WHAT LIES BENEATH THE ADIRONDACK LOWLANDS? INSIGHTS FROM U-PB ZIRCON GEOCHRONOLOGY

The Adirondack Lowlands are part of a larger region of the southern Grenville Province, characterized by a thick, marble-rich supracrustal succession, the Central Metasedimentary Belt, that can be traced from Canada to the New York-New Jersey Highlands and adjacent areas. In the Lowlands, the stratigraphic succession, while deformed and intruded, is relatively intact and well-described due to the world-class zinc deposits it hosts. The sequence is dominated by calcitic and dolomitic marbles and calc-silicate gneisses, also including meta-evaporitic units and metamorphosed clastic rocks consisting of quartzites, pelitic gneisses, meta-arkoses, meta-greywackes, and meta-siltstones deposited during back-arc extension through orogenic collapse (ca. 1300-1250 Ma). As a consequence of the Elzevirian and Shawinigan orogenies (ca. 1250-1150 Ma), the sequence was deformed and metamorphosed to mid-upper amphibolite facies but escaped later widespread deformation associated with the Ottawan event. Previous tectonic studies suggest that the Lowlands stratigraphic package is allochthonous, as basement rocks are unknown or unrecognized. We applied U-Pb zircon geochronology of crosscutting plutonic rocks to better understand the age of possible basement rocks. A sample from the Hyde School gneiss near Canton yielded zircon xenocrysts with an age of 1322.5 ±2.1 Ma, whereas a sample from the Gouverneur Body of the HSG yielded a range of xenocrystic core dates from ca. 1363-1227 Ma. The third sample was collected from pegmatite cutting breccia within the Carthage-Colton Shear Zone and yields a xenocrystic core age of 1328.5 ±8.3 Ma. The data indicate that basement rocks to the Lowlands sequence are similar in age to those in Dysart-Mt. Holly Suite, which is exposed in the Southern Adirondack Terrane, the Green Mountains of Vermont, the NY-NJ Highlands, and in the Central Metasedimentary Belt Boundary Zone of Ontario. A model involving thin-skinned deformation resulting in the southeastward thrusting and nappe development over tonalitic basement gneisses of the Southern Adirondacks is envisioned.