Central Appalachian Exotic Terranes and Exposures of Former Orogenic Middle Crust

Abstract:

In the northern and southern Appalachians, rocks that formed Paleozoic orogenic middle crust mostly are exposed within or directly inboard of terranes that originated near Gondwana. Most outcrops of the Paleozoic orogenic middle crust of the eastern edge of Laurentia occur adjacent to these exotic terranes. However, a narrow belt of Paleozoic orogenic middle crust is exposed in the Piedmont of the central Appalachians despite the absence of recognized exotic terranes. The presence of these deformed, amphibolite facies rocks raises the questions: “Did central Appalachian orogeny occur in the absence of accreted exotic terranes?” and, more generally, “Is exotic terrane collision required for exhumation of Appalachian former middle crust?”

Previous U/Pb isotopic dating of spots in detrital zircon revealed the presence of Gondwanan terranes in three locations in the central Appalachians: central Virginia, central Maryland, and southeastern Pennsylvania. Two new samples collected near the discovery locations in Virginia and Maryland yielded prominent peaks in zircon U/Pb age distributions at ca. 630-610 Ma, confirming the Gondwanan affinity of these rocks. Hf isotopic compositions of spots in these upper Neoproterozoic zircon grains range to both more and less depleted than spots in zircon from the few possible Laurentian granitic sources, consistent with derivation of the zircon from Gondwana. Abundant 1700-1000 Ma detrital zircon rules out the West Africa Craton as a potential source; Amazonia is the most likely ultimate source of the zircon.

The extent of the exotic terrane(s) in the central Appalachian Piedmont remains enigmatic due to uncertain connections between isolated exposures of the terrane(s). Nevertheless, the discovery of one or more exotic terranes in the central Appalachian Piedmont underscores the relationship between exotic terranes and exposed former middle crust in the Appalachians. This relationship may be a feature of several other major Phanerozoic orogens.