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A garnet-hornblende Fe-Mg exchange geothermometer has been calibrated against the garnet-clinopyroxene geothermometer of Ellis & Green (1979) using data on coexisting garnet + hornblende + clinopyroxene in amphibolite and granulite facies metamorphic assemblages. Data for the Fe-Mg exchange reaction between garnet and hornblende have been fitted to the equation.where K, is the Fe-Mg distribution coefficient, using a robust regression approach, giving a thermometer of the form:with very satisfactory agreement between garnet-hornblende and garnet-clinopyroxene temperatures. The thermometer is applicable below about 850 "C to rocks with Mn-poor garnet and common hornblende of widely varying chemistry metamorphosed at low sol.Application of the garnet-hornblende geothermometer to Dalradian garnet amphibolites gives temperatures in good agreement with those predicted by pelite petrogenetic grids, ranging from 520°C for the lower garnet zone to 565-610°C for the staurolite to kyanite zones. These results suggest that systematic errors introduced by closure temperature problems in the application of the garnet4nopyroxene geothermometer to the 'calibration' data set are not serious. Application to 'eclogitic' garnet amphibolites suggests that garnet and hornblende seldom attain Fe-Mg exchange equilibrium in these rocks.Quartzo-feldspathic and mafic schists of the Pelona Schist on Sierra Pelona, Southern California, were metamorphosed under high Present address:

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