Mineral Chemistry and Metasomatic Growth of Aluminous Enclaves in Gedrite--Cordierite-Gneiss from Southwestern New Hampshire, USA

The aluminous enclaves occur in gedrite-cordierite-gneissesof the Middle Ordovician Ammonoosuc Volcanics, and are composedof combinations of the aluminous minerals sillimanite (Sill),kyanite, corundum (Cor), staurolite (St), sapphirine (Sa), andspinel (Sp), which are set in a matrix of cordierite (Crd) orplagioclase (Plag). Generally, where plagioclase is present,both it and the aluminous minerals are separated from gedrite(Ged) and rare hornblende (Hbl) by cordierite. The enclavesarc interpreted to have formed near the peak of Acadian (Devonian)metamorphism at sillimanite-staurolite-muscovite grade by reactionsthat were encountered during the pressure decrease which accompaniedthe rise of gneiss domes in the region. The enclaves are divided into two main types: (1) enclaves ofcordierite surrounding aluminous minerals; and (2) enclavesof cordierite and plagioclase surrounding aluminuous minerals.Sapphirine grains contain between 9?2 and 9?3 Al atoms per formulacalculated to 14 cations. Staurolites from the enclaves areMg-rich and have (Fe²⁺+ Mn)/(Fe²⁺+Mn+Mg) ratios of 0-59–0?64. The textures and mineralogy of the enclaves suggest that theserocks originally consisted of Ged+Sill?Qz?Hbl?Sp?Plag. Theseminerals reacted to form Crd+Aluminous Minerals?Plag. The mineralogyof both main types of enclaves can be explained by two analogoussets of continuous Fe-Mg reactions:The structure of the enclavessuggests that the mineral growth by the above reactions wasdiffusion controlled, which would have resulted from oversteppingthe above reactions (i.e. the P change exceeded the reactionrate). Therefore, chemical potential gradients (relative mobilityof diffusing components) between gedrite and sillimanite controlledthe location of mineral growth. The Fe-Mg ratio of the bulkcomposition and the proportions of non-Fe-Mg minerals (quartzand sillimanite) appear to determine which continuous Fe-Mgreactions 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 themineral 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 importanttool for understanding and illustrating the development of theenclaves. These diagrams allow one to view simultaneously allthe discontinuous and continuous Fe-Mg reactions along a P–µ_H2O(or T) rock path. With this information it is possible to determinequalitatively which reactions and what sequence of reactionsmight be encountered by bulk compositions with variable Fe-Mgratios and modal proportions of phases.