DETRITAL ZIRCON EVIDENCE FOR EARLY PALEOZOIC SEDIMENT SOURCES ON THE LAURENTIAN MARGIN, EMPHASIS ON PENNSYLVANIA

Detrital zircon U-Pb age distributions for Cambrian rift-to-drift sequences along the Laurentian margin from Newfoundland to North Carolina show remarkable consistency. The dominant Mesoproterozoic age is consistent with derivation from the Grenville orogen (ca. 900-1500 Ma). There are generally some Paleoproterozoic and Archean zircons, as well as few latest Proterozoic to earliest Cambrian. This suggests a Laurentian sediment source with little or no outside input.

While falling within the parameters described above, zircon distributions from rift-drift rocks in the Pennsylvania Piedmont have distinctive characteristics, among them a younger maximum age (ca. 530 Ma) than other parts of the margin. The Antietam/Harpers Fm, early drift clastics indisputably deposited on the Laurentian margin, and likely slope-rise sediments of the Octoraro Fm have dominant Mesoproterozoic peaks of 1300-1500 Ma and 1000-1300 Ma, respectively. The dominant older peak is unusual. The Setters Fm and overlying West Grove Metamorphic Suite (Doe Run Schist and Mt. Cuba Gneiss) form a separate rift-drift sequence lying to the southeast. They contain additional zircons with 600-900 Ma ages not found in the western units, perhaps tapping sources that saw early Iapetan rifting. Amphibolites with rift- and MORB-like compositions interlayered with West Grove metasediments suggest rift-to-drift volcanism continued into the earliest Cambrian.

The Peters Creek Schist, in a fault-bounded outcrop belt between the Octoraro and West Grove, exhibits both Mesoproterozoic peaks. The Peters Creek, which retains turbiditic textures and is host to greenstones with ocean-floor chemistry, has been interpreted as rift-related. Alternatively, it may be a younger unit containing reworked elements of the older margin sequence, deposited during renewed instability along the margin. Its zircon distribution is most similar to those of the local Taconic allochthons (Dauphin and Linglestown Fms), which are also dominated by bimodal Mesoproterozoic peaks. Despite conodont evidence indicating that the allochthons were originally deposited in a trench far removed from Laurentia, the sediments recycled into them do not include elements exotic to the Grenville orogen.