Empirical garnet–muscovite geothermometry in metapelites

Two empirical garnet–muscovite geothermometers, assuming no ferric iron (Model A) and 50% ferric iron (Model B) in muscovite, respectively, were calibrated under the physical conditions of P=3.0–14.0 kbar and T=530–700 °C. The input temperatures and pressures were determined by simultaneously applying the garnet–biotite thermometer [Am. Mineral. 85 (2000) 881.] and the GASP geobarometer [Am. Mineral. 86 (2001) 1117.] to natural metapelites. To confirm internal thermodynamic consistency, Holdawayʹs [Am. Mineral. 85 (2000) 881.] garnet mixing properties were adopted. Muscovite was treated as a symmetric Fe–Mg–AlVI ternary solid solution, and its Margules parameters were derived in this work. The resulting two formulae reproduced the input garnet–biotite temperatures well within ±50 °C, and gave identical results for a great body of natural samples. Moreover, they successfully distinguished the systematic changes of temperatures of different grade rocks from a prograde sequence, inverted metamorphic zone, and thermal contact aureole. Pressure estimation has almost no effect on the two formalisms of the garnet–muscovite geothermometer. Assuming analytical error of ±5% for the relevant components of both garnet and muscovite, the total random uncertainty of the two formulations will generally be within ±5 °C. The two thermometers derived in this work may be used as practical tools to metamorphic pelites under the conditions of 480 to 700 °C, low- to high-pressure, in the composition ranges Xalm=0.51–0.82, Xpyr=0.04–0.22, and Xgros=0.03–0.24 in garnet, and Fetot=0.03–0.17, and Mg=0.04–0.14 atoms p.f.u. in muscovite.