Hornblende-garnet-plagioclase thermobarometry: a natural assemblage calibration of the thermodynamics of hornblende

Calibrations are presented for an independent set of four equilibria betwee n end-members of garnet, hornblende, plagioclase and quartz. Thermodynamic data from a large internally-consistent thermodynamic dataset are used to d etermine the DeltaG degrees of the equilibria. Then, with the known mixing properties of garnet and plagioclase, the non-ideal mixing in amphibole is derived from a set of 74 natural garnet-amphibole-plagioclase-quartz assemb lages crystallised in the range 4-13 kbar and 500-800 degreesC. The advanta ge of using known thermodynamic data to calculate DeltaG degrees is that co rrelated variations of composition with temperature and pressure are not ma nifested in fictive derived entropies and volumes, but are accounted for wi th non-ideal mixing terms. The amphibole is modelled using a set of ten ind ependent end-members whose mixing parameters are in good agreement with the small amount of data available in the literature. The equilibria used to c alibrate the amphibole non-ideal mixing reproduce pressures and temperature s with average absolute deviations of 1.1 kbar and 35 degreesC using an ave rage pressure-temperature approach, and 0.8 kbar with an average pressure a pproach. The mixing data provide not only a basis for thermobarometry invol ving additional phases, but also for calculation of phase diagrams in compl ex amphibole-bearing systems.