TERRANE TRANSFER DURING THE GRENVILLIAN ASSEMBLY OF RODINIA: TRACING THE AMAZONIAN ANCESTRY OF SOUTHERN APPALACHIAN BASEMENT THROUGH PB AND ND ISOTOPES

Whole rock Pb isotope data can be used to determine the provenance of different blocks within the Rodinia supercontinent, providing a test for paleogeographic reconstructions. Using U-Pb zircon data as an anchor point, we calculate values for the initial Pb present at the time of rock crystallization (mu2) using published whole rock and feldspar data from the Grenville Province of Canada. The calculated values of mu2 values are remarkably coherent for the entire Grenville Province, including the Adirondacks of the northern U.S.A. and Grenvillian rocks of Texas and adjacent New Mexico ca.3000 km away, suggestive of a homogeneous mantle source for this entire region. A distinctly different source region characterizes “Grenvillian” basement rocks of the southern Appalachians (Blue Ridge/Mars Hill), suggesting that these rocks are not of Laurentian origin.

In order to test for a Rodinian connection between Laurentia and Amazonia, the Pb fingerprint of 54 whole rocks samples from the “Grenvillian” basement rocks of the SW Amazon craton (Rondônia, Brazil), are compared to those from the Grenville belt of North America and Grenvillian basement inliers in the southern Appalachians. Both the SW Amazon basement and the allochthonous Blue Ridge/Mars Hill terrane are defined by highly radiogenic mu2 values of ~10, well above the crustal average. In contrast, the greater Grenville Province of Laurentia is characterized by lower mu2 values of ~9.5. Published U-Pb zircon ages and Sm-Nd for the Blue Ridge/Mars Hill terrane also suggest an ancient provenance very different from the rest of the adjacent Grenville belt, which is dominated by juvenile, 1.3-1.5 Ga rocks. The presence of mature continental material in rocks older than 1.15 Ga in the Blue Ridge/Mars Hill terrane (Epsilon Nd > 0) is consistent with the signature of Amazon basement rocks. We propose that this portion of the S. Appalachian basement is derived from Amazonia and was transferred to Laurentia during Grenvillian orogenesis at ~1.15 Ga. The presence of these Amazonian rocks in southeastern Laurentia records the northward passage of the Amazon craton along the Laurentian margin, following the original collision with southernmost Laurentia at ca. 1.2 Ga.