Abstract
Mineral compositions and reaction textures found in migmatitic gneisses from near Gilsum, New Hampshire, constrain both peak metamorphic pressure ( P ) and temperature ( T ) conditions and the P - T path. Large K-feldspar porphyroblasts indicate isobaric heating of the samples at P P > 4 kb on the cooling path. Four generations of monazite have been identified, three of which have been linked to specific whole-rock reactions. Monazite (4) (the last generation) was produced with xenotime ± apatite during melt crystallization and consumption of garnet and cordierite. Monazite (3) grew in a xenotime-absent mineral assemblage as garnet + muscovite reacted to form sillimanite + biotite. Monazite (2) grew in a xenotime-bearing ± garnet + biotite + chlorite assemblage, as xenotime and chlorite were consumed during garnet production. Monazite (1) has not been linked to a specific reaction; it may be detrital, record an earlier metamorphic event, or represent disequilibrium overgrowths of xenotime. YAG-monazite, YAG-xenotime, and monazite-xenotime thermometry for monazite generations (2)–(4) yield temperatures consistent with major-phase thermometers.
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