Spencer High Point, a Unique Mafic Vent Field Characterized by Two Distinct Types of Basalt on the Eastern Snake River Plain, Idaho

Two types of basalts are found within the Spencer High Point volcanic field (SHP), a ~1700 sq km region of Pleistocene mafic vents and lava flows on the eastern Snake River Plain (ESRP), Idaho. Type 1 basalt is olivine tholeiite typical of the ESRP, whereas, type 2 is chemically evolved basalt similar to that found at Craters of the Moon, Idaho. Type 1 basalts are olivine and plagioclase phyric, diktytaxitic and have MgO = 6.1-10.7, TiO₂ = 1.3-3.6, and FeO(t) = 9.5-14.9 wt%; and Zr = 122-479, Ba = 253-848 and Cr = 35-551 ppm. Type 2 basalts are glassy, with olivine and resorbed plagioclase phenocrysts, and often contain crustal xenoliths. Chemically, type 2 basalts have MgO = 2.7-4.2, TiO₂ = 2.0-3.3, and FeO(t) = 10.7-14.2 wt%; and Zr = 680-1160, Ba = 1250-1940 and Cr = 2-18 ppm. These compositional differences are reflected in two different overall eruptive styles and geomorphology. Type 1 vents have shallow slopes and spatter ramparts, and produce tube-fed lava flows reaching 10-20 km. Type 2 vents are topographically higher features with steep slopes; and characterized by cinder cones and short, low-volume lava flows. Vents and surficial alignments over the entire field, evident in aerial photos and DEMs, are roughly perpendicular to the Basin and Range and Rocky Mountains fold and thrust belt on the northern margin of the field area. However, eruptive fissure alignments are independent of geochemical types, and both types can be observed in a single fissure system. The existence of two independent geochemical and eruptive types within the same eruptive cycle make the SHP field a unique system on the ESRP in which to study the evolution of small vent fields.