ALLEGHANIAN MAGMATISM IN THE SOUTHERN APPALACHIANS: IMPLICATIONS FOR THE ASSEMBLY OF PANGEA

The last major magmatic expression of the assembly of Pangea in North America is a suite of epizonal-mesozonal, I- S- and A-type, late syn- to post-tectonic granitoids emplaced in Appalachian terranes from Nova Scotia to Florida. Attempts to date these granitoids by U-Pb zircon methods have been successful in some cases, but are often plagued by numerous xenocrysts. In-situ (SHRIMP and LA-ICP) U-Pb ages of magmatic zircons and monazites from “Alleghanian” granites in the southern Appalachian orogen (SAO) yield a bimodal age distribution. Two of the granitoids (Stone Mountain and Tyrone) intrude the Inner Piedmont and yield early Alleghanian/post-Neo-Acadian ages (~335 Ma). Zircon xenocrysts from the Stone Mountain granite are dominantly from the host Lithonia orthogneiss (440 Ma) and probable Grenville basement (~1100 Ma). Six younger granitoid plutons intrude the Inner Piedmont, Carolinia/Kiokee, Pine Mountain, and Suwannee terranes, and collectively yield a limited range of ages (293-302 Ma). The two most southerly plutons intrude undeformed strata of the Gondwanan Suwannee basin and contain Neoproterozoic xenocrysts rarely seen in plutons north of the Suwannee suture; however, Grenville-age grains are also present. The 300-330 Ma, post-tectonic, granitoids north and south of the Suwannee suture zone are clearly products of crustal anatexis, not supra-subduction magmatism. The lack of Alleghanian supra-subduction magmatism strongly suggests accretion of the Suwannee terrane occurred during transpressional convergence between Laurentia and a combined African-Amazonian craton. The intervening Iapetan, Rheic, and Theic oceanic lithosphere was either dispersed along this transpressional boundary or subducted at the leading edge(s) of other blocks of Gondwanan lithosphere (e.g., Wiggins, Sabine, Maya, etc.) that accreted during the late Paleozoic. Xenocrystic zircons in the Alleghanian plutons along with detrital zircons from Paleozoic Suwannee strata form the basis for comparing the Suwannee terrane to other Gondwanan elements, including the Bove Basin (Africa) and parts of Iberia. Mesoproterozoic Nd model ages, however, are more indicative of an Amazonian origin for the bulk of the Suwannee lithosphere.