Abstract

The northwest-dipping Carthage-Colton shear zone, located near the eastern margin of the Grenville province in northern New York, separates the Adirondack Lowlands of the Metasedimentary Belt from the Adirondack Highlands of the Granulite Terrane. Published U/Pb sphene ages are 1156–1103 Ma in the Lowlands and 1050–982 Ma in the Highlands, indicating an ∼100 m.y. offset in metamorphic ages across the boundary. Our reported hornblende 40Ar-39Ar ages are ∼1060 Ma in the Lowlands and ∼950 Ma in the Highlands. This confirms the offsets in sphene ages across the Carthage-Colton shear zone and indicates an average cooling rate in both the Lowlands and the Highlands of 1°–2°/m.y. over ∼100 m.y. Biotite 40Ar-39Ar ages from this study yield no apparent age difference, thus approximating the time of final Carthage-Colton shear-zone extension. These 40Ar-39Ar data suggest that extensional motion along the Carthage-Colton shear zone occurred between 950 and 920 Ma, postdating the latest recorded compressional activity between 1060 and 1030 Ma and orogenic collapse between 1045 and 1030 Ma in the Metasedimentary Belt. Extensional motion occurring at least 100 m.y. after the last compressional event is probably not related to orogenic collapse, but rather may be related to another, as yet undefined, extensional event in eastern Laurentia during Late Proterozoic time.

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