ORIGIN, DISTRIBUTION, AND SIGNIFICANCE OF MESOPROTEROZOIC CRUST IN THE SOUTHERN APPALACHIANS: RODINIA REVISTED

The eastern margin of North American Laurentia retains evidence of numerous orogenic events beginning in the Mesoproterozoic (Grenville). As a result of extensive overlying sedimentary rocks of the Coastal Plain and post Grenville accreted terranes (e.g., Carolinia), only limited exposures of Mesoproterozoic crust are present. Nonetheless, knowledge of the extent of Mesoproterozoic crust is critical to understanding the origin of that crust, persistence, and extent of sub-continental lithospheric mantle. Constraining the extent of Mesoproterozoic crust and lithosphere is critical to evaluating the extent of juvenile crust produced during the Appalachian. Some insight into the difficult problem of quantifying Paleozoic crustal growth vs. crustal recycling can be gained by measuring the isotopic compositions of Nd and Hf (zircon) of post Grenville magmatic rocks. From north to south, Hf isotopic compositions of zircons from Mesoproterozoic rocks indicate a mixture of juvenile and recycled crust. In Paleozoic magmatic rocks (i.e., Taconic, Acadian, Alleghanian), similar mixed signals. This mixed source/recycling signal extends to the accreted exotic terranes, such Carolinia, Suwannee, Uchee, etc. These observations have led to proposals that exotic, accreted terranes (e.g., Suwannee) contain Mesoproterozoic crust-lithosphere that evolved as part of Gondwana, most often Amazonia. A totality of the evidence, however, does not support an exotic origin for the southern Appalachian Mesoproterozoic inliers for several reasons, including a comparison of detrital zircon age distributions in post-Grenville sedimentary rocks of the Appalachians vs. rocks of equivalent age from Amazonia (e.g., Sunsas orogen). Instead, we propose that the variations in isotopic compositions of Mesoproterozoic igneous rocks and detrital zircon are more compatible with continental arc. In this model, an origin in a Mesoproterozoic continental marginal arc and that magmas generated in this environment reflect the age of the juxtaposed terrane as evident in their detrital zircons age spectra. Mars Hill, for example, would represent a section of the Grenvillian arc that incorporated detritus form the Congo craton, the oldest and most isotopically evolved of the possible conjugates.