HISTORY OF EMPLACEMENT AND DEFORMATION OF ANORTHOSITE BODIES IN THE EASTERN MARCY MASSIF, ADIRONDACK MOUNTAINS, NEW YORK

The eastern part of the Grenvillian granulite-facies Marcy metanorthosite massif and host-country, supracrustal complexes displays at least three periods of anorthosite emplacement, each followed by regional ductile deformation. The emplacement sequence is: (1) emplacement of igneous rocks and deposition of sedimentary rocks (order of emplacement is not evident)=Rocky Branch complex to the north and New Russia complex to the east, with possible subsequent metamorphism and shearing; (2) emplacement (No. 1) of anorthosite, gabbroic anorthosite (leuconorite, leucogabbro), and anorthositic gabbro into country rock as thin foliated sheets and dikes and undeformed layered bodies; (3) metamorphism and accompanying ductile shearing (No. 1) to form the Elizabethtown ductile deformation zone (EDDZ); (4) emplacement (No. 2) of the Westport, Jay, Keeseville domes, and possibly a buried dome as layered anorthosite diapirs with warping and infolding of the country rock and the EDDZ; (5) deformation (No. 2) within the Westport dome and emplacement of the Lynch Mountain layered slabs, and undeformed bodies at Hurricane Mt., Baxter Mt., Spread Eagle Mt., the Sentinel Range, and McKenzie Mt. as allochthonous gabbroic anorthosite bodies during the formation of the Keene Valley ductile deformation zone (KDDZ), and possible emplacement of small gabbro bodies along the KDDZ; (6) intrusion (No. 3) of the Giant Mountain complex (GMC) of anorthosite and gabbroic anorthosite; (7) imposition of regional ductile deformation (No. 3) that bounds the southwestern margin of the Marcy massif and cuts through the GMC. Metagabbro bodies are spatially associated with shear zones and may have been intruded at several times during these events. This sequence of emplacement and deformation is displayed by shear- and intrusive-crosscutting relations that only provide relative-age assignments. The absolute ages may extend from 1160 Ma to 980 Ma.