PETROLOGY OF THE SHOULDERBONE ULTRAMAFIC BODY, HANCOCK COUNTY, GA

The "Shoulderbone" body (33o20'35"N, 83o05'10"W) is a small (150 x 500 m), lens shaped ultramafic body embedded in the biotite gneisses of the eastern-most Piedmont. The mineralogy of this body is complex and includes remnants of a primary igneous mineral, olivine, as well as metamorphic minerals (anthophyllite, zoned calcic amphiboles [pargasite to actinolite], chlorite) that record a decrease in metamorphic grade with mineral growth. The rock unit is heterogeneous and includes fine-grained to coarse-grained chlorite- and amphibole-rich lithologies. One of the more striking textures is randomly oriented, cm-sized intergrowths of amphibole and oxide which we interpret to be psuedomorphs after igneous pyroxene. One sample contains corroded remnants of relatively Fe-rich olivine (Fo66-67). A published bulk rock analysis shows that this body is relatively Fe-, Ca- and Al-rich compared to many of the better known dunitic or serpentinized ultramafics of the Appalachian belt and the protolith for the Shoulderbone body was probably pyroxene-rich. Zoned calcic amphiboles record two metamorphic events. Relatively aluminous cores reflect amphibolite grade metamorphism and relatively thin actinolitic rims attest to lower grade conditions. This sequence of metamorphic events may record decreasing pressure and temperature in the evolving mountain belt . The primary igneous lithology probably reflects accumulation of magmatic crystals from an intermediate composition magma in an arc environment.