ELECTRON MICROPROBE AGE DATING OF MONAZITE FROM THE BRETTON WOODS PLUTON, PRESIDENTIAL RANGE, NEW HAMPSHIRE

Sixty-three electron microprobe ages of monazite sampled from the Bretton Woods pluton in northern New Hampshire constrain the pluton to be 368.9+/-5.8 Ma or late Devonian. The pluton is a two-mica, medium- to fine-grained, Concord-type member of the New Hampshire Plutonic Series. It cross cuts D1, D2 and D3 deformation fabrics in the adjacent Littleton Formation schists and may be related to D4 deformation. This and other nearby plutons of similar age record the waning stages of the Acadian orogeny in the Presidential Range. Two samples from the center of the Bretton Woods pluton were analyzed. For each sample two of the largest (30-90um) and most complex chemically zoned monazite grains were selected. Trace element weight % concentrations for the monazite grains were as follows: Th 4.88 – 9.54; Pb .09 - .19; and U .20- 1.13. Chemical zoning consists of low Th core domains enveloped by high or intermediate Th rim domains. The domains are interpreted to have formed during the crystallization of the granite. Monazite 1 from the first sample yielded six ages with a mean of 367.1+/-9.4 Ma. Monazite 2 from the same sample yielded three ages with a mean of 371.0+/-7.6 Ma. Monazite 3 from the second sample yielded thirty ages with a mean of 368.5+/3.2 Ma. Monazite 4, also from the second sample, yielded twenty-four ages with a mean of 370.8 +/- 4.8 Ma. Despite distinct U, Th and/or Pb chemical zonation, Monazites 1, 2 and 3 showed no corresponding age domain variations. In fact, all ages for all chemical domains agreed within 2-sigma uncertainty. Monazite 4 showed the most complex zoning and was the only grain to exhibit age domains. The high and low Th domains in the core of the grain gave comparable ages of 381.0+/-8.8 Ma and 379.5+/-3.6 Ma respectively. The intermediate Th domain gave an age of 363.7+/-4.8Ma. The older age domains likely represent relict metamorphic monazite that was overgrown by igneous monazite.