ULTRAMAFIC ROCKS IN NORTH CAROLINA: USING SPINEL-GROUP MINERAL CHEMISTRY TO UNDERSTAND THEIR TECTONIC SIGNIFICANCE

The Appalachian and Caledonian mountains, which extend from Norway in the north to Alabama in the south, record the closure of the Iapetus Ocean during the early Paleozoic. Ultramafic rocks (igneous rocks containing low SiO₂ contents) occur within the Appalachian-Caledonian system and are a major constituent of the lowermost portions of ophiolites, which are fragments of oceanic lithosphere. Understanding the origin of ultramafic rocks in mountain belts may provide insight into the formation and evolution of convergent margins. Our research compares ultramafic rocks from opposite ends of the Appalachian-Caledonian system: the Leka Ophiolite Complex (LOC) in Norway; and the Buck Creek and Webster-Addie bodies in North Carolina. We present the results of petrographic observations and spinel-group mineral chemistry, with the aim of understanding the tectonic significance of the North Carolina ultramafic rocks. Spinel grains from the Leka samples showed variation in two key chemical proxies: Fe²⁺ (Fe²⁺/[Fe²⁺+Mg]) and Fe³⁺ (Fe³⁺/[Fe³⁺ + Al + Cr]), with Fe²⁺–numbers ranging from 0.49-0.99 and Fe³⁺–numbers from 0.05-0.09 Fe³⁺. The low Fe²⁺ and Fe³⁺ values represent primary compositions and correspond to spinel grain cores, while the increasing values reflect progressive alteration. North Carolina samples have Fe²⁺ values ranging from 0.48-0.94 and Fe³⁺ values ranging from 0.05-0.43. All North Carolina data overlap with secondary compositions analyzed from Leka, suggesting that the samples have undergone heavy metamorphism. Comparison of resulting chemical data and use of Cr # (molar Cr/([Cr+Al]) and TiO₂ (wt. %) yield two possible conclusions: (1) The North Carolina samples formed as cumulates in a continental magma chamber; or (2) the North Carolina samples indicate a trend of alteration undergone by ophiolites.