40AR/39AR CONSTRAINTS ON THE AGE OF FABRIC DEVELOPMENT IN THE WESTMINSTER TERRANE, NORTH-CENTRAL MARYLAND

The Westminster terrane in Maryland and Pennsylvania is one of the least understood tectonostratigraphic units in the central Atlantic Piedmont. This terrane is bounded on the northwest by the Martic Line Fault and on the southeast by the Pleasant Grove- Huntington Valley faults. The rocks within the Westminster terrane have been interpreted to be part of a slope-rise deep-water prism that was deposited in the Iapetus ocean off the Larentian margin. The Westminster terrane is thought to have been thrust over the unmetamorphosed, Cambro-Ordocician Frederick Valley Limestone along the Martic Line fault onto the Laurentian margin during the Ordovician Taconic orogeny, and its phyllites have been metamorphosed at lower greenschist facies conditions.

Petrographic analysis of samples collected along an E-W traverse within the Westminster terrane in the Urbana and Damascus quadrangles in north-central Maryland documents the presence of multiple fabrics defined by muscovite-chlorite intergrowths. In addition, most of the samples appear to preserve a usually minor detrital (<1%)component of muscovite. The muscovite-chlorite intergrowths are interpreted to have grown within the chlorite zone, below the closure temperature for argon diffusion in muscovite (<350ºC). ⁴⁰Ar/³⁹Ar age spectra of white micas from these samples are complex, and variations in the dominant ages in the age spectra are striking. Samples in the western part of the Westminster terrane suggest growth of fabric forming white mica in the Early Silurian (~430 Ma), contrasting strongly with those in its eastern part which suggest Middle Devonian (~375 Ma) growth. A single sample from the western, older portion of the Westminster terrane contains a high proportion of detrital white mica. The highest temperature steps of the age spectrum of this sample record ages that are consistant with cooling from Grenvillian metamorphism and support our interpretation that metamorphic temperatures were never high enough to reset white micas in the Westminster terrane.