is presented that closely matches the measured profile.