PLAGIOCLASE CORONAS AROUND GARNETS FROM THE SAWTOOTH METAMORPHIC COMPLEX, IDAHO, USA: EVIDENCE OF COOLING IN THE PRESENCE OF SMALL VOLUME MELTS

Plagioclase-rich coronas surrounding garnets in high-grade aluminous gneisses from the Sawtooth Metamorphic Complex (SMC), Idaho, USA appear to have formed by crystallization of melt that was restricted to porphyroblast-matrix boundaries. The coronas, surrounding embayed garnets (grt), are up to 1 cm in diameter, consist dominantly of anhedral plagioclase (pl) + quartz (qtz) + biotite (bt) with minor sillimanite (sil) and differ by texture and in modal amounts from the bt-sil-rich matrix. Peak pressure (P)-temperature (T) conditions for the samples are near 7 kb and 750^oC based on thermobarometry.

Plagioclase-rich coronas around garnet are commonly attributed to grt growth in rocks due to heating or grt consumption due to decompression. In contrast, calculations using Theriak-Domino with the HP2 dataset (Pattison & DeBuhr, 2015, JMG) suggest that the SMC coronas are produced by a two-stage process involving: (1) removal of significant amounts of melt formed during heating or decompression to produce highly aluminous, low potassium restite bulk compositions that are unsaturated with respect to muscovite (ms); and (2) cooling of small amounts of melt located near grt at mid-crustal pressures. It is unclear whether the small amount of melt near grt was a residual from the extensive, early melting event or was a second melting episode that produced small amounts of melting in local domains near grt. In either case, the absence of ms profoundly alters the orientation of calculated grt volume isopleths for these rocks. When ms is present, the volume of grt is strongly affected by P, i.e. decreasing P dissolves grt and produces pl. However, when ms is absent, the volume of grt in equilibrium with melt is primarily controlled by T and is nearly insensitive to P. In this scenario, cooling is the only way to dissolve grt and produce pl from melt at 7 kb/750^oC for the SMC bulk composition. Consequently, we interpret these textures to indicate that the SMC rocks were extensively melted and the melt was removed, producing a low potassium, high-aluminum bulk composition with a small amount of residual or new melt localized around garnet. This melt then consumed grt and produced pl-rich coronas around the grt as it cooled and crystalized in the middle crust, providing a different mechanism for formation of pl-rich coronas around grt.