U-PB DATING OF GRANITES WITH INHERITED ZIRCON - CONVENTIONAL AND ION MICROPROBE RESULTS FROM 2 PALEOZOIC PLUTONS, CANADIAN APPALACHIANS

Two Paleozoic granites from the Gander Zone, New Brunswick, contain inherited zircon. U-Pb dating of these granites by three different methods (multi-grain and single-grain conventional techniques and ion microprobe) highlights the strengths and weaknesses of each technique. Zircon morphology determined by optical microscopy of sectioned grains is shown to be a useful but not totally reliable technique for distinguishing magmatic and inherited zircons. Backscattered electron imaging with an electron microprobe can clarify the compositional zoning in zircons. Conventional analyses of carefully selected single grains and multi-grain fractions provide the most precise magmatic ages whereas ion microprobe dating is the most effective technique for defining populations of inherited grains. The magmatic ages of the Meridian Brook and Pabineau Falls granites are 472.3+/-4.0 and 397.2+/-1.9 Ma, respectively. Ages of inherited zircons range from 480 to 1850 Ma in the Meridian Brook granite and 435 to 1750 Ma in the Pabineau Falls granite.