Mineral chemistry and metasomatic growth of aluminous enclaves in gedrite-cordierite-gneiss from Southwestern new hampshire, USA

The aluminous enclaves occur in gedrite-cordierite-gneisses of the Middle Ordovician Ammonoosuc Volcanics, and are composed of combinations of the aluminous minerals sillimanite (Sill), kyanite, corundum (Cor), staurolite (St), sapphirine (Sa), and spinel (Sp), which are set in a matrix of cordierite (Crd) or plagioclase (Plag). Generally, where plagioclase is present, both it and the aluminous minerals are separated from gedrite (Ged) and rare hornblende (Hbl) by cordierite. The enclaves arc interpreted to have formed near the peak of Acadian (Devonian) metamorphism at sillimanite-staurolite-muscovite grade by reactions that were encountered during the pressure decrease which accompanied the rise of gneiss domes in the region.The enclaves are divided into two main types: (1) enclaves of cordierite surrounding aluminous minerals; and (2) enclaves of cordierite and plagioclase surrounding aluminuous minerals. Sapphirine grains contain between 9·2 and 9·3 Al atoms per formula calculated to 14 cations. Staurolites from the enclaves are Mg-rich and have (Fe²⁺+ Mn)/(Fe²⁺+Mn+Mg) ratios of 0-59-0·64.The textures and mineralogy of the enclaves suggest that these rocks originally consisted of Ged+Sill±Qz±Hbl±Sp±Plag. These minerals reacted to form Crd+Aluminous Minerals±Plag. The mineralogy of both main types of enclaves can be explained by two analogous sets of continuous Fe-Mg reactions:Cordierite enclavesCordierite-plagioclase enclavesGed + Sill + Qz = CrHbl + Sill + Qz = Crd + PlagGed + Sill = Crd + SaHbl + Sill = Crd + Sa + PlagGed + Sill = Crd + CorHbl + Sill = Crd + Cor + PlagGed + Sill = Crd + StHbl + Sill = Crd + St + PlagThe structure of the enclaves suggests that the mineral growth by the above reactions was diffusion controlled, which would have resulted from overstepping the above reactions (i.e. The P change exceeded the reaction rate). Therefore, chemical potential gradients (relative mobility of diffusing components) between gedrite and sillimanite controlled the location of mineral growth. The Fe-Mg ratio of the bulk composition and the proportions of non-Fe-Mg minerals (quartz and sillimanite) appear to determine which continuous Fe-Mg reactions were encountered.Examples of mineral sequences in the cordierite enclaves are: Sill (core)/St+Crd/Ged (matrix); Cor+Crd (core)/Ged (matrix), and Sill (core)/St+Crd/Sa+Crd/Ged (matrix). Examples of the mineral sequences in the cordierite-plagioclase enclaves are: Sill (core)/St+Plag/Plag+Crd/Hbl+Ged (matrix); Cor+Plag (core)/St+Plag/Sa+Plag/Ged + Hbl (matrix); and St+Plag (core)/Plag+Crd/Ged+Hbl (matrix).P-μ_FeMg-1 diagrams proved to be an important tool for understanding and illustrating the development of the enclaves. These diagrams allow one to view simultaneously all the discontinuous and continuous Fe-Mg reactions along a P-μ_H2O (or T) rock path. With this information it is possible to determine qualitatively which reactions and what sequence of reactions might be encountered by bulk compositions with variable Fe-Mg ratios and modal proportions of phases.