GEOLOGIC MAP AND CROSS SECTIONS OF THE SOUTHEAST QUARTER OF THE WITHERBEE QUADRANGLE; DEFINING THE PRECAMBRIAN GEOLOGY OF WITHERBEE AREA, NORTHEAST ADIRONDACK MOUNTAINS, NEW YORK

Geologic mapping of a portion of the Witherbee Quadrangle, North East Adirondack Mountains, gives insight as to field relationships between the Marcy anorthosite massif and the surrounding New Russia gneiss complex. Rocks within the mapping area are part of the anorthosite-mangerite-charnockite-granite (AMCG) suite within the Grenville Province of North America. A comprehensive field and petrologic study provides evidence that multiple phases of anorthosite exist within the massif. Massive to foliated anorthosite and gabbroic anorthosite exist as a large plutonic core and is surrounded by a zone of gabbroic anorthosite gneiss. The fabric of the gabbroic anorthosite gneiss, which is juxtaposed to the New Russia gneiss complex, is equivalent to the orientation of the contact margin. An overturned syncline has been mapped within the New Russia gneiss complex and occurs along the margin of the massif. Migmatites occur within the New Russia gneiss complex along the margin of the massif. The leucosomes show evidence of parallel extension to foliation and igneous textures that cross cut foliation at low to high angles. Petrographic data shows that plagioclase has been alterated to sericitie as a result of fluid migration from emplacement of anorthosite. Alteration occurs within a zone adjacent to the contact between the massif and New Russia gneiss complex and shows a progression from altered plagioclase to unaltered plagioclase. From these field relationships and petrologic data, inferences are made that the Marcy anorthosite massif intruded as a multiphase event forming a metamorphic aureole within the host rock causing deformation and anatexis.