DETRITAL ZIRCON AGES FROM THE WESTERN PIEDMONT OF PENNSYLVANIA: IMPLICATIONS FOR TERRANE BOUNDARIES AND THE EARLY PALEOZOIC LAURENTIAN MARGIN

Although major orogen-parallel faults in the western Piedmont of Pennsylvania have been interpreted as terrane boundaries, their significance and amount of displacement remains controversial. Two of these faults cross the study area in Lancaster County. The northwesternmost is the Martic Fault, long considered to be a major overthrust. Recent mapping (Blackmer, 2007) reassigned the northern belt of quartzose pelitic schist and minor metasandstone of the Octoraro Fm, south of the Martic Fault, to the Antietam-Harpers Formations, part of a sequence of Cambrian to early Ordovician clastic and carbonate rocks of the Laurentian margin found north of the fault. Based on this mapping, the Martic Fault may be part of an imbricate fan. The Pleasant Grove Shear Zone (PGSZ) separates the Octoraro Fm from quartz schist and pebbly metasandstone of the Peters Creek Schist to the south. The Octoraro and Peters Creek have been interpreted as associated with the Laurentian margin, or as sediments deposited in a narrow ocean basin between Laurentia and a microcontinent. The PGSZ is a broad high-strain zone, but the displacement along it is unclear.

To further clarify relationships between the metaclastic units and constrain the significance of the faults, detrital zircon ages were obtained from 5 samples: Antietam-Harpers north of the Martic Fault, Octoraro/Antietam-Harpers south of the Martic Fault (2), Octoraro Fm at the PGSZ, and Peters Creek pebbly metasandstone south of the PGSZ. All age distributions are essentially the same – a large peak at 950-1500 Ma, a few zircons at 530 Ma, and a few zircons at about 2750 Ma. Consistency of age distributions suggests that all units had a similar sediment source, and were likely deposited in the same basin, limiting displacement along the faults and calling into question their terrane boundary status. The Antietam-Harpers is generally considered part of the Cambrian rift-to-drift transgressive sequence. By association, that ties the Octoraro and Peters Creek to the Laurentian margin. Laurentia was likely the primary sediment source, contributing the Mesoproterozoic zircons. Archean zircons suggest minor input from a non-Laurentian source, perhaps a fragment of Gondwana across a narrow basin. The early Cambrian zircons are a puzzle, but do provide a maximum depositional age for these units.