THE SMITH RIVER ALLOCHTHON-LYNCHBURG GROUP CONTACT, REVISITED: DOES IT REPRESENT CLOSURE OF THE IAPETUS OCEAN OR A LAURENTIAN RIFT-BASIN?

The Smith River Allochthon (SRA) of the southern Appalachian Piedmont zone lies in fault contact with the underlying Lynchburg Group (LG), a rift sequence deposited on attenuated Laurentian (LA) basement during the opening of the Iapetus Ocean (IO). Like other Piedmont zone terranes, the SRA's origin and relationship with LA rocks are controversial. Some workers depict the SRA as having an early tectonothermal history exotic to the LA margin and believe that its contact with the LG represents closure of the IO. Other workers hypothesize that the SRA is a distal component of the LG, implying that their contact indicates closure of a LA rift basin. New geochronological, geochemical, and metamorphic data provide insight into the origin of the SRA, its tectonothermal history, and the nature and timing of its emplacement upon the LG.

Detrital zircon age data from the SRA range from 0.9-1.5 Ga, which is strikingly similar to the 0.9-1.4 Ga ages obtained from the LG, suggesting that these terranes were derived from similar source rocks. Mafic rocks from the SRA and LG lack the negative Nb-Ta anomaly indicative of arc affinity and are not depleted in highly incompatible elements as is N-MORB (i.e. oceanic crust). Instead, these suites are most similar to continental rift basalts. These data imply that the SRA originated along the attenuated LA margin. Thus the SRA-LG contact likely represents closure of a LA rift basin rather than closure of the IO.

P-T-t paths for the SRA and LG are being constructed by integrating structural and petrographic observations with 1) MnNaCaKFMASHTi phase equilibria modeling, 2) garnet growth P-T estimates, 3) laserprobe Ar-Ar geochronology, and 4) EPMA monazite geochronology. These paths will not only provide valuable insight into the tectonothermal histories of each terrane, but also will allow for direct comparison of their histories, which will help determine the nature and timing of the emplacement of the SRA upon the LG. Preliminary data suggests that the SRA was involved in a progressive low P, high T event (580 C, 4 kb) that occurred from 500 to 460 Ma while the LG experienced Barrovian metamorphism at ~ 460 Ma. These data also suggest that the SRA was emplaced upon the LG between 390 and 425 Ma at greenschist facies conditions (~ 530 C, < 4 kb). Both terranes also record a later transtensional (?) event at ~ 340 Ma.