ND ISOTOPIC EVIDENCE FOR JUVENILE CRUST IN THE CAROLINA TERRANE, SOUTHERN APPALACHIANS

Nd isotopic analyses of whole-rock samples from the older portion of t he Carolina terrane, one of the largest terranes in the Appalachian or ogen, demonstrate that part of this terrane is composed of juvenile, m antle-derived crust. These data suggest that the terrane may not have originally been built upon old, evolved basement material but rather m ay have been built upon oceanic crust. A recent study by other workers demonstrates a more crustally evolved Nd isotopic signature for young er components of the Carolina terrane. These data may indicate that th e terrane interacted with evolved crust at a later time, possibly by a malgamation with a more evolved crustal fragment before final accretio n to Laurentia, rather than indicating a primary old basement. A juven ile nature for the older portion of the terrane contrasts with models that suggest it is an evolved crustal fragment that formed in a contin ental margin setting - a scenario proposed to explain the high proport ion of felsic volcanic rocks within the terrane. It is herein suggeste d that: Carolina is a chemically evolved but isotopically juvenile cru stal fragment, because it remained in an oceanic setting for an unusua lly long time. In this regard the Carolina terrane is similar to some of the large accreted terranes in the Canadian Cordillera, such as Wra ngellia and Alexander. The presence of juvenile crust in the Carolina terrane documents that at least part of the southern Appalachian oroge n 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 t han previously thought. However, until additional major Appalachian te rranes have been isotopically characterized the volume of juvenile cru st in the whole orogen remains unknown. The isotopic make-up of a terr ane can be an important aspect of terrane analysis as different terran es may have significantly different isotopic compositions, while even widespread pieces of a single terrane should have very similar isotopi c characteristics. The Nd isotopic data for the Carolina terrane form the beginning of an isotope database for terranes in the southern Appa lachians.