A model for the origin of rhyolites from South Mountain, Pennsylvania; implications for rhyolites associated with large igneous provinces

High-silica rhyolites, ubiquitous features of continental volcanism, continue to evoke controversy as to their petrogenesis and evolution. We utilized the geochemical characteristics of late Vendian high-silica rhyolites erupted in the Catoctin Volcanic Province at South Mountain in Pennsylvania to probe the origin of the parental magmas and assess heterogeneities in the subsequent fractionation paths. We identified high- and low-Ti signatures within the South Mountain rhyolites, a common feature in many large igneous provinces, and these signatures are suggestive of a genetic link between basalts and rhyolites erupted in the Catoctin Volcanic Province. Two evolutionary trends are superimposed on the Ti-based subdivisions that reflect variable control of plagioclase and amphibole in the fractionating assemblage of the South Mountain rhyolites. Such distinctive evolutionary trends are evident in rhyolites from other tectonic settings (e.g., arcs), where they have been interpreted in terms of cold-wet and hot-dry conditions within the differentiating magmas. We interpret the amphibole-dominated fractionation path of the South Mountain rhyolites as following a cold-wet fractionation path compared to the hot-dry plagioclase-dominated trends. This study, which examines the geochemical implications of cryptic amphibole fractionation, has implications for assessing the role of amphibole and volatile content in the development of rhyolites in other large igneous provinces.