A TRAVERSE ACROSS THE PIEDMONT OF PENNSYLVANIA, DELAWARE, AND MARYLAND: THERMOCHRONOLOGIC CONSTRAINTS ON THE TIMING OF METAMORPHISM

The metamorphic history and assembly of the rocks of the mid-Atlantic Piedmont in PA, DE, and MD are poorly understood, in part due to limited data. Here is discussed white mica (WM) and amphibole ⁴⁰Ar/³⁹Ar and monazite U/Pb results from samples collected on a traverse from the Wissahickon Fm (WF) near Philadelphia in the east, to the Martic thrust zone in the west that have been previously mapped as the Potomac composite (PCT) and Westminster terranes (WT).

Rocks of the WF in the PCT have been heated as high as amphibolite facies and record monazite ages of 386-390 Ma. To the west, members of the proposed West Grove metamorphic suite, the Mt Cuba Gneiss, and the Doe Run Schist record monazite ages as young as 415 Ma and 406 Ma, respectively. Amphibole from rocks within these units cool through argon closure (~500°C) at ~400 Ma, and WM cooled through closure (~350°C) at ~365 Ma. The data are consistent with these rocks cooling as a unit from ~400 Ma to the present, much the same as the Sykesville Fm to the SW.

Age spectra of WM from rocks of the Peters Creek Fm between the Baltimore Mafic Complex (BMC) and the Pleasant Grove-Huntington Valley shear zone (PVHG) appear to contain variable amounts of excess argon and record minimum ages that range from 342- 330 Ma. These ages are interpreted to represent maxima for cooling through closure for these samples. These ages are significantly younger than ages within the PCT further to the SW. Perhaps their cooling was delayed by later thrusting of a now eroded portion of the BMC over top of these rocks.

Rocks of the Octoraro Fm in the WT west of the PVHG reached at least garnet zone, significantly hotter than rocks in the WT in MD to the southwest. WM age spectra from these rocks are complex, and are interpreted to reflect a mixture of WMs that grew above and below closure. Sigmoidal age spectra from these samples have a minimum age in the saddles of 370 Ma, and a maximum high-temperature age of 436 Ma. The 436 Ma age is interpreted to represent the minimum time of cooling through WM closure and the 370 Ma age to represent a maximum age for the growth of the youngest generation of WM. Observed age differences between the western (430 Ma) and eastern (371 Ma) parts of the WT to the SW in MD do not occur in the WT in PA, instead both events are recorded across the entire width of the WT in PA.