The Carthage-Colton Mylonite Zone (Adirondack Mountains, New York): The Site of a Cryptic Suture in the Grenville Orogen?

Abstract

U-Pb ages were determined on metamorphic sphenes and monazites from the Late Proterozoic Adirondack Highlands and Lowlands in the vicinity of the Carthage-Colton mylonite zone. Monazites were extracted from metapelites, and sphenes were separated from marbles, calc-silicate gneisses, and granite gneisses in order to determine the timing and the duration of metamorphism as well as the cooling histories for rocks on either side of the mylonite zone. Monazite ages from the Lowlands range from 1171-1137 Ma; sphene ages in the Lowlands range from 1156-1103 Ma, those from the Highlands immediately to the east of the mylonite zone range from 1050-982 Ma. The ages indicate that the last high-grade metamorphism in the Highlands is ca. 100 m.y. younger than in the Lowlands and that both terranes had separate cooling histories at least until ca. 1000 Ma. Sphenes from within the Carthage-Colton mylonite zone yield ages of about 1098 Ma, which are distinct from sphene ages on either side of the shear zone. The mineral ages, structures, and metamorphic histories suggests that the Carthage-Colton mylonite zone is a fundamental tectonic boundary within the Proterozoic Grenville Orogen of North America.