WESTERN BLUE RIDGE BASEMENT OF NORTHEAST TN AND NORTHWESTERN NC: AGE, GEOCHEMISTRY, AND POSSIBLE RELATIONSHIPS TO PROTEROZOIC ROCKS OF THE SOUTHEASTERN USA

The extensive Proterozoic basement of the Western Blue Ridge (WBR) has been widely recognized, but its age and elemental and isotopic characteristics, and hence its relationships to other terranes, have remained poorly characterized. Our ion probe U-Pb age dating and geochemical reconnaissance studies in the WBR between I-40 and Johnson City constrain possible tectonic linkages among ancient rocks. The pre-Neoproterozoic basement of the WBR is almost entirely composed of granitoids and granitoid gneisses. Zircons from these rocks are almost entirely magmatic - we have identified only a single inherited core (1.2 Ga) in 9 samples, and only a single sample displayed clear evidence for any metamorphic overgrowths (1.0 Ga). Magmatic ages define two major episodes: 1050-1080 Ma (4 samples, including Max Patch granite) and 1.14-1.19 Ga (4 samples). In addition, a single gneiss from the Fork Ridge thrust sheet on US 23 yielded a crystallization age of 1.36 Ga, to our knowledge the first reported age between 1.3 and 1.6 Ga in the southern Appalachians. Although uniformly granitic, the WBR samples are quite variable in composition; SiO2 ranges from 59-74 wt% and REE patterns and incompatible element concentrations span a wide range. Granitoids from the 1.14-1.19 Ga pulse have initial e_Nd of –1 to +1 and Nd T_DM ages of 1.5-1.8 Ga, but the only analyzed younger granitoid (1.08 Ga Blowing Rock gneiss) has e_Nd of +3 and a model age of 1.3 Ga.

The WBR is structurally overlain to the SE by the enigmatic Mars Hill Terrane (MHT), which is in turn overlain by the Eastern Blue Ridge (EBR). The EBR is broadly similar in composition to the WBR and shares ~1.16 Ga magmatism and similar T_DM, but it is somewhat less variable compositionally and appears to lack the ~1.07 and ~1.4 Ga magmatic episodes (Carrigan et al., in press). The MHT is older (magmatic ages of 1.2-1.3 Ga and 1.6-1.8 Ga, T_DM ~2.0 Ga; Ownby et al., in review) and extremely diverse in composition. Zircons from both the MHT and EBR have common 1.03 Ga metamorphic overgrowths. The WBR thus experienced a significantly different early history than the other components of the southern Appalachian basement. It also differs from the likely adjacent parts of Laurentia to the northwest, the juvenile ~1.5 Ga crust of the mid-continent and the ~1.3 Ga crust of the Grenville belt.