Isograds and the Role of H2O in Metamorphic Facies of Orthogneisses of the Northwest Adirondack Area, New York

Abstract

Four isograds are shown on a geologic map of the northwest Adirondack area based on the first development of: (1) regional metamorphic garnet in biotite-quartz-oligoclase paragneiss; (2) orthopyroxene as a major member of the mafic assemblage in amphibolitic gneiss, and the disappearance of sphene in hornblende quartz syenite and old granite orthogneisses; (3) garnet in amphibolitic orthogneiss and metagabbro; and (4) garnet in pyroxene quartz syenitic and hornblende quartz syenitic orthogneisses. The successive new mineral assemblages are inferred to represent development under conditions of increasing temperatures and load pressures. An attempt is made to correlate them with the metamorphic facies of progressive metamorphism. The orthogneisses between any two isograds show a variety of mineral assemblages which are best interpreted in terms of development under a wide range of P H 2 O . The hypothesis of partial pressure of H 2 O about equivalent to load presure for the origin of the Adirondack orthogneisse gives results inconsistent with the data. In each of the metamorphic zones from that of almandine amphibolite facies to that of the pyroxene granulite subfacies, igneous rocks that were pyroxene syenite or quartz syenite have been metamorphosed to equivalent pyroxenic gneisses, and hornblende quartz syenite or hornblende granite to equivalent hornblende gneisses. The rocks have, in general, been impermeable either to access or to loss of H 2 O during plastic flow and any subsequent recrystallization. Evidence is presented that most of the hornblende-hypersthene amphibolites, with or without garnet, associated with rocks of the pyroxene granulite subfacies were formed at the same T and Pi as this facies, but at relatively high P H 2 O . The late-stage development of a network of rescrystallized garnetiferous shear zones in otherwise nongarnetiferous host rock is discussed.