POSSIBLE PRESERVATION OF SALINIC CLEAVAGES IN THE PENNSYLVANIA EMBAYMENT: 40AR/39AR EVIDENCE FOR BOTH EARLY SILURIAN AND LATE DEVONIAN CLEAVAGES IN THE WESTMINSTER TERRANE, MARYLAND

In an attempt to constrain the age(s) of cleavage-forming events in the rocks of the Westminster terrane (WT) in the western Piedmont of Maryland, we present new ⁴⁰Ar/³⁹Ar age spectra from whole rock and white mica concentrate samples from the WT and adjacent rocks. The argon data from these samples has been a challenge to interpret because muscovite in many of them is intimately intergrown with chlorite, and the rocks contain cleavage micas of multiple ages as well as detrital Proterozoic muscovite. The WT rocks are dominantly phyllites and are in sharp contrast with the migmatitic (Taconic) Potomac terrane rocks present across the Pleasant Grove fault zone to the east.

Our ⁴⁰Ar/³⁹Ar results from nine new samples from the eastern WT (including the Blockhouse Point domain of Kunk et al., 2005) confirm a late Devonian age (375-365 Ma) (Acadian) for the time of crystallization of the dominant cleavage there. Similar late Devonian ages have been obtained from Frederick Valley synclinorium rocks west of the Martic fault. These ages are in sharp contrast with those present in the western WT, where several thoroughly recrystallized samples of micaceous phyllites show >50 percent of the ³⁹Ar_K gas as early Silurian (~430 Ma), ten other spectra are consistent with this Salinic(?) age. The northeast striking Parrs Ridge fault separates the eastern and western sub-domains within the WT.

The fault rocks separating these domains are everywhere phyllitic and commonly host dismembered quartz veins. Age spectra from some samples collected in the Pleasant Grove and Parrs Ridge fault zones yield Late Pennsylvanian apparent ages, suggesting that final juxtaposition of these belts of rocks occurred in the Alleghanian orogeny. We are tantalized with the possibility that these slices of weakly metamorphosed shelf and slope sediments found safe haven in the ‘harbor' of the Pennsylvanian embayment of Thomas (2006) during right-lateral strike-slip motion around the New York promontory during the Salinic, Acadian, and Alleghanian orogenies.