VOLCANOLOGY OF THE NAVAJO LAKE VOLCANIC FIELD, SOUTHWESTERN UTAH

The Navajo Lake volcanic field (NLVF) extends from Navajo Lake (NL), just east of Cedar City, Utah to Panguitch Lake (PL). The volcanic field is characterized by numerous basalt flows that erupted from more than 28 cinder cones. Many of the cones are aligned in a SE – NW direction. Three stages of basalt flows are recognized by mineralogy and morphology. Additionally, andesite occurs in the PL area. Although precise age of flows is unknown (⁴⁰Ar/³⁹Ar dating is currently underway), many appear to be young (Recent). They are devoid of vegetation and have flow fronts 10 to 100 meters high. Flows erupted from the base and on the flanks of several cinder cones that are generally well covered by vegetation. In the PL area, the northernmost A’a flow is composed of a basal basalt flow and an upper andesite flow. These flows appear to have erupted from a common fissure system. The lava flow just north of Navajo Lake covers 15 km² and erupted from two vents on the west flank of a well vegetated cone. Aligned depressions in the western and central parts of NLVF probably reflect a network of lava tubes. Tertiary Brian Head and Claron formations and Isom formation pyroclastic flows underlie the NLVF. The volcanic rocks of the NLVF are calc-alkalic basalts and andesites. Stage 1, 2, and 3 basalts vary between 49 and 56% SiO₂, whereas andesite contains 57-61% SiO₂. Trace-element distribution diagrams show enrichment in Ba and Pb along with La and Ce compared to primitive mantle. These plots also show depletion in Nb and Ta suggesting a source in the lithospheric mantle. High Pb and K perhaps indicate crustal contamination. Initial ⁸⁷Sr/⁸⁶Sr of 13 samples are remarkably similar (0.704 +/- 0.0005). Tentatively, similar ⁸⁷Sr/⁸⁶Sr values may suggest that basalts and andesites of the NLVF are cogenetic.