PARAGENESIS OF TOURMALINES IN EARLY PALEOZOIC METASEDIMENTS OF THE MARYLAND PIEDMONT
Based on geochemical results from microprobe analyses, tourmalines from the quartzite and schist members of the Setters Formation and associated granitoid intrusions are predominatly schorl (Fe/Mg>1 and Ca/Na<1) with lesser amounts of feruvite (Fe/Mg>1 and Ca/Na>1). Tourmaline compositions within the Cockeysville Marble range from dravite (Fe/Mg<1 and Ca/Na<1) to uvite (Fe/Mg<1 and Ca/Na>1). Most of the analyzed tourmaline grains exhibit a range of compositional complexities. The most common variability in grains from the Setters Formation is core-to-rim compositional zoning, typically changing from schorl (core) to feruvite (rim). In tourmaline grains from the Cockeysville Marble, compositional zoning is more complex and many grains contain domains of both dravite and uvite.
These changes in composition, particularly the core-to-rim variations recorded in the Setters Formation tourmalines, coupled with petrographic observations, suggest changing fluid composition and possibly multiple stages of growth for many of these tourmalines. The origin of the fluids from which the tourmalines crystallized is still an open-ended question. However, our results suggest that tourmaline composition is strongly dependent on the chemical composition of the host rock. This requires a significant host-rock derived component for the formational fluids.