ORIGIN OF TOURMALINE IN THE SETTERS FORMATION, MARYLAND: EVIDENCE FROM MAJOR AND TRACE ELEMENT, BORON ISOTOPE, AND RARE EARTH ELEMENT CHARACTERISTICS

Little research has been conducted to elucidate the origin of tourmaline within the Setters Formation of the Glenarm Series in the Maryland Piedmont. The Setters Formation is easily recognized by abundant tourmaline (up to 10 modal%) in domains defining, in part, the mica-rich foliation. The focus of this study has been to provide geochemical and petrologic evidence to gain an understanding of the conditions under which the tourmaline from the Woodstock Dome in the Setters Formation developed. Tourmaline is growing in popularity as tool to infer petrologic history. Several unique chemical and physical characteristics are responsible for its use as a single-crystal indicator of conditions of formation and geologic processes, including B source reservoirs, based on major element chemistry and boron isotope characteristics. Major- and minor-element chemistry of the tourmaline analyzed as part of this work is consistent with substitution primarily along the dravite-schorl solid-solution series. Individual grains are zoned with Fe/Mg ranging from 2.61 to 0.72 pafu from core to rim. Discontinuous sector zoning, and patchy patterned convolute zoning with discrete rims are common as evidenced by BSE imaging. High-resolution x-rays maps confirm this. Natural tourmaline samples have a ∂¹¹B range from -30‰ to +30‰: individual samples may retain information indicative of boron source reservoirs. Single collector LA-ICP-MS analysis was performed: tourmaline of the Setters Formation are isotopically heavy with a ∂¹¹B range of 4.21-14.24‰. LA-ICP-MS in situ measurements of trace elements provided a range of ∑REE from 0.31 to 24.97 ppm, where the relatively wide range may reflect distinct growth horizons within the evolution of the tourmalines. The REE display a distinct concave upward pattern with slight HREE enrichment, slight LREE depletion and a positive Eu anomaly. This is consistent with REE trends previously observed in tourmaline from metapelitic host rocks. The solid-solution series identified for the analyzed tourmaline, in addition to the range of ∂¹¹B values and REE characteristics, are consistent with the tourmaline developing during prograde metamorphism within a metapelitic host rock.