CHARACTERIZING THE UNSORTED DACITE UNITS OF YELLOWSTONE: IMPLICATIONS FOR AN EARLIER VOLCANIC PAST

The Greater Yellowstone Region (GYR) is often recognized for its periods of Eocene and Quaternary volcanism, with the earliest substantial volcanic activity beginning with the Washburn group of the Absaroka Volcanic Supergroup (AVS), approximately 53 Mya. However, dacite units appearing throughout the GYR have been variably mapped since the 1970’s, with a few maps referring to select dacite units as “Cretaceous” and most maps describing the units as “Tertiary.” These age disagreements have yet to be clarified in earlier studies, where diverse dating techniques spread over several decades on select samples have led to inconsistent results on which dacite bodies fall under Cretaceous or Tertiary ages. In addition, the descriptive names for these rocks differ greatly, from “dacite porphyry”, to “quartz latite”, to “dacite (?) intrusive” and more. This study aims to better characterize the “dacites” in the northwestern GYR using textural analysis, whole rock geochemistry and U-Pb dating on zircon. Twelve “dacites” were sampled from the GYR and three of those samples were sent for dating. While two samples returned early Eocene dates (49 and 48 Ma), the third sample returned a Late Cretaceous date, 74 Ma, providing concrete evidence of an earlier phase of volcanism in the region. Geochemically, the twelve “dacite” samples range from basaltic trachyandesite to dacite, with the basaltic trachyandesite representing the Cretaceous date. However, when compared against regional geochemistry of both Cretaceous and Paleogene igneous rocks, the trace element signatures (e.g., Ba, Rb, Zr) of all twelve “dacites” consistently plot within the AVS Washburn group geochemical field. Texturally, the “dacites” seem have been intruded as shallow sills, evidenced by high modal percentages of large crystals and lack of glass, though a few samples do seem to have been eruptive flows. The mix of age, geochemistry and texture indicate that the Cretaceous basaltic trachyandesite may have been an earlier product of the Washburn group or might represent a completely separate volcanic period in this region. The presence of Cretaceous volcanic rocks in the GYR challenges our current geologic knowledge of the area and further work is required to place the older volcanic presence into the framework of regional tectonics and volcanism of the period.