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Colgate University, Hamilton, NY 13346, USA
The Adirondack Mountains are subdivided into the highlands lying south-east of the Carthage-Colton mylonite zone (CCMZ, Fig. 1) and the lowlands situated north-west of the CCMZ. The lowlands are underlain by upper amphibolite-facies assemblages developed in a dominantly metasedimentary terrain, while the highlands consist principally of orthogneisses of hornblende-granulite grade. The application of a wide range of geothermometers/geobarometers (Bohlen et al. 1985) indicate peak metamorphic temperatures of 600–750°C for the lowlands and highlands, respectively, with maximum temperatures centering on the northwestern prong of the Marcy anorthosite massif (Fig. 1). Metamorphic pressures increase from 6.5 to 7.0 kbar in the lowlands to 7.5–8.0 kbar in the highlands. Retrograde garnet rims are interpreted by Bohlen et al. (1985) as indicating appreciable cooling prior to decompression, hence an approximately isobaric, counter-clockwise P-T-t path implying significant magmatic heating during metamorphism.
Models for the metamorphic evolution of the Adirondacks must consider whether the assemblages analysed are products of a single metamorphism (presumably the Ottawan phase, c. 1050 Ma, of the Grenville orogenic cycle (Moore & Thompson 1980)) or contain previously recrystallized phases. It is therefore important to determine the extent to which the Adirondacks constitute a polymetamorphic terrain.
Ashwal & Wooden (1983) summarize earlier geochronological data that date Adirondack granulite-facies metamorphism at 1000–1100 Ma. These results agree with U-Pb ages for metamorphic zircons reported by Chiarenzelli et al. (1987) as well as those given by Silver (1969). Chiarenzelli et al. (1987) also give U-Pb zircon ages for emplacement of the mangerite-charnockite suite of the highlands
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