PARAGENESIS OF TOURMALINES IN EARLY PALEOZOIC METASEDIMENTS OF THE MARYLAND PIEDMONT

The composition and origin of tourmalines in the amphibolite-grade metasediments of Maryland Piedmont are poorly understood. Here we present results that suggest a complex paragenetic history for this accessory phase. Tourmalines from three units within the Glen Arm Supergroup and associated granitoid intrusions outside of Baltimore, Maryland, were sampled. The basal Setters Formation contains distinctive black tourmalines up to several centimeters in length; long axes lie within the metamorphic foliation but c-axes do not form a lineation. These tourmalines are strongly pleochroic from light yellow to olive-green. Cores of prismatic sections contain fine, acicular inclusions parallel to the tourmalines' c-axes. Tourmalines of the Cockeysville Marble are very weakly pleochroic in pale yellow; most are subhedral to anhedral with irregular patterns of fine to coarse inclusions.

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.