VOLCANOLOGY OF THE SANTA CLARA AND DIAMOND VALLEY LAVA FLOWS, SNOW CANYON STATE PARK, UTAH

The Santa Clara and Diamond Valley tholeiitic basalt flows (2 to 20 ka) are the youngest units of the Pine Valley volcanic field near St. George, Utah. This field has been active since 2.43 Ma, and is an example of extension-related mafic volcanism on the eastern margin of the Basin and Range province. The Santa Clara flow originated from the Snow Canyon cone (SCC) located north of Snow Canyon State Park. The history of the SCC is complex and includes both Hawaiian and Strombolian eruptions from at least three vents. Two of these vents are located along a dike south of the main cone, and one is at the base of the cone. The main cone is primarily composed of scoria but a small spatter cone is located on its south flank. Large rafted blocks of massive basalt representing remnants of a lava lake are adjacent to the north flank of the cone. The Santa Clara flow cascaded into the upper reaches of Snow Canyon. Within the canyon, the flow split six times, weaving around sandstone ridges, before combining to form two large lobes near the town of Ivins, Utah. The flow is A'a in character, and includes at least two flow units. Lava tubes were important in the development of the flow; one such tube is approximately 5 meters in diameter and at least 500 meters in length. The Diamond Valley cone (DVC) is located approximately 1200 meters north of the SCC. The eruptive history of the DVC is characterized by Strombolian eruptions from at least two vents. The Diamond Valley flow originated from the DVC and was previously considered part of the Santa Clara flow. This flow is volumetrically minor and is confined to Diamond Valley. It is an A'a flow and characterized by small lava tubes and large pressure ridges. Despite the complex volcanic history, the Santa Clara and Diamond Valley flows are relatively uniform in chemistry, showing little evidence for differentiation. SiO₂ and MgO variation is less than 1.8 wt. %, while FeO varies less than 0.7 wt. %. Trace element compositions and isotopic ratios are remarkably consistent; Yb values vary by 0.11 ppm, Th by 0.51 ppm, Nd by 1.52 ppm, Rb by 2.8 ppm, ⁸⁷Sr/⁸⁶Sr values vary by 1.61E-4, while epsilon Nd varies by 0.72. Average isotope values of 0.7045 for ⁸⁷Sr/⁸⁶Sr and -3.4 for epsilon Nd suggest involvement of both lithospheric and asthenospheric mantle in the petrogenesis of the magma.