Abstract

Middle Proterozoic metasedimentary and metavolcanic rocks in the northwest Adirondacks are assigned into four lithostratigraphic units: basal Hyde School Gneiss that herein is given formational status, lower marble, Popple Hill Gneiss Formation, and upper marble. A variety of stratigraphic columns have been proposed for this complexly deformed terrane, the most recent being a column by Wiener and others (1984). Their basal formation, the Alexandria Bay Gneiss, is defined from rocks that include the Rockport granite gneiss near Rockport, Ontario, and Alexandria Bay, New York, as well as leucogneisses in 14 structural domes throughout the northwest Adirondacks. We believe that these rocks have a diverse origin, one being intrusive, the other volcanic ash. Their uppermost formation, the Pleasant Lake Gneiss, includes granitoid gneisses near Rossie village and Pleasant Lake that they suggest are illitic arkoses, but we give field, geochemical, and geochronological evidence for an intrusive origin. The gneisses near Rossie village have a whole-rock Rb-Sr age of 1,160±42 m.y. (IR = 0.7036 ± 0.0004), and similar gneisses near Antwerp village are 1,197 ± 53 m.y. (IR = 0.7034 ± 0.0005). These ages are younger than the zircon U-Pb ages of 1,280-1,250 m.y. generally obtained for metasedimentary and metavolcanic rocks in the Central Metasedimentary Belt of Ontario.

We urge abandonment of the five formations and two Groups proposed by Wiener and others (1984) on the basis of an intrusive origin for the gneisses near Rockport and Rossie villages and on stratigraphic correlations of recent studies. The stratigraphy of four formations proposed herein with a generalized geologic map of the northwest Adirondacks may lead to correlation with carbonate sequences in the Central Metasedimentary Belt and with rocks of the Adirondack highlands to the east.

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