REVISITING ALUMINOUS ENCLAVES IN GEDRITE-CORDIERITE-GNEISS FROM SOUTHWESTERN NEW HAMPSHIRE, USA

Aluminous enclaves from gedrite-cordierite-gneisses of the Ordovician Ammonoosuc Volcanics (Robinson & Jaffe, 1969; Schumacher & Robinson, 1987) are combinations of the aluminous minerals sillimanite (SIL), kyanite, corundum (CRN), staurolite (ST), sapphirine (SPR), and spinel, set either in a matrix of cordierite (CRD) or plagioclase (PL). The PL enclosing the aluminous minerals is separated from gedrite (GED) by a moat of CRD. The enclaves formed during 370-350 Ma amphibolite-facies metamorphism; volume increase associated with CRD production suggests its formation is associated with decompression that accompanied the formation of gneiss domes. Textures indicate SIL + GED via diffusion-controlled reactions form aluminous minerals (most commonly CRN) + CRD. Some enclaves have relict SIL, but in some only the Al-rich minerals remain in enclosed in CRD enclaves suggesting diffusion-controlled reactions (disequilibrium) progressed to completion (attained equilibrium). Since the original study of these rocks, advances in thermodynamic modeling allows assessment of the phase relations of these rocks, but disequilibrium (e.g., relict SIL), uncertainty in estimating local bulk composition, and assumptions about fluid content complicate the calculations. Modeling was done on whole-rock analyses of two rocks: one with PL-CRD enclaves and one bearing PL-free enclaves. To approximate the effective local bulk composition in the enclaves, synthetic compositions were created by mixing in various proportions of SIL with the whole-rock analyses. Assumptions about fluid behavior also strongly affect the mineral assemblages of the model; the calculations suggest the rocks were not saturated with water when the enclaves formed. For all the rocks the major change to the equilibrium assemblage would have been chlorite (CHL) + quartz (QZ) to GED + CRD. For the rocks with PL-bearing enclaves, changes to equilibrium assemblage would have include PL and hornblende (HBL) as additional phases. Durning the transition from CHL + QZ ± HBL to OAM + CRD ± PL, aluminosilicate (ALS), talc, and/or ST are transitional phases predicted to form and then be consumed. These transitional minerals plus QZ are uncommon, but are found as relicts in these rocks rendering the modeled phase relations plausible.