GEOCHEMICAL AND ND-SR-PB ISOTOPIC COMPOSITION OF ALLEGHANIAN GRANITESOF THE SOUTHERN APPALACHIANS - ORIGIN, TECTONIC SETTING, AND SOURCE CHARACTERIZATION

We report major element, trace element, and Nd and Sr isotopic ratios for 35 samples of Late Mississippian-Early Permian Alleghanian granite s that intrude the Carolina terrane, Eastern slate belt, Inner Piedmon t, and Kiokee belt of the southern Appalachians. Major element composi tions indicate that most plutons are high-potassium (K2O greater than or equal to 4%) metaluminous granites. Initial epsilon(Nd), depleted m antle model ages, and initial Sr-87/Sr-86 ratios, respectively, of the plutons range from -8.2 to -3.4, 940-1020 Ma, and 0.70642-0.72798 for the Inner Piedmont; -2.3 to +2.0, 660-1130 Ma, and 0.70421-0.71035 fo r the Kiokee belt; -2.7 to +2.4, 660-870 Ma, and 0.70353-0.70816 for t he Eastern slate belt; and -6.7 to +1.9, 690-1140 Ma, and 0.70391-0.70 739 for the Carolina terrane. The chemical and isotopic data are most consistent with formation of the granites by anatexis of continental c rust, rather than derivation from a depleted mantle source. Rocks of t he Carolina terrane have epsilon(Nd)(300 Ma) values and Pb isotopic ra tios indistinguishable from those of most Alleghanian granites, sugges ting it is a likely source. Some Alleghanian granites within the Carol ina terrane require a more evolved source than exposed Carolina terran e rocks, suggesting that evolved crust, possibly Grenville basement, o ccurs structurally below or within the terrane. The chemical and isoto pic compositions of granites intruding the Kiokee belt, Eastern slate belt, and Carolina terrane are similar, suggesting that these terranes are geochemically similar. In contrast, Alleghanian granites within t he Inner Piedmont have higher initial Sr-87/Sr-86 and lower Nd-143/Nd- 144 ratios, indicating that their sources are isotopically more evolve d. The virtual absence of Alleghanian plutons with mafic-intermediate compositions, and the lack of geographic trends in chemical and isotop ic composition across the orogen, makes a subduction origin for the gr anites unlikely. The consistency of the geochemical data with a crusta l anatectic origin of the granites, together with the observation that they intruded synchronously with Alleghanian thrusting in the Valley and Ridge Province, suggests strongly that they are collisional in ori gin. The crustal heating events that produced the granites might have been caused by delamination of mantle lithosphere during collision of Laurentia with Gondwana [1,2]. Alternatively, adequate heating may hav e occurred by crustal thickening during emplacement of thrust sheets [ 3].