A Report on a Biotite‐Calcic Hornblende Geothermometer

Abstract  This paper presents a biotite‐calcic hornblende geothermometer which was empirically calibrated based on the garnet‐biotite geothermometer and the garnet‐plagioclase‐hornblende‐quartz geobarometer, in the ranges of 560–800°C (T) and 0.26–1.4 GPa (P) using the data of metadolerite, amphibolite, metagabbro, and metapelite collected from the literature. Biotite was treated as symmetric Fe‐Mg‐AlVI‐Ti quaternary solid solution, and calcic hornblende was simplified as symmetric Fe‐Mg binary solid solution. The resulting thermometer may rebuild the input garnet‐biotite temperatures well within an uncertainty of ±50°C. Errors of ±0.2 GPa for input pressure, along with analytical errors of ±5% for the relevant mineral compositions, may lead to a random error of ±16°C for this thermometer, so that the thermometer is almost independent of pressure estimates. The thermometer may clearly discriminate different rocks of lower amphibolite, upper amphibolite and granulite facies on a high confidence level. It is assumed that there is a ferric iron content of 11.6% in biotite, and that the iron content in calcic hornblende may be calculated according to the method of Dale et al. (2000). This thermometer can be used for medium‐ to high‐grade metabasites and metapelites.