DETRITAL ZIRCONS OF CAMBRO-ORDOVICIAN SANDSTONE UNITS IN EASTERN ONTARIO AND NORTHERN NEW YORK

Detrital zircons from seven Middle Cambrian to Lower Ordovician sandstone units in eastern Ontario and northern New York have been analyzed by LA-MC-ICP-MS. Sampling sites extend west across the Frontenac Axis from near Lake Champlain to Sunbury, Ontario, and include various stratigraphic levels within the Altona Formation, Potsdam Group, and March (Theresa-equivalent) Formation. Shallow marine (Altona), aeolian (Hannawa Falls) and fluvial (Ausable) sandstones near the northern fringe of the Adirondack Mountains yield similar populations, displaying symmetric U-Pb age distributions with peaks near ca.1160 Ma, attesting to provenance from local AMCG plutonic rocks. Zircon ages from both the Altona and Ausable formations range between 1000-1300 Ma, whereas at Hannawa Falls the range is between 1100-1400 Ma, except for a few grains from 1400-1600 Ma. The Altona and Asuable samples are strongly influenced by nearby basement exposures, with little evidence of far-traveled grains, likely indicating derivation from uplifted areas to the south. The Hannawa Falls zircon population may reflect its location in Adirondack Lowlands, which escaped the metamorphism and plutonism associated with the Ottawan phase (ca. 1020-1080 Ma) of the Grenvillian Orogeny, and its windblown source. Eastern Ontario samples from predominantly shallow-marine environments at Charleston Lake (basal Potsdam), Sunbury (Nepean), and Ottawa (Nepean and March) yield polymodal age-distribution patterns that presumably reflect proximity to older provinces of the Canadian Shield. All samples contain zircons with ages consistent with derivation from Ottawan and Shawinigan/AMCG plutonism (ca. 1150-1200 Ma) and contain a few Archean grains (2.5-2.8 Ga). Other peaks include 1350, 1400, 1450, 1550, 1625, and 1850 Ma, suggesting limited contributions from basement terranes to the east (e.g. Pinware and Labradorian). Major differences in zircon populations across the Frontenac axis are lacking, possibly because the arch was not an effective barrier to sand dispersal. Zircon population complexity increases up-section, perhaps as a consequence of basin expansion that allowed additional regions to be sampled through time.