ORIGINS OF MAFIC-ULTRAMAFIC COMPLEXES OF THE EASTERN BLUE RIDGE, SOUTHERN APPALACHIANS: GEOCHRONOLOGIC AND GEOCHEMICAL CONSTRAINTS

Abundant mafic-ultramafic complexes (MUMC) of the Eastern Blue Ridge are interpreted to represent in situ intrusions and/or allocthonous tectonic fragments of either arc or oceanic crust. Deciphering ages and origins of MUMC is critical to identifying accreted terranes and their boundaries and understanding assembly of the southern Appalachians.

Major MUMC appear to be broadly divisible into two groups: Type I record high-P, high-T metamorphism (to granulite and eclogite facies) and commonly preserve coarse plutonic textures (e.g., Lake Chatuge, NC-GA; Buck Creek, NC; Bakersville eclogite, NC); Type II are mostly amphibolite with little evidence for high-P metamorphism or plutonic textures (e.g., Lake Burton, Dahlonega Gold Belt, GA; Hillabee Greenstone, AL). Ion probe studies of zircon from interlayered felsic units indicate Type II metavolcanics are ~470 Ma (Hillabee: 470±4 Ma, McClellan and Miller, 2000; Lake Burton: 468±6 Ma, this study). Grenville-age inherited zircons from Lake Burton suggest proximity to a Laurentia-like landmass during magmatism. Magmatic ages of Type I MUMC have not been undetermined, but B. Miller et al. (2000) dated Bakersville eclogite facies zircon growth at 460 Ma. Type I MUMC compositions are similar to MORB, with very low incompatible element concentrations, no relative depletion in Ti, Ta, Nb, and other HFSE, LREE-depleted rare earth patterns, and (for Lake Chatuge) high eNd (+6) and low (0.7028) ⁸⁷Sr/⁸⁶Sr (Long, 1984; Slusser et al, 1998; B. Miller et al., 2000; Shaw and Wasserburg, 1984; this study). Type II MUMC are geochemically similar to arc magmas, with strong depletion in HFSE (Tull and Stow, 1980; Hopson, 1989; this study).

Existing data suggest that Type I MUMC formed in oceanic environments (MOR or back arc basin) and were transported to a continent margin, then subducted or deeply underthrust during mid-Ordovician (Taconian) orogenesis. We are attempting to determine magmatic ages of this oceanic crust by further ion probe studies. Type II MUMC apparently represent Taconian arc(s) that formed at/near the margin of Laurentia before subsequent accretion.