THERMOMETRY, COOLING RATES, AND MONAZITE AGES OF THE SOUTHERN ADIRONDACKS

Rocks collected from five outcrops in the Southern Adirondacks between Peck Lake and Upper Benson, NY, provide textural and chemical evidence for dehydration melting. Leucosomes and symplectites are numerous in these rocks. The absence of muscovite suggests progress of the reaction Ms + Pl + Qtz=As + Kfs + L. In addition, the escape of some portion of melt from the rock is required to prevent back-reacting which would have formed late muscovite upon cooling. Furthermore, the presence of garnets in leucosomes indicates progress of the reaction Bt + As + Pl + Qtz=Grt + Kfs + L.

EMP analysis of a biotite inclusion shielded by a quartz inclusion within garnet, coupled with the garnet core composition, yields a temperature of around 800 °C. This temperature should reflect conditions at the peak of metamorphism. There is a large discrepancy between this result and the thermal Bullseye of Bohlen and Essene (1977), which gives temperatures of around 675 °C for southern Adirondack rocks.

Biotite inclusions in contact with garnet have undergone Fe-Mg exchange, as evidenced by the decreased Fe-Mg ratio for larger inclusions. Linear isopleths, which plot on a graph of apparent temperature vs. log of the inclusion diameter, are calculated for different cooling rates using a numerical program that models diffusion in garnet. Biotite inclusion and garnet core compositions, determined from EMP analyses, give apparent temperatures for the sample, and the diameters were measured. From the plot, a cooling rate of 1 -10 °C/Ma was determined for this sample.

Monazites from one southern Adirondack sample were dated using the EMP. The monazites display a complex, patchy zoning in Y, Th, U, and Pb. The results show at least two age populations—1160 ± 50 Ma and 1040 ± 40 Ma. The older age is possibly resolvable into two separate ages of 1170 ± 40 Ma and 1130 ± 20 Ma. In addition to the Grenvillian ages, a few Taconian and Acadian dates were obtained from monazite rims (500 ± 25 Ma and 390 ± 10 Ma). Such young ages have rarely been documented in the Adirondacks.