A COMPARISON OF U-TH-TOTAL PB MICROPROBE AGES AND APATITE FISSION-TRACK AGES FOR CLOSURE TEMPERATURE ESTIMATES IN THE SOUTHERN AND EASTERN ADIRONDACKS

Apatite fission-track (AFT) ages were determined for four samples from the southern and eastern Adirondack Mountains to better constrain the closure temperature and timing of allanite formation in these areas. Storm and Spear (2002) and Lupulescu and Pyle (2004) measured Mesozoic U-Th-total Pb microprobe ages on allanite rims on monazite of 144-155 Ma and 165 Ma for samples from Peck Lake migmatitic metapelite and Mineville, respectively. These results suggested a Mesozoic hydrothermal event with an estimated closure temperature of 200-300° C for allanite formation. AFT ages, which indicate a closure temperature of 100° C, for the Peck Lake metapelite and Mineville apatite are within error, 123 and 113 Ma, respectively. These AFT ages are consistent with others from the same Adirondack regions. In the southern Adirondacks, AFT ages determined in this study include the Canada Lake charnockite and the Rooster Hill granite, 125 and 88 Ma, respectively. AFT ages previously determined for samples from Crown Point and Port Henry in the eastern Adirondacks near Mineville range from 123 to 84 Ma. Initial track length measurements on both the Peck Lake metapelite and Mineville apatites yield mean track lengths of 12.6-12.7 ± 1.6 microns suggesting relatively slow cooling from Early Cretaceous to the present. More track length measurements and modeling will be done to establish the thermal history of these samples below 100°C. AFT ages younger than ages for allanite formation support the suggested closure temperature estimates. Evidence for a Mesozoic hydrothermal event is inconclusive based on our current data.