THE GEOCHEMISTRY AND PETROLOGY OF THE BALD HILL BENTONITES IN SOUTHWESTERN PENNSYLVANIA

The early Devonian (middle Lochkovian) Bald Hill Bentonites (BHBs) of central Appalachia have been examined through field, geochemical, and petrographic analyses to determine their tectonomagmatic setting. To the best of our knowledge, minimal trace element geochemistry exists on these K-bentonites; obtaining trace element geochemistry allowed for the testing of petrogenetic models/hypotheses to determine the type-eruptive source of the bentonites. Field work was undertaken at the New Paris Limestone Quarry in southwestern Pennsylvania, where three BHB units (BHB-B, BHB-B’, and BHB-C; respectively, oldest-to-youngest) outcropped in mined sections of the Helderberg group. The BHB-B unit was observed to be ~1 ft thick and consists of basal calcite nodules, a stratified section, and an upper arkosic layer. A previously unidentified BHB unit (BHB-B’) is situated between other known BHB units (BHB-B and BHB-C) and has been examined during this study. Both BHB-B’ and BHB-C were observed to be ~3 in thick and consisted of only a stratified section. Representative samples were collected and submitted for bulk whole rock geochemical analyses for major, minor, and trace elements as well as thin-section production. Geochemical results were examined on multi-element variation (spider) diagrams, petrologic rock description plots, and petrogenetic discrimination diagrams. Major element data exhibit a shift in composition from felsic to mafic as the bentonite units get younger (from trachyandesite-andesite to foiditic), suggesting that progressive eruptions became increasingly more silica undersaturated. Rare Earth Element (REE) geochemistry plotted with respect to chondrite reveals a slight HREE enrichment, creating an overall broad U-shaped trend which could be attributed to secondary hydrothermal alteration. Trace element data plotted on petrogenetic discrimination diagrams supports that the magma composition likely originated from a volcanic island arc with some data plotting in the collisional source field. The timing and geochemistry support an association with early Acadian volcanic island arcs colliding with the eastern margin of North America and could provide insight on the tectonomagmatic dynamics of this orogenic event.