Early history of the Carthage-Colton Shear Zone, Grenville Province, Northwest Adirondacks, New York (USA)

The Adirondack Mountains expose two distinct tectonic elements of the Prote rozoic Grenville Province of northeastern North America: the Adirondack Low lands and Highlands. The Lowlands are located along the eastern edge of the Metasedimentary Belt, and the Highlands form the western portion of the Gr anulite Terrane. The two are separated by the Carthage-Colton Shear Zone (C CSZ). U/Pb titanite and Ar-40-Ar-39 hornblende ages in the Lowlands are ca. 100 m.yr. older than in the Highlands, across the CCSZ. While both the Low lands and the Highlands record a history of metamorphism during the Elzevir ian Orogeny (ca. 1150 Ma), only the Highlands record evidence of a major ph ase of Grenvillian thermotectonic activity (Ottawan Orogeny) at ca. 1090-10 30 Ma. Proposed tectonic models require that the Lowlands were either later ally separated from the Highlands or were at structurally higher levels dur ing this metamorphism. New U/Pb and Ar-40-Ar-39 ages from titanite and horn blende-bearing mylonites in the Dana Hill metagabbro within the CCSZ constr ain the timing of early shearing and offer a possible solution explaining t he discrepancy in metamorphic ages between the Highlands and Lowlands. Prec ise U/Pb titanite ages at 1020 Ma are coupled with Ar-40-Ar-39 hornblende a ges at 1000 Ma within the shear zone. Thermobarometric constraints indicate that deformation occurred under granulite-facies conditions. Therefore, ti tanite and hornblende ages are interpreted as cooling ages from a ca. 1050- 1030-Ma event. An early transpressive shearing event at ca. 1040 Ma, combin ed with structural data in the Dana Hill metagabbro that indicate strike-sl ip displacement along the CCSZ, suggests significant lateral displacement b etween the Lowlands and the Highlands during the ca. 1090-1030-Ma Ottawan O rogeny, prior to later extension along the CCSZ.