THE SKIFF MOUNTAIN TECTONIC UNIT REVISITED: EASTERN ADIRONDACK MOUNTAINS, NY

In 1962 I spent six weeks mapping the 12 square kilometers of Skiff Mountain on a scale of 1:10,000 at the behest of my graduate advisor, Matt Walton, because Walton suspected the structure was complex. The mountain consists of mappable units of layered biotite granite gneiss, clinopyroxene granite gneiss, metagabbro, and an albite-quartz rock associated with low-Ti magnetite deposits. No charnockite has been found. Lithologies are quite similar to those described for the Lyon Mountain Gneiss in the Au Sable Forks quadrangle by Whitney and Olmsted (1993). The NW end of the mountain consists of a synform of metasedimentary rocks conformably enclosed by the gneisses. Unpublished structural studies show at least three fold sets, including several isoclinal folds coaxially refolded by tightly appressed similar folds. The granite layers contain a great deal of leucosome and pegmatite in shear zones is common. Unpublished whole rock Rb/Sr analyses by Mose (pers. comm., 1980) failed to establish an isochron for Skiff Mountain rocks and strongly suggested that the magnetite-enriched rocks had been open to metasomatic activity. Hypotheses for the origin of the layered granites and associated magnetite deposits have ranged from ancient basement to highly metamorphosed felsic volcanics to igneous intrusives. Published work suggests that the folded magnetite layers are hydrothermal in origin (McLelland et al., 2002). The layering and mappable folds indicate that the tectonic unit likely originated as layered felsic volcanics. Ubiquitous metagabbro sills could have been the source of hydrothermal fluids. An over-arching issue regards the tectonic history of the mountain: Could it have been allochthonous prior to folding?