Zircon U-Pb geochronology of the Ottawan Orogeny, Adirondack Highlands, New York: regional and tectonic implications

Both single and multigrain U-Pb zircon thermal ionization mass spectrometry (TIMS) as well as sensitive high resolution ion microprobe (SHRIMP ll) dating of two suites of Adirondack granites have yielded ages constraining the principal tectonomagmatic events of the Ottawan Orogeny to the interval ca 1090–1035 Ma. The earliest of these consists of mildly A-type hornblende granites of the Hawkeye granite suite, multigrain samples of which define a tight age cluster of ca 1103–1093 Ma. Assemblages and fabrics in this suite demonstrate that it experienced the high-grade effects of the Ottawan Orogeny, thereby fixing the maximum age of the latter at ca 1090 Ma. The second suite consists of Lyon Mt. Granitic Gneiss, six samples of which cluster tightly at ca 1060–1045 Ma. Fabrics associated with this suite indicate a late- to post-tectonic origin thus fixing the minimum age for the Ottawan Orogeny. Especially critical are two samples of ca 1047 Ma fayalite granite that are essentially undeformed and must post-date tectonism. In addition, an undeformed pegmatite dike yields an age of 10348 Ma confirming the termination of Ottawan orogenesis by that time. It is suggested that the genesis of the Hawkeye suite is related to athenospheric heating of the crust due to far-field effects of contemporaneous magmatic events at the Midcontinent rift. Lyon Mt. Granitic Gneiss is interpreted as the result of deep crustal melting following delamination of the overthickened Ottawan orogen. Together with the results of metamorphic investigations, these two suites define a counterclockwis