REINTERPRETING THE PROVENANCE OF EASTERN LAURENTIAN LATE PROTEROZOIC RIFT FILL: NEW WHOLE-ROCK PB ISOTOPIC CONTRAINTS

Southern Appalachian Blue Ridge and western Inner Piedmont metasedimentary sequences were deposited as rift-to-drift fill along the Laurentian margin during the breakup of Rodinia. These sediments may have been derived from Laurentia, the conjugate margin, or both. Previous work using detrital zircon ages and Nd compositions from these metasedimentary sequences did not identify obvious non-Laurentian characteristics and concluded that the sediments likely had Laurentian provenance. However, this is not a unique solution, as multiple cratons contain rocks with similar U/Pb ages and Nd compositions.

Recent work demonstrates that some cratons have distinct whole-rock Pb isotopic signatures. New whole-rock Pb isotopic data were obtained for 20 samples from a previous detrital zircon and Nd isotopic study. Of these samples, 17 have ²⁰⁷Pb/²⁰⁴Pb values > 15.65 and most ²⁰⁶Pb/²⁰⁴Pb values are between 18.2 and 20.2. The metasedimentary rocks have higher ²⁰⁷Pb/²⁰⁴Pb values for given ²⁰⁶Pb/²⁰⁴Pb values relative to rocks from the mid-continent ~1.4 Ga Granite-Rhyolite and 1.0-1.3 Ga Grenville terranes of New York and Texas. This distinct Pb isotopic signature of the metasedimentary rocks precludes incorporation of significant components of Laurentian material and necessitates an alternate source.

Amazonia is the southeastern Laurentia conjugate rift margin in most Rodinia reconstruction and it contains rocks with similar U/Pb ages and Nd and Pb isotopic signatures to samples from the southern Appalachians. The U/Pb detrital ages and Nd and Pb isotopic signatures of the metasedimentary sequence are also strikingly similar to those of the underlying southern and central Appalachian basement (scAB). Distinct scAB signatures were used to argue that it was an exotic block that was transferred from Amazonia to Laurentia during the Grenville Orogeny. Thus, Blue Ridge and western Inner Piedmont metasedimentary sequences were mostly derived from: (1) a post-Rodinian Laurentian margin that included accreted Amazonian material (scAB); (2) the Amazonian conjugate margin; or (3) both margins. All three scenarios imply that during Rodinia breakup Laurentia included a rifted margin dominated by extended Amazonian blocks and possibly a continental interior that was submerged, topographically subdued, or isolated in terms of its drainage pattern.