Abstract

Around the Adirondack (New York) and Morin (Quebec) anorthosite masses the assemblage orthopyroxene–plagioclase is replaced to some extent by the compositionally equivalent assemblage garnet-plagioclase(less calcic)-quartz(-clinopyroxene). Although the transformation is partial, the latter assemblage has been considered typomorphic for metamorphism of the (hornblende-) garnet–clinopyroxene subfacies. Characteristic for this "subfacies" are the symplectites of garnet and quartz that commonly separate (i) orthopyroxene from plagioclase and/or (ii) iron oxide from plagioclase. Clinopyroxene is generally present in rocks either with or without the symplectites.Published experimental work shows that an increase in load pressure toward the anorthosite masses at constant or slightly increasing temperature could account for the symplectites in and around these masses, and their absence remote from them. There is, however, very little independent consistent evidence for such an increase in load pressure.We offer as an alternative the explanation that the anorthosite–mangerite suite completed solidification under a load pressure of the order of 8–10 kb and retarded regional cooling sufficiently to permit the garnet–quartz-forming reactions to take place, in itself and in contiguous rocks. This explanation is supported by the fabric of the rocks, suggesting syn- and late-kinematic (re-) crystallization in the hornblende–orthopyroxene–plagioclase subfacies, and partial postkinematic adjustment to the physical conditions of the hornblende–garnet–clinopyroxene "subfacies".

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