The Na-K exchange reaction between plagioclase and muscovite has been formulated as a geothermometer utilizing a ternary-feldspar subregular solution model, a modified binary white mica solution model, and available thermochemical data that account for high and low structural states of feldspar. The plagioclase-muscovite geothermometer has been applied to pelitic metasedimentary rocks and peraluminous granitoids that have equilibrated at temperatures of 490-750°C and at pressures of 2-13 kbar. The equilibration temperatures of pelitic rocks, predicted from compositions of coexisting plagioclase and muscovite, are generally similar to those predicted by gamet-biotite geothermometry. The plagioclase-muscovite temperatures of peraluminous granitoids derived using the compositions of secondary muscovites are distinctly subsolidus, whereas temperatures estimated using primary muscovite compositions are near liquidus. Application of the plagioclase-muscovite thermometer extends the range of pelitic metasedimentary rocks and peraluminous granitoids for which equilibration temperatures may be estimated.

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