DATING DEFORMATION WITH MONAZITE

Although it is now possible to date single grains of monazite with the ion and electron microprobes, and to resolve mulitple age domains within single grains, it is commonly difficult to demonstrate that individually dated grains or domains grew during the development of specific deformation fabrics. Because careful field observations can link fabrics with map-scale folds and faults, it is vital to develop sound microstructural criteria to date monazite growth relative to fabrics in order to establish a meaningful structural chronology. The approach we used was to establish the timing of monazite growth relative to fabric development was to compare monazite characteristics in similar lithologies across a zone with a large strain gradient in the Chester dome, Vermont.

It is widely accepted that rocks in southeastern Vermont were affected by both the Ordovician Taconic and Devonian Acadian orogenies. Monazite is a common accessory phase in pelitic schist and grains are typically irregularly shaped and approximately equant. Monazite grains from three samples of Late Proterozoic to Cambrian schist near the Chester dome, but outside of a high strain zone (HSZ), are chemically zoned and contain two or more distinct age domains. Electron microprobe (EMP) ages show multi-modal distributions consistent with metamorphic growth during the Taconic, Acadian, and Pennsylvanian Alleghenian orogenies. In sharp contrast, monazite grains in lithologically similar samples within the HSZ that rims the Chester dome are elongated parallel to the mylonitic fabric and are chemically homogeneous. EMP ages from two HSZ samples from Townshend Dam both show a normal distribution that give averages of 380 +/- 31 Ma (standard deviation) and +/-3 Ma (standard error) (n=124) and 384 +/- 36 Ma (s.d.) and +/-5 Ma (s.e.) (n=64). Monazite that predated mylonitization of these rocks was, presumably, thoroughly recrystallized during intense deformation in the HSZ. Lithologic units are dramatically thinned or entirely absent in the HSZ, compared with similar units elsewhere in southeastern Vermont, and we interpret the HSZ as a normal ductile shear zone that formed during Acadian extension at approximately 380 Ma.