PROVENANCE OF MIDDLE JURASSIC HIGH-K RHYOLITE VOLCANIC CLASTS FROM THE TERTIARY RIM GRAVELS, COLORADO PLATEAU, ARIZONA

The Paleogene “rim gravels” of northern Arizona are part of a northeast-flowing drainage system that was eventually succeeded by the southwest-flowing Colorado River around 5 million years ago. Understanding the stratigraphy, sedimentology and provenance of the rim gravels is important to understanding the ~2 kilometers of uplift of the Colorado Plateau and reorganization of drainage patterns since the Late Cretaceous. The rim gravels consist of exotic arkosic sediments derived from Laramide highlands that formerly bordered the southern and western margin of the Colorado Plateau. Three rhyolite volcanic clasts collected from a volcanic-clast-rich conglomerate facies in the fluvial sequence near Long Point (Music Mountain Formation) yield indistinguishable Middle Jurassic zircon U-Pb ages (160.4±4 Ma, 161.3±3 Ma, and 163.0±3 Ma). These Middle Jurassic ages are significantly older than previously reported K-Ar volcanic clast dates from the rim gravels that ranged from 117-65 Ma. The Middle Jurassic volcanic clasts are all ultrahigh-K rhyolites based on whole rock X-ray fluorescence analysis (K₂O ranging from ~9 to 12 wt %) with Na₂O contents <1wt% suggesting potassium metasomatism resulting in the enrichment of potassium and depletion of sodium. The early Mesozoic continental arc that borders the Colorado Plateau to the south and west contains an abundance of Middle Jurassic rhyolite and represent a potential source for the rim gravel volcanic clasts. However published rhyolite compositions from the continental arc have K₂O contents more typically in the range of 3-7 wt% K₂O. Further work is needed to determine the extent of the Middle Jurassic high-K volcanic rocks in the rim gravels and possible provenance linkages.