LAKE CHATUGE REVISITED: CRYSTALLIZATION IN A SHALLOW OCEANIC MAGMA CHAMBER

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.