Early History of the Carthage‐Colton Shear Zone, Grenville Province, Northwest Adirondacks, New York (U.S.A.)

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Abstract
The Adirondack Mountains expose two distinct tectonic elements of the Proterozoic Grenville Province of north- eastern North America: the Adirondack Lowlands and Highlands. The Lowlands are located along the eastern edge of the Metasedimentary Belt, and the Highlands form the western portion of the Granulite Terrane. The two are separated by the Carthage-Colton Shear Zone (CCSZ). U/Pb titanite and 40 Ar- 39 Ar hornblende ages in the Lowlands are ca. 100 m.yr. older than in the Highlands, across the CCSZ. While both the Lowlands and the Highlands record a history of metamorphism during the Elzevirian Orogeny (ca. 1150 Ma), only the Highlands record evidence of a major phase of Grenvillian thermotectonic activity (Ottawan Orogeny) at ca. 1090-1030 Ma. Proposed tectonic models require that the Lowlands were either laterally separated from the Highlands or were at structurally higher levels during this metamorphism. New U/Pb and 40 Ar- 39 Ar ages from titanite and hornblende-bearing mylonites in the Dana Hill metagabbro within the CCSZ constrain the timing of early shearing and offer a possible solution explaining the discrepancy in metamorphic ages between the Highlands and Lowlands. Precise U/Pb titanite ages at 1020 Ma are coupled with 40 Ar- 39 Ar hornblende ages at 1000 Ma within the shear zone. Thermobarometric constraints indicate that deformation occurred under granulite-facies conditions. Therefore, titanite and hornblende ages are interpreted as cooling ages from a ca. 1050-1030-Ma event. An early transpressive shearing event at ca. 1040 Ma, combined with structural data in the Dana Hill metagabbro that indicate strike-slip displacement along the CCSZ, suggests significant lateral displacement between the Lowlands and the Highlands during the ca. 1090-1030-Ma Ottawan Orogeny, prior to later extension along the CCSZ.

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