THE TIMING OF THE LOW-TEMPERATURE THERMAL EVOLUTION OF LOWER PALEOZOIC COVER STRATA AND GRENVILLE BASEMENT, EASTERN NEW YORK STATE

A new approach to FT dating of zircon grains has been developed (SEM-HDFT dating) that allows dating of Precambrian and Paleozoic cooling events, it has therefore focused our attention on the low-temperature thermal evolution of the Grenville basement and Cambro-Ordovician cover rock in New York State. We have used this technique on detrital zircon from lower Paleozoic strata (mainly Cambrian and Ordovician clastics) around the Adirondacks, the Mohawk Corridor and in the Taconics. On the west side of the Adirondacks, detrital zircon from the Cambrian Potsdam Formation are completely unreset and reveal detailed information about the underlying Grenville terrane. This result is consistent with paleothermometry that show the cover strata on western side of the Adirondacks have not been heated significantly (CAI 2). The detrital zircons have U/Pb ages that are almost exclusively Grenville (c. 1.0-1.1 Ga), but ZFT cooling ages that reflect post-Grenville exhumation (780 Ma) and a subsequent widespread thermal event likely associated with rifting of the Iapetus (540 Ma). On the east side of the Adirondacks, a number of paleothermometers indicate that the lower Paleozoic strata were subjected to the some of the highest post-depositional temperatures in the region. Our ZFT grain ages from the eastern Adirondacks are largely unreset, and therefore maximum temperatures did not exceed 200-300°C. A small fraction of reset radiation-damaged zircon grains provides information about post-depositional thermal events. Virtually all the reset grains yield Taconic cooling ages. However, a small fraction of the reset grains (15%) indicate that these rocks experienced subtle heating related to the Alleghenian orogeny. The effects of the subtle Alleghenian thermal anomaly are better developed to south in the Taconic allochthon. Our ZFT grain ages from five samples within the center part of the allochthon show resetting at c. 264 Ma. Farther to the south, in Pennsylvania, all detrital zircon from the Cambrian Chickies Formation have ZFT grain ages that are fully reset to c. 270 Ma. These ZFT data provide a more detailed understanding of the temporal and spatial effects of the thermal veil related to Alleghenian orogenesis that undoubtedly affected migration and maturation of hydrocarbons in Paleozoic strata.