40AR/39 AR DATING OF WHITE MICAS ALONG THE SUSQUEHANNA RIVER IN THE PIEDMONT OF PENNSYLVANIA AND MARYLAND, AND POINTS EAST: A TRAVERSE ACROSS THE WESTMINSTER AND POTOMAC COMPOSITE TERRANES

The mid-Atlantic Piedmont in SE Pennsylvania, NE Maryland, and northern Delaware is a complex amalgam of NE trending tectonic terranes that were metamorphosed and deformed at various times during the Paleozoic. Our new ⁴⁰Ar/³⁹Ar data is from the northern portions of the Westminster (WT) and Potomac Composite (PCT) terranes along the Susquehanna River and points east.

Samples from the WT near its western and northern limits at the Martic thrust zone in PA have multiple generations of white mica. The oldest of these grew above the closure for argon diffusion in white mica and record cooling through ~350°C. The youngest generation grew below this temperature and records its crystallization age. The age spectra of these samples are sigmoidal in shape with a minimum age in the saddles of 370 Ma and a maximum age in high temperature steps of 436 Ma. We interpret the 436 Ma age to be a minimum for cooling through closure and the 370 Ma age to represent the maximum for growth of the youngest generation of white mica in these rocks. While these ages are broadly consistent with those from the WT further SW in MD and VA, the rocks in this area were hotter and do not show the difference in age seen between eastern (371) Ma and western (430 Ma) parts of the terrane further SW.

White mica samples from the PCT between the Pleasant Grove-Huntington Valley shear zone and the Baltimore Mafic Complex (BMC) to the SE have age spectra that we interpret to indicate the presence of excess argon. Minimum ages in these spectra, which range from 342 to 330 Ma are interpreted to represent the maximum age for cooling of these samples through ~350°C. These ages are significantly younger than any cooling ages found in the PCT further to the SW. Perhaps these rocks were buried by a NW extension of the BMC that delayed their exhumation and has since eroded away.

To the north and east of the BMC in PA and northern DE some rocks of the PCT are at amphibolite to granulite facies. Amphiboles from these rocks cooled through argon closure (~500°C) at ~400 Ma, similar to amphibolites in the Sykesville Formation to the SW. We interpret white mica ⁴⁰Ar/³⁹Ar ages from samples across this entire area to represent cooling through closure (~350°C) at about 365 Ma, again similar to the timing of cooling of white micas in the Sykesville Formation further to the SW.