CONSTRAINTS ON BLUE RIDGE AND INNER PIEDMONT PARAGNEISS PROVENANCE: SYNTHESIS OF DETRITAL ZIRCON AND WHOLE-ROCK PB ISOTOPIC DATA

Thick sequences of rift- and passive-margin sediments were deposited during the breakup of Rodinia. Multiple Paleozoic orogenic events later modified the Laurentian margin and erased or obscured much of the margin's early history. As a result, almost all surface and sub-surface lithotectonic boundaries within the southern Appalachian crystalline core are the subject of variable, and mostly incompatible, interpretations. Detrital zircon and whole-rock Nd-isotopic data for crystalline core samples demonstrate that the Blue Ridge and western Inner Piedmont (wIP) were derived from rocks like those of the Grenville basement exposed in the southern Appalachians. Data from the eastern Inner Piedmont metasedimentary samples suggest that it has a mixed provenance largely comprised of rocks derived from the Carolina superterrane and reworked Blue Ridge-wIP material as young as Ordovician in age.

Whole-rock Pb isotopic data from 20 Blue Ridge and Inner Piedmont metasedimentary samples was obtained to further constrain the relationship of these terranes to Laurentia. These samples have higher ²⁰⁷Pb/²⁰⁴Pb values for given ²⁰⁶Pb/²⁰⁴Pb values as compared to mid-continent 1.4 Ga Granite-Rhyolite and 1.0-1.3 Ga Grenville exposures in New York and Texas and are more compatible with an Amazonian source than a pre-Rodinian Laurentian source. Not surprisingly, the Pb-isotopic data from metasedimentary cover samples is remarkably similar to southern and central Appalachian basement. Incorporation of published whole-rock Pb-isotopic data from North and South America with new data from surface and sub-surface eastern North American basement samples (Fisher et al., 2006) and these metasedimentary samples suggests that either a sub-surface suture exists between the pre-Rodinian Laurentian margin and an exotic, likely Amazonian, crustal fragment or that the Rodinian rift basins along this margin were filled entirely by the conjugate margin.