GEOTHERMOMETRY AND GEOBAROMETRY OF PLAGIOCLASE-HORNBLENDE BEARING ASSEMBLAGES

Abstract

The reaction Hb+Zo(Ep)+H2O+CO2⇌Pl+ Chl+Cc+Q was studied under hydrothermal conditions at P total=2, 4, 6 and 8 kb at Xco2=0.1. The continious transition from tremolite (actinolite) to Al-rich hornblende was fixed along the equilibrium curve of the reaction, providing a complete solid solution in the calcic amphibole series. A dependence of Al content in Ca-amphiboles and coexisting plagioclases on PT-conditions of their crystallization, determined for a wide range of temperature (450–650° C) and pressure (2–8 kb), has been used for construction of the experimental geothermobarometer. This may be employed to deduce temperature and pressure conditions of metamorphism of the albite-epidote-amphibolite and the amphibolite facies metabasites, including zoisite (or epidote)-bearing assemblages. An application of the Hb-Pl geothermobarometer is illustrated on the Patom Highland amphibolites and also on the well-known mafic schists of Vermont.