ANOMALOUS ANCIENT CRUST IN THE SOUTHEASTERN USA: IMPLICATIONS FOR THE ASSEMBLY OF NORTH AMERICA AND RODINIA

New evidence from SHRIMP U-Pb ages and Nd isotopic data, combined with previous whole-rock Pb isotope data, indicates that pre-1.0 Ga basement of the southern Appalachian orogen is anomalous with respect to simple reconstructions of Laurentia. Southeastward-younging belts in the Laurentian craton document regular growth. Extrapolation of these belts across the covered interior SE of Missouri suggests continuity of the 1.4-1.5 Ga mid-continent granite-rhyolite terrane and the ~1.2-1.4 Ga juvenile Grenvillian margin into the Southeast, but such crust is rare or absent in the Appalachians.

Exposed >1.0 Ga basement in the Blue Ridge of NC, TN, SC, and northwest GA is mostly granitoid gneiss with crystallization ages of 1.14-1.19 Ga. Pb isotopic compositions are distinct from those of exposed portions of the Grenville margin (Sinha & McLelland, 1999; Loewy et al., 2003), and Nd T_DMs of ~1.5-1.7 Ga suggest older crustal heritage than either the Grenville margin or the adjacent mid-continent terrane. Some of the northwesternmost basement exposures, in the western Blue Ridge, reveal ~1.07 Ga granitoids with T_DM of ~1.3-1.5 Ga, and we have identified a single 1.38 Ga orthogneiss; no Pb isotopic data are yet available for these rocks. The most distinctive southeastern basement, the Mars Hill terrane of NC-TN, includes highly variable paragneisses and mafic and felsic orthogneisses. Most magmatic crystallization ages are ~1.20-1.25 Ga, but ~1.8 Ga orthogneiss is locally exposed, most T_DMs are ~1.8-2.0 Ga, Paleoproterozoic detrital and inherited zircons are common, and elevated ²⁰⁷Pb/²⁰⁴Pb ratios indicate Paleoproterozoic or older crustal heritage (Sinha et al., 1996).

With the possible exception of some western Blue Ridge basement that may represent Laurentian margin outliers, southern Appalachian basement appears to consist of relict non-Laurentian crust; based on a shared 1.03 Ga metamorphic overprint and ~750 Ma dikes, it likely accreted during Grenvillian assembly of Rodinia. Pb and Nd isotopic signatures and U-Pb crystallization ages suggest possible Gondwanan correlatives in South America (Amazonia [Loewy et al., 2003], Arequipa-Antofalla) or Africa (Kalahari).