Geochemical and Nd‒Sr‒Pb isotopic composition of Alleghanian granites of 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 granites that intrude the Carolina terrane, Eastern slate belt, Inner Piedmont, and Kiokee belt of the southern Appalachians. Major element compositions indicate that most plutons are high-potassium (K₂O ≥ 4%) metaluminous granites. Initial ε_Nd, depleted mantle model ages, and initial ⁸⁷Sr/⁸⁶Sr 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 for the Kiokee belt; -2.7 to +2.4, 660-870 Ma, and 0.70353-0.70816 for the Eastern slate belt; and -6.7 to +1.9, 690-1140 Ma, and 0.70391-0.70739 for the Carolina terrane. The chemical and isotopic data are most consistent with formation of the granites by anatexis of continental crust, rather than derivation from a depleted mantle source. Rocks of the Carolina terrane have ε_Nd(300 Ma) values and Pb isotopic ratios indistinguishable from those of most Alleghanian granites, suggesting it is a likely source. Some Alleghanian granites within the Carolina terrane require a more evolved source than exposed Carolina terrane rocks, suggesting that evolved crust, possibly Grenville basement, occurs structurally below or within the terrane. The chemical and isotopic 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 the Inner Piedmont have higher initial ⁸⁷Sr/⁸⁶Sr and lower ¹⁴³Nd/¹⁴⁴Nd ratios, indicating that their sources are isotopically more evolved. The virtual absence of Alleghanian plutons with mafic-intermediate compositions, and the lack of geographic trends in chemical and isotopic composition across the orogen, makes a subduction origin for the granites unlikely. The consistency of the geochemical data with a crustal 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 origin. 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 have occurred by crustal thickening during emplacement of thrust sheets [3].