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

Abstract

A B S T R A C TThe Adirondack Mountains expose two distinct tectonic elements of the Proterozoic Grenville Province of northeastern 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 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 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.

Tectonic Evolution of the Adirondack Mountains and Grenville Orogen Inliers within the USA

“…Ar ages indicating that Ottawan temperatures in the Lowlands did not exceed 500°C (Streepey et al 2001). Similar arguments can be made for the CMB in general, one of two major examples of the Ottawan Orogenic Lid proposed by Rivers (2008).…”

“…1.09-1.03 Ma Ottawan orogenic phase ( Fig. 2) Ar cooling ages as old as 1.1-1.05 Ga demonstrate that the terrane did not experience temperatures exceeding ~500° C during the Ottawan Orogeny (Streepey et al 2001;Dahl et al 2004). Titanite ages as old as 1.16 Ga lead to a similar conclusion.…”

“…1.03 Ga and hornblende 40 Ar/ 39 Ar ages to ca. 0.98 Ga to the southeast of the Carthage-Colton shear zone (Mezger et al 1991(Mezger et al , 1992Streepey et al 2001;Heumann et al 2006).…”

“…Recently, Bonamici et al (2011) used δ 18 O zoning profiles in titanite to compute cooling rates in the CCZ, and their results showed that cooling took place at 6-60 times previous estimates with the best fit being a cooling rate of 30-50°C/m.y., a rate vastly in excess of earlier estimates of 2°C/m.y. (Mezger et al 1991;Streepey et al 2001). It is inferred that the differences result from time-averaging of several rapid pulses of down-faulting and cooling followed by longer intervals of quiescence.…”

Shawinigan arc magmatism in the Adirondack Lowlands as a consequence of closure of the Trans-Adirondack backarc basin

“…Because of this mining history, it is one of the best-studied areas in the Grenville Province. In addition, it is at lower grade than the adjacent Adirondack Highlands (Mezger et al, 1991;Streepey et al, 2001), contains considerably less volume of AMCG plutonic rocks, and lacks an Ottawan overprint (Heumann et al, 2006;Rivers, 2008). Thus the Adirondack Lowlands presents a window into the events that preceded the Ottawan phase of the Grenville orogeny, and contains supracrustal rock deposited before, and perhaps during, the Shawinigan orogeny (Rivers, 2008).…”