Status of thermobarometry in granitic batholiths

Abstract

Most granitic batholiths contain plutons which are composed of low-variance mineral assemblages amenable to quantification of theP–conditions that characterise emplacement. Some mineral thermometers, such as those based on two feldspars or two Fe–Ti oxides, commonly undergo subsolidus re-equilibration. Others are more robust, including hornblende–plagioclase, hornblende–clinopyroxene, pyroxene–ilmenite, pyroxene–biotite, garnet–hornblende, muscovite-biotite and garnet–biotite. The quality of their calibration is variable and a major challenge resides in the large range of liquidus to solidus crystallisation temperatures that are incompletely preserved in mineral profiles. Further, the addition of components that affect Kdrelations between non-ideal solutions remains inadequately understood. Estimation of solidus and near-solidus conditions derived from exchange thermometry often yield results >700°C and above that expected for crystallisation in the presence of an H2O-rich volatile phase. These results suggest that the assumption of crystallisation on an H2O-saturated solidus may not be an accurate characterisation of some granitic rocks.