FIELD AND GEOCHEMICAL COMPARISON OF THE WEBSTER-ADDIE AND BALSAM GAP ULTRAMAFIC BODIES, EASTERN BLUE RIDGE, NORTH CAROLINA

The Webster-Addie (WA) and Balsam Gap (BG) ultramafic bodies of the eastern Blue Ridge are different in outcrop pattern and lithologies; however, geochemical similarities suggest a possible common source. The WA body has a ring-like outcrop pattern of variable thickness and continuity and is interpreted as a structural dome. The smaller lenticular BG ultramafic body lies 6 km to the northeast. The ultramafic rocks are enclosed within migmatitic biotite gneisses and amphibolites of the Tallulah Falls Formation, although amphibolite is only locally in contact with the ultramafic units. The primary igneous mineralogy in both the WA and BG dunites is olivine plus chromite with minor enstatite. The WA dunite also contains pods, bands, and lenses of orthopyroxenite, clinopyroxenite, and websterite. Others contain both clino- and ortho-amphiboles. The ultramafic units have undergone multiple metamorphic episodes that produced hydrous mineral assemblages from the primary mineralogy such as serpentine, anthophyllite, talc, and tremolite. Recrystallization textures showing annealed olivine with 120° grain boundaries are common in both deposits.

WA and BG are characterized by very similar major and trace element geochemistry. DCP analyses show primarily MgO and SiO₂ with minor amounts of FeO and CrO. The slightly higher CaO and Al₂O₃ abundances in the WA rocks reflect its greater abundance of cpx. There are no clear geochemical or physical connections between the amphibolites and ultramafic rocks. The amphibolite compositions are inconsistent with interpretation as mafic cumulate compositions. Geochemistry of the ultramafic rocks is more suggestive of residual mantle assemblages (i.e., they have high Ni, Cr, and low Sr, Ba). The nearly identical major and trace element chemistries are consistent with a similar source for the two bodies. While our data for key trace elements is currently limited, flat to U-shaped REE patterns point to a more LREE-enriched source region. Such signatures are typical of metasomatized ultramafic rocks (Frey and McDonough 1988). The source of these metasomatizing fluids may be from the tonalites and pegmatites that cut through the ultramafic bodies.