Calibration of the SGAM thermobarometer for pelitic rocks using data from phase-equilibrium experiments and natural assemblages

The assemblage quartz (S) Garnet Aluminosilicate Mica (SGAM) is common in amphibolite-grade metapelitic rocks; yet the equilibrium Alm + Ms = Ann + 2 Al2SiO5 + Qtz is rarely used as a barometer because of difficulties relating primarily to the lack of experimental calibration, We present a calibration of the above equilibrium determined using linear programming analysis (LIP) of data from phase-equilibrium experiments and natural assemblages. Pressure and temperature of the natural assemblages were constrained independently by aluminosilicate polymorph occurrences, and by Grossular Aluminosilicate quartz (S) Plagioclase (GASP) barometry and garnet-biotite thermometry. The calibration of the above equilibrium yields a slope almost identical to that of GASP. Application to several sets of published compositions shows that this calibration offers distinct improvements over previous calibrations. Pressures determined using the new calibration are consistent with the aluminosilicate polymorph present and with GASP barometry, where available. Also, some SGAM results show systematic spatial variations, suggestive of field gradients, which are not evident in the GASP results. Some differences between the two barometers probably reflect contrasting kinetics of the two systems,

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