Understanding the origin and subsequent history of metamorphosed mafic and ultramafic rocks can provide critical clues elucidating the tectonic evolution of the southern Appalachians. The Lake Chatuge mafic-ultramafic complex (LCC), located in the Eastern Blue Ridge of NC-GA, has attracted attention because it has a wide range of compositions and textures representing magmatism and multiple generations of metamorphic crystallization and recrystallization, including an episode of high-P metamorphism; like other mafic complexes, its history is controversial. It has been interpreted to have crystallized (1) in situ as a sill (Hartley, 1973); (2) ~40 km deep, near the base of the continental crust (Meen, 1988); or (3) in primitive arc or MORB crust (Long & Miller, 1983; Shaw & Wasserburg, 1984). (2) and (3) imply tectonic emplacement.

We have tested deep vs. shallow crystallization models by application of the MELTS program (Ghiorso & Sack, 1995). Well preserved magmatic textures in gabbros and troctolites reveal the crystallization sequence olivine-plagioclase-cpx, and new and previously reported elemental data indicate a picritic magma (~45 wt% SiO₂). MELTS demonstrates that this crystallization sequence can only be achieved at P<5 kb; high-P crystallization suppresses olivine + plagioclase and favors early cpx (+/- garnet).

Elemental data demonstrate that the LCC magma was derived from a MORB-like source. REE patterns reveal LREE depletion, incompatible element concentrations are very low (e.g. K₂O <0.1 wt%, Rb ~1-3 ppm), and high field strength elements (e.g. Nb, Ta, Ti) are relatively undepleted. Isotopic compositions also indicate long-term depletion in the source region (e.g. Thomas et al., this volume). These characteristics are incompatible with a continental or within-arc environment and appear to require a back arc basin or mid-ocean ridge setting.

We conclude that the LCC (1) formed as a shallow intrusion in an oceanic setting; (2) was underthrust/subducted and emplaced into a continent-derived lithologic assemblage (with partial recrystallization to high-P granulite facies); and (3) was locally deformed and retrogressively metamorphosed to amphibolite facies.