40AR/39AR TIMING CONSTRAINTS ON THE PALEOZOIC ASSEMBLY OF THE MID-ATLANTIC PIEDMONT, USA

The Rock Creek shear zone (RCSZ) is the easternmost of a number of exposed faults located in the mid-Atlantic Piedmont that separate various domains within the Potomac composite terrane in Washington, D. C., Maryland, and Virginia. It lies between the rocks of the Sykesville Formation to the west and those of the Laurel Formation to the east. ⁴⁰Ar/³⁹Ar age spectrum data from samples collected within the RCSZ are used to constrain the timing of its ductile and brittle deformation.

Amphibole from massive amphibolite in the Laurel Formation yielded a plateau age of 400 ± 2 Ma representing the time of cooling through its closure temperature (500 ± 50°) from peak metamorphic conditions. This age agrees well with previously published amphibole ages of ca 400 Ma from both the Laurel and Sykesville formations suggesting that these rocks were at about the same crustal level in the Early Devonan.

Age spectra of white mica samples are somewhat more complex. Rocks from the Sykesville Formation contain multiple age populations of white mica, some of which grew below the argon closure temperatures for white mica (350 ± 50°) during Alleghanian shearing, while all of the white mica on the east side of the RCSZ grew in conditions above its argon closure temperature. ⁴⁰Ar/³⁹Ar data from these samples indicate that the rocks on the east side of the RCSZ cooled later than those on the west side with the eastern Laurel Formation moving up relative to the Sykesville Formation through much of the Devonian and Carboniferous. Argon data from one randomly oriented white mica sample from the Laurel Formation, with a maximum crystallization age of 306 Ma, suggests brittle faulting consistent with the movement of hot fluids in the Laurel Formation and an Alleghanian timing of thermal convergence relative to the Sykesville Formation, virtually the same time as motion along the Plummers Island fault that bounds the Sykesville Formation to the west.

Our new data, together with previously published data from the Potomac composite terrane indicate that while the Taconic and Acadian orogenies had variable thermal effects on these different crustal blocks, their final assembly definitely occurred during transpressive events in the Alleghanian.