THERMAL EVOLUTION OF ULTRAMAFIC BODIES IN THE EASTERN BLUE RIDGE MOUNTAINS, NC

Microstructural evidence of the timing and conditions of metamorphism can lead to new understanding of orogeny and its role in the emplacement and modification of ophiolites and other components. The Ashe Metamorphic Suite (AMS) of the eastern Blue Ridge Mountains of North Carolina contains bodies of mafic and ultramafic rock that underwent varying grades of metamorphism during and after the multiple orogenies. In our broader study of the area, we sampled and analyzed the petrography and geochemistry of multiple ultramafic bodies from the Blue Ridge Mountains north of Boone, NC to determine their metamorphic history.

Here, we examine 30 samples collected from the two largest ultramafic bodies in the AMS, the Edmonds and Todd bodies. Although these bodies were originally interpreted as two distinctive “type localities” of mineral assemblages, we find no major mineralogical distinction between these bodies. Ten of the Edmonds samples contain the assemblage antigorite, tremolite, forsterite, and chlorite, indicating a temperature range of 400-600°C in the lower amphibolite facies. Six Edmonds and four Todd samples contain the assemblage tremolite, forsterite, and talc, similarly indicating 400-650°C. One Edmonds and three Todd samples contain the assemblage chlorite, anthophyllite, tremolite, while one Todd sample contains chlorite, anthophyllite, tremolite, and forsterite, indicating a minimum temperature constraint of 650-700°C in the upper amphibolite facies. Most higher-grade assemblages also contain retrograde antigorite.

Many ultramafic bodies in the AMS have a strong foliation defined by tremolite, chlorite and sometimes anthophyllite. Some elongate amphibole crystals crosscut the foliation and porphyroblasts of tremolite, which contain pseudomorphs of amphibole inclusions that are at a high angle to the foliation, indicating multiple stages of growth of tremolite and anthophyllite. Many chlorite-rich samples display crenulations; tremolite in these samples is parallel to foliation and sometimes forms polygonal arcs with respect to the crenulations. These differing temperature constraints and microstructural relationships indicate that at least some of the deformation occurred in the amphibolite facies and that peak metamorphism was reached during and after the deformation event.