Plagioclase Corona Around Garnets: Implications of Pressure-Temperature Paths in Aluminous Gneisses

Abstract

Textures found in metamorphic rocks provide insights into the metamorphic and tectonic history of the region. Plagioclase coronas around resorbed garnets are observed in metapelites and amphibolites encompassing the globe. These textures are common in the high-grade rocks of the Sawtooth Metamorphic Complex (SMC), Idaho. Because textures are commonly ambiguous and difficult to interpret, this study evaluates the development of these textures due to: melt formation, using Theriak/Domino to calculate the changes of mineral assemblages and modes; and solid state material transfer, using irreversible thermodynamic modeling (SEG). Six aluminous gneiss samples from the SMC displaying corona textures were studied in detail. All samples contain mineral assemblages of quartz, plagioclase, biotite, garnet, and sillimanite, with traces of ilmenite, graphite, apatite, zircon and monazite and secondary muscovite. The matrix contains melanosomes of foliated biotite and lineated sillimanite, and leucosome of predominantly plagioclase with minor amounts of biotite and quartz. Within the matrix lie the corona textures, where garnets exhibit varying degrees of resorption and are surrounded by rims of plagioclase, biotite and quartz. Pressure-Temperature conditions, calculated by Garnet-Biotite- Plagioclase-Quartz (GBPQ), Garnet-Biotite (GB)and Garnet-Aluminum silicate-Plagioclase (GASP), suggest peak conditions between 6.7 – 9.2 kb (± 0.5 kb) and 700-750⁰C (± 25⁰C). Phase equilibria modeling is consistent with these P-T conditions. Two different modeling techniques elucidate the textural formation. Modeling the garnet dissolution in a matrix consisting of the bt+sil+plag+qtz by solid state material transfer (Foster, 1999) produces multiple mantles surrounding the garnets with distinct mineral assemblages and modes. Zones adjacent to the garnet are dominated by biotite unlike coronas observed. Analysis of mineral assemblages and modes from whole rock chemistry along a decompression path, using Theriak/Domino, suggest garnet moles decrease as melt volumes increases. In addition, moles of plagioclase and An content increases. Based on this modeling study in these rocks, it appears that the plagioclase coronas surrounding embayed garnets form as a results of melt interactions.