COMMON PB AND SM-ND ISOTOPIC SIGNATURES OF THE SUWANNEE TERRANE AND ADJACENT TERRANES OF THE SOUTHEASTERN US: COMPARISON AND SPECULATIONS ON INTERTERRANE CORRELATIONS

The Suwannee, Uchee, and Carolina terranes of the southeastern USA have similar characteristics, most notably the presence of Neoproterozoic igneous suites, and paleomagnetic, paleontologic, and geochronologic data indicating Gondwanan origins. All bear whole-rock Sm-Nd and/or zircon U-Pb isotopic evidence of a 1.0-1.2 Ga component, which could be interpreted as either Laurentian (Grenvillian) or Gondwanan (Amazonian or Kalaharian) in origin. Here we discuss the implications of common Pb isotopic data for samples from these terranes. Common Pb isotopic ratios for Neoproterozoic igneous rocks of the Suwannee terrane (ST) show a linear trend on plots of 207Pb/204Pb vs. 206Pb/204Pb, with a slope corresponding to an age of ca. 1.1 Ga. The majority of the samples also have Mesoproterozoic Nd depleted mantle model ages (1.0 to 1.6 Ga), and 1 Ga xenocrystic zircons have been found in Alleghanian ST granites. The age of 1.0 to 1.2 Ga corresponds to the age of the Grenville event in North America, as well as to the Orinoquian/Sunsas events in Amazonia. Neoproterozoic samples from the Uchee terrane (UT) lie on the same Pb array, and are indistinguishable in Pb-Pb space from the ST rocks. In addition, Pb data from samples taken north of the UT, from the Pine Mountain block, fall on a less-radiogenic extension of this same array. All of the aforementioned samples lie above the 1.2 Ga Pb-Pb trend reported by Sinha et al. (1996) for basement units of the central and southern Appalachians, and instead plot within the same field as Amazonian basement rocks, thus strengthening the connection of these terranes to Amazonia. Both the ST and the UT, therefore, appear to have been generated from similar lithosphere, and have similar lithostratigraphy through the Neoproterozoic. As described by Horton et al. (1989), the two are separated from each other by the buried Charleston terrane, but the terrane boundaries are not well-defined or well-understood. We suggest that the ST and the UT were derived predominantly from Mesoproterozoic Amazonian lithosphere, and that they are closely related, if not the same terrane subdivided by later faulting. Likewise, Pb whole-rock isotopic data from basalts of the Carolina and Suwannee terranes are similar, suggesting mutual origins within Amazonian lithosphere and a close relationship among all three terranes.