Late Proterozoic (ca. 930 Ma) extension in eastern Laurentia

The northwest-dipping Carthage-Colton shear zone, located near the eastern margin of the Grenville province in northern New York, separates the Adiron dack Lowlands of the Metasedimentary Belt from the Adirondack Highlands of the Granulite Terrane. Published U/Pb sphene ages are 1156-1103 Ma in the L owlands and 1050-982 Ma in the Highlands, indicating an similar to 100 m.y. offset in metamorphic ages across the boundary. Our reported hornblende Ar -40-Ar-39 ages are similar to 1060 Ma in the Lowlands and similar to 950 Ma in the Highlands. This confirms the offsets in sphene ages across the Cart hage-Colton shear zone and indicates an average cooling rate in both the Lo wlands and the Highlands of 1 degrees-2 degrees/ m.y. over similar to 100 m .v. Biotite Ar-40-Ar-39 ages from this study yield no apparent age differen ce, thus approximating the time of final Carthage-Colton shear-zone extensi on, These Ar-40-Ar-39 data suggest that extensional motion along the Cartha ge-Colton shear zone occurred between 950 and 920 Ma, postdating the latest recorded compressional activity between 1060 and 1030 Ma and orogenic coll apse between 1045 and 1030 Ma in the Metasedimentary Belt. Extensional moti on occurring at least 100 m.y. after the last compressional event is probab ly not related to orogenic collapse, but rather may be related to another, as Set undefined, extensional event in eastern Laurentia during Late Proter ozoic time.