Abstract

Cathedral Cliffs are located in the Heart Mountain Detachment of northwest Wyoming. The Heart Mountain Detachment is one of the largest documented subaerial rockslides. Cathedral Cliffs contain Paleozoic carbonate rocks and volcanics of the Tertiary Absaroka volcanic province that are both intruded by mafic dikes. Major element geochemistry indicates the dikes are mostly basaltic trachyandesite and genetically related with evidence of pyroxene and plagioclase fractionation. The δ¹⁸O(WR) of the dikes at Cathedral Cliffs ranges from 4.4–7.8‰ with an average of 6.30±0.71‰ (n=32). Samples collected from locations where the dike is hosted in the Tertiary volcanics tend to have lower δ¹⁸O(WR) than those samples hosted in the Paleozoic sedimentary rocks. There is no correlation of major element geochemistry with whole-rock δ¹⁸O suggesting the dikes experienced an alteration event affecting the oxygen isotope geochemistry without significantly changing major element geochemistry or petrography. This study documents that at Cathedral Cliffs, whole-rock δ¹⁸O values of mafic dikes sampled hundreds of meters above the detachment plane are heterogeneous and suggest both low and high δ¹⁸O fluids are needed to interact with the Eocene mafic dikes. The source of these fluids cannot be a single composition fluid migrating on the detachment plane but rather heated pore fluids derived from variable composition host rocks and meteoric waters. Heated pore fluids are also suggested to play a role initiating movement of the upper plate along the detachment surface.