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 11) dat ing 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 g ranites 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 Orog eny, 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 t ightly at ca 1060-1045 Ma. Fabrics associated with this suite indicate a la te- to post-tectonic origin thus fixing the minimum age for the Ottawan Oro geny. Especially critical are two samples of ca 1047 Ma fayalite granite th at are essentially undeformed and must post-date tectonism. In addition, an undeformed pegmatite dike yields an age of 1034 +/- 8 Ma confirming the te rmination of Ottawan orogenesis by that time. It is suggested that the gene sis of the Hawkeye suite is related to athenospheric heating of the crust d ue to far-field effects of contemporaneous magmatic events at the Midcontin ent rift. Lyon Mt. Granitic Gneiss is interpreted as the result of deep cru stal melting following delamination of the overthickened Ottawan orogen. To gether with the results of metamorphic investigations, these two suites def ine a counterclockwise P-T-t loop for the Adirondacks during the Ottawan Or ogeny. Geochronological and tectonic investigations from the Grenville Prov ince of Canada and the northern Blue Ridge Province of the Appalachians dem onstrate the presence of strong Ottawan deformation, magmatism, and metamor phism in these areas and emphasize the large scale and marked intensity of this event. In Canada, the crust responded to the Ottawan collision princip ally by imbricating into large northwest-directed thrust slices rather than the fold nappes of the Adirondacks. This is consistent with the apparent a bsence in the Canadian foreland of Hawkeye age magmatism and resultant theo logical weakening of the crust. (C) 2001 Elsevier Science B.V. All rights r eserved.