GEOCHEMISTRY OF SILURIAN-DEVONIAN AND “ATYPICAL ALLEGHANIAN” GRANITES IN THE CAROLINA TERRANE, SOUTHERN APPALACHIANS: IMPLICATIONS FOR SOURCES AND TERRANE ACCRETIONARY HISTORY

The Carolina Terrane in the southern Appalachian is host to a large number of granitic intrusions. Three of these intrusions, the Cloud's Creek Granite, Newburry granite and Lake Murray granite gneiss, were previously thought to be Alleghanian in age. U-Pb analysis of zircon from these granites, however, yield ages of 414.9+2.1/-1.7 Ma, 414±3 Ma and 421+4/-2.9 Ma respectively (Samson & Secor, 2001). Two of these granites (Cloud's Creek and Lake Murray) are chemically significantly distinct from the vast majority of granites of proven Alleghanian age. These Silurian/Devonian granites have higher ¹⁴⁷Sm/¹⁴⁴Nd (> 0.12) than values typical of the majority of Alleghanian granites (i.e. ≤ 0.10). In addition to their less steep REE patterns these older granites also have significantly lower total REE contents compared to the average Alleghanian value (ΣREE = 178 ppm). These pre-Alleghanian rocks also have Lu/Hf ratios > 0.07, essentially double the average Alleghanian value of 0.04. There are other undated granites intruding into Carolina Terrane, many of which have also been assumed to be Alleghanian, that are chemically and isotopically more similar to the known Silurian-Devonian intrusions and thus may also predate the Alleghanian event by ~ 100 Ma. Regardless of age, the chemical disparity suggests that these granites may have been derived from different source materials. Specifically, the sources appear to be more depleted in REE (assuming comparable degrees of partial melting) and have higher MREE/LREE, yet have higher average Cs and Rb concentrations than the Alleghanian source. The juxtaposition of geochemically distinct granites could be explained if the “atypical Alleghanian” plutons were formed solely by partial melting of Carolina Terrane crust prior to the accretion of Carolina to the Laurentian margin. Melting during or post accretion may have involved components of native Laurentian crust (i.e. Grenville basement) in addition to terrane curst, thus providing different source characteristics. Demonstration of consistently older ages for the ‘atypical' granites would support this view. Thus the petrogenetic details of Alleghanian versus potentially ‘pre-Alleghanian' granites might hold clues to the enigmatic accretionary history of the Carolina Terrane.