Nd isotopic evidence for juvenile crust in the Carolina terrane, southern Appalachians

Nd isotopic analyses of whole-rock samples from the older portion of the Carolina terrane, one of the largest terranes in the Appalachian orogen, demonstrate that part of this terrane is composed of juvenile, mantle-derived crust. These data suggest that the terrane may not have originally been built upon old, evolved basement material but rather may have been built upon oceanic crust. A recent study by other workers demonstrates a more crustally evolved Nd isotopic signature for younger components of the Carolina terrane. These data may indicate that the terrane interacted with evolved crust at a later time, possibly by amalgamation with a more evolved crustal fragment before final accretion to Laurentia, rather than indicating a primary old basement. A juvenile nature for the older portion of the terrane contrasts with models that suggest it is an evolved crustal fragment that formed in a continental margin setting - a scenario proposed to explain the high proportion of felsic volcanic rocks within the terrane. It is herein suggested that Carolina is a chemically evolved but isotopically juvenile crustal fragment, because it remained in an oceanic setting for an unusually long time. In this regard the Carolina terrane is similar to some of the large accreted terranes in the Canadian Cordillera, such as Wrangellia and Alexander. The presence of juvenile crust in the Carolina terrane documents that at least part of the southern Appalachian orogen is not composed solely of reactivated pre-existing continental crust. The importance of this part of the orogen in terms of the volume of juvenile Phanerozoic crustal material in North America may be larger than previously thought. However, until additional major Appalachian terranes have been isotopically characterized the volume of juvenile crust in the whole orogen remains unknown. The isotopic make-up of a terrane can be an important aspect of terrane analysis as different terranes may have significantly different isotopic compositions, while even widespread pieces of a single terrane should have very similar isotopic characteristics. The Nd isotopic data for the Carolina terrane form the beginning of an isotope database for terranes in the southern Appalachians.