REMAPPING SLICES OF THE DISMEMBERED OPHIOLITE IN THE BLUE RIDGE MOUNTAINS, SOUTHEAST US

The Ashe Metamorphic Suite (AMS) in the Blue Ridge Mountains of the southern Appalachians likely represents a sequence of dismembered ophiolites emplaced onto the Laurentian margin during the Taconic orogeny in the Ordovician. This suite is made up of mafic hornblende-rich amphibolites, pelitic schists, quartzofeldspathic schists and gneisses, and small (10 m to a few km) tabular-shaped bodies of meta-ultramafic rock. To better understand the emplacement and modification of these ophiolite fragments, we present mapping and petrology of the two largest ultramafic bodies in the AMS: the Todd and Edmonds bodies.

The largest discrepancy between previous mapping efforts and this study relates to the shape and dimensions of the Todd body. Rather than one large mapped body, the ultramafics are a series of smaller tabular shapes within a quartzofeldspathic gneiss. Mapping of mineral assemblages in the ultramafic rocks reveals compositional layering (alternating olivine-bearing and anthophyllite-bearing assemblages).

Unlike the ultramafic slices recognized as the Todd body, the Edmonds ultramafic rocks form a large (2 km by >10 km), coherent body. The Edmonds body also displays compositional layering, with bands of chlorite-tremolite rocks interlayered with wider zones of olivine-bearing assemblages. These likely represent pyroxenite or gabbroic layers within a peridotite. Superimposed on this layering is a north-south metamorphic gradient. The central olivine-rich zones grade into soapstones and serpentinites in the north. The southern margin is mainly chlorite schist.

While fluid flow is undoubtedly involved in the metamorphism of these rocks, the range of chemical compositions and mineral assemblages in the ultramafic and mafic units can be explained by a heterogeneous protolith. A combination of original layering and intense folding during orogeny is responsible for the current patterns of exposure. Due to the remapping of the Todd body, the Edmonds body is now the only piece of this dismembered ophiolite with a width > 1 km. Possible explanations for this lack of segmentation could include: 1) differences in the protolith composition or metamorphic conditions lead to a different response to stress; 2) the Edmonds body represents the core ophiolite slice, from which the other bodies have been sheared off.