ANOTHER LOOK AT ALUMINOUS ENCLAVES IN GEDRITE-CORDIERITE-GNEISS FROM THE UPPER ORDOVICIAN AMMONOOSUC VOLCANICS, SOUTHWESTERN NEW HAMPSHIRE, USA

Peter Robinson discovered gedrite gneisses containing the aluminous enclaves while mapping the Bronson Hill Anticlinorium during his PhD fieldwork. Later in collaboration with Howard Jaffe (Robinson & Jaffe, 1969) they described them as combinations of the aluminous minerals sillimanite (SIL), kyanite, corundum (CRN), staurolite, and spinel, set either in matrices of cordierite (CRD) or plagioclase (PLG). Where PLG encloses aluminous minerals, the PLG, which is commonly rich in anorthite, is separated from gedrite (GED) by a moat of CRD. In a later microprobe study of the enclaves (Schumacher & Robinson, 1987) sapphirine (SPR) was discovered in two samples with the other aluminous minerals both in CRD and PLG. The enclaves formed during 370-350 Ma amphibolite-facies metamorphism. Late in this event, decompression that accompanied gneiss-dome formation and probably led to extensive CRD production in these rocks. Textures suggest 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) eliminated SIL (attained chemical equilibrium). Modelling of the phase relations suggests that at the start of decompression SIL and GED formed primarily from Na-mica and chlorite (CHL). As decompression continued, CHL + quartz (QZ) ± PLG => CRD + GED and CHL + SIL + QZ => CRD. These reactions consumed all the QZ in the rock leaving SIL-bearing and SIL-free local bulk compositions. Mixtures of SIL with the whole-rock compositions were used to approximate these local bulk compositions for modelling. With further decompression, SIL reacted with GED to form CRD + CRN. Continued decompression led to rare reactions like GED + CRN => CRD + SPR. The aluminous enclaves that contain PLG require reactions in addition to those already described. Minor (<1%) hornblende (HBL) or cummingtonite (CUM), both of which contain Ca and Na, are present in these rocks. If either of these amphiboles along with GED react with SIL, the result would produce CRD + PLG + CRN. Similarly, GED + HBL/CUM + CRN could yield CRD + PLG + SPR.