ALLEGHANIAN MAGMATISM IN THE SOUTHERN APPALACHIANS: IMPLICATIONS FOR PANGEAN TECTONIC MODELS

The last major magmatic expression of the assembly of Pangea in North America is a suite of generally epizonal-mesozonal, I- and S-type, late syn- to post-tectonic granitoids emplaced in Appalachian terranes from Nova Scotia to Florida. The majority of age-dates for these rocks are based on Rb-Sr whole-rock and/or K-Ar and 40Ar-39Ar determinations on micas. Many of the Rb-Sr and K-Ar ages are precise and essentially equivalent, indicating both are likely to be cooling ages, either directly (K-Ar/40-39) or indirectly (high Rb/Sr samples that control isochron slopes). Attempts to date these granitoids by U-Pb zircon methods have been successful in some cases, but are often plagued by numerous xenocrysts. New, in-situ (SHRIMP and LA-ICP) U-Pb ages of magmatic zircons and monazites from 8 “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). The age of the Stone Mountain granite was measured on monazite (337+/-4 Ma) and is >40 Ma older than its Rb-Sr whole-rock isochron age (291 Ma) and 10 Ma younger than the oldest reported multi-grain U-Pb zircon age (350 Ma). Zircon xenocrysts from the Stone Mountain granite are dominantly from the host Lithonia orthogneiss (440 Ma) and probable Grenville basement (~1100 Ma). The six younger granitoids 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 intruded the undeformed stratigraphy 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 age and distribution of xenocrysts, the narrow time-frame of emplacement, elemental and isotopic compositions indicating mainly crustal sources, and the wide geographic area over which the true Alleghanian granitoids occur restrict possible tectonic environments in which these melts were generated. These factors in conjunction with the pattern of emplacement may reflect the loci of crustal thickening associated with Alleghanian thrust sheets and/or orogenic collapse during phases of transcurrent faulting.