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

The aluminous enclaves from gedrite-cordierite-gneisses of the Upper Ordovician Ammonoosuc Volcanics (Robinson & Jaffe, 1969; Schumacher & Robinson, 1987) are combinations of the aluminous minerals sillimanite (SIL), kyanite, corundum (CRN), staurolite, 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 rim of CRD. The enclaves appear to have formed during Quaboagian (370-350 Ma) amphibolite-facies metamorphism, and because of the volume increase associated with CRD production, the reactions are associated with decompression that accompanied the rise of gneiss domes in the region. The textures suggest these are diffusion-controlled reactions during which SIL + GED (matrix) reacted to produce aluminous minerals (most commonly CRN) + CRD. Some enclaves have relict SIL, but others have only the Al-rich reaction products in the cores of the enclaves suggesting diffusion-controlled reactions progressed to completion (equilibrium?). Advances since the original study of these rocks, make thermodynamic modeling of the phase relations of these rocks possible, but disequilibrium (relict SIL), uncertainty in estimating local bulk composition, and assumptions about fluid saturation complicate these types of calculations. Modeling was done on whole-rock analyses of two rocks: one with PL-CRD enclaves (I34EXA) and one bearing PL-free enclaves (W95JS). To approximate the effective local bulk composition in the enclaves, synthetic compositions were created by mixing in various proportions of SIL (in Wt %) with the whole-rock analyses. Calculations were done both assuming fluid saturation and fixed amounts of fluid. Regardless of the assumptions, several common features of the models emerge. Aluminosilicate (ALS) is stable with chlorite (CHL) at T mostly below 600°C (P dependent); ALS + CHL + PL react to form CRD + GED (e.g., at about 600°C and 0.6 GPa); Most of the GED and CRD in the rock forms by this reaction; Composition of PL, which is < 2% volumetrically, varies greatly and can be driven to high anorthite contents as Na-Al is incorporated into GED; SPR + SIL is not stable in rocks of these compositions at these conditions, as was assumed in a previous study. The SPR forms at the expense of CHL and CRN.