Timing of metamorphism and deformation in southeastern Pennsylvania and northern Delaware

Blackmer, Gale C., Bureau of Topographic and Geologic Survey, 3240 Schoolhouse Rd, Middletown, PA 17057, gblackmer@state.pa.us, Kunk, Michael J., USGS, MS 926A, National Center, Reston, VA 20192, Southworth, Scott, U.S. Geol Survey, 926A National Center, Reston, VA 20192, and Bosbyshell, Howell, Department of Geology and Astronomy, West Chester Univ, 750 South Church Street, West Chester, PA 19383

New 40Ar/39Ar age data from southeastern Pennsylvania and northern Delaware, together with published monazite data, suggest that development of the dominant regional fabrics and cooling from metamorphic conditions happened in the Devonian, with only localized, minor white mica growth during the Pennsylvanian. The study area is a portion of the central Appalachian Piedmont consisting of greenschist to granulite facies metamorphic rocks, including three provisional divisions of the Wissahickon Formation, separated into structural blocks by major NE-trending faults. Units were sampled as follows, from SE to NW: type Wissahickon Fm SE of the Rosemont Fault (Block 1); Baltimore Gneiss, Mt. Cuba Wissahickon fm, and Setters Fm between the Rosemont and Street Road faults (Block 2); Glenarm Wissahickon fm between the Street Road and Embreeville faults (Block 3); Peters Creek Schist and Octoraro Fm between the Embreeville and Martic faults (Block 4); and Antietam and Harpers formations, undivided, NW of the Martic Fault (Block 5). Amphiboles from amphibolite to granulite facies rocks in Block 2 yield 40Ar/39Ar ages indicating cooling through the ~500ºC isotherm at ~400-375 Ma, suggesting that the Baltimore Gneiss (basement) and the Mt. Cuba Wissahickon fm were at roughly the same structural level at about this time. In comparison, monazite that grew during formation of the regional fabric in garnet-grade rocks in Block 3 yields microprobe ages of ~400 Ma. White mica from lowermost greenschist facies rocks in Block 5 yields a growth age similar to the amphibole cooling and monazite growth ages. This may indicate a time of thrusting on the Martic Fault. White micas from Blocks 1-4 yield 40Ar/39Ar cooling ages of about 365 Ma in all units. We interpret these ages to represent cooling through ~350ºC. No regional patterns are apparent. Age spectra of white micas from the vicinity of the Pleasant Grove-Huntingdon Valley shear zone, within Block 4, are more complex and suggest minor recrystallization during Alleghanian shearing. Regionally, white mica ages from Block 4 are comparable with results from correlative units in the Potomac Terrane of Virginia and Maryland. The amphibole cooling ages from Block 2 are similar to amphibole ages from the Sykesville and Laurel formations of the Potomac Terrane.

Poster presentation presented at the 42nd Annual Meeting (12–14 March 2007) of the Northeastern Geological Society of America, Durham, New Hampshire.