TACONIAN AND ACADIAN RIMS ON SOUTHERN ADIRONDACK MONAZITES: HYDROTHERMAL FLUIDS FROM APPALACHIAN METAMORPHISM

Electron Microprobe dating of southern Adirondack pelites has revealed Taconian and Acadian rims on monazite that were formed initially during the high-grade granulite facies metamorphism of the Ottawan Orogeny. The monazites display complex patchy zoning in U, Th, Y, Pb, and Ca. Although chemical zoning does not mimic the age distribution, Paleozoic rims are easily distinguishable from Proterozoic cores in both backscatter and x-ray imaging. The compositions of the young rims are not identical, chemically, and show changes from one monazite to the next. Monazites mantled by young rims have been found only in the matrix, and not as inclusions within garnet.

The Adirondacks have experienced multiple orogenic events, with the main, granulite facies metamorphism reaching peak temperatures of 700 - 800°C during the Ottawan Orogeny (1070 - 1030 Ma). ⁴⁰Ar/³⁹Ar data suggests that the rocks were cooled to less than 400°C by 800 Ma. Young rims on monazite from three samples yield ages of around 395 and 505 Ma, which correspond to the timing of metamorphism to the east in the northern Appalachians. The low inferred temperature in addition to the similarity of the rim ages to the timing of Appalachian metamorphism suggest that the monazite rims represent growth from the infiltration of Appalachian-derived hydrothermal fluids.

Additional evidence for the travel of hydrothermal fluids through the rocks is manifested by reaction rims on monazite and other accessories. In highly sericitized and partially chloritized samples, monazite is mantled by alteration rims of Th-rich allanite yielding ages of 353 and 144 Ma. The allanite rims are not present in fresh samples and are interpreted to be a reaction product of the hydrothermal alteration. The allanite ages (353 and 144 Ma) correspond to the waning stages of the Acadian Orogeny and Jurassic rifting, respectively.

The textural, chemical, and geochronological evidence given by monazite and allanite imply their involvement with late hydrothermal fluids and show how these minerals can be used as tools to date periods of fluid movement through the crust. The ability to date the passage of hydrothermal fluids through the rocks is a powerful capability that could reveal information as fundamental as the rate of fluid movement through the crust.