A GONDWANAN TERRANE IN MARYLAND?

Accretion of peri-Gondwanan terranes onto the eastern margin of Laurentia is a key part of the Paleozoic Appalachian paradigm. Continental fragments that originated near Gondwana have long been recognized in the northern and southern Appalachians, but no exotic terranes have been confirmed between the New York City area and central Virginia, making tectonic links through this gap tenuous.

We dated spots in detrital zircons from metasandstones exposed across the central Maryland Piedmont to search for exotic terranes in this portion of the Appalachians. U/Pb dates were acquired by laser ablation mass spectrometry. Whereas sediment derived from Laurentia typically contains zircons with a major age peak between 1200 and 1000 Ma and very few dates between 680 and 580 Ma, zircons in Gondwanan detritus characteristically yield an age spike at c. 630-610 Ma. 17 of 18 studied samples yielded detrital zircon ages consistent with derivation from Laurentia. In contrast, the 130 dated zircons in the remaining sample produced an age spectrum with a single prominent spike at c. 630 Ma, very similar to the age spectra from some metasandstones in the peri-Gondwanan terranes Avalonia and Carolinia to the north and south, respectively. Pending confirmation from other samples, we tentatively conclude that the sediment in this sample was derived from Gondwana. If correct, this is one of the first discoveries of sediment with a Gondwanan provenance in the central Appalachians.

These potentially exotic rocks occur within the Potomac terrane west of Baltimore. However, other Potomac terrane samples 4 km to the east and west have a Laurentian detrital zircon age signature, suggesting that a peri-Gondwanan terrane, if it exists, must be no more than 8 km wide in map view. The presence of these peri-Gondwanan rocks inboard of the Baltimore terrane and Wilmington arc suggests that geologists might reconsider the possibility that these more outboard terranes are exotic with respect to Laurentia.