The McCullough Range preserves a unique record of Miocene volcanism in the western Lake Mead area of Nevada. The basal part of the volcanic section is composed of interbedded basalt and dacite of the McClanahan Spring, Cactus Hill, and McCullough Wash volcanoes (Eldorado Valley volcanic section), and the Colony volcano, which is age-equivalent to, but does not crop out within, the Eldorado Valley volcanic section (18.5–15.2 Ma). These units lie on Precambrian basement and locally on the Peach Springs Tuff (18.5 Ma). Over 400 m of andesite lava, agglomerate, and breccia of the Farmer Canyon volcanic section forms the McCullough stratovolcano. Eruptions occurring after 15.2 Ma were lower in volume and are mainly present on the flanks of the McCullough stratovolcano. These include the eruption of (1) the McCullough Pass caldera and outflow tuff (14.1 Ma), (2) Hidden Valley andesite, including 300 m of andesite lavas erupted from local centers (mainly cinder cones), (3) four Sloan volcanoes on the west flank of the McCullough stratovolcano (Mount Ian, Mount Sutor, Center Mountain, and Mount Hanna) (13.1 Ma), and (4) the Hender son dome complex on the northern flank of the McCullough stratovolcano. The volcanic rocks in the McCullough Range are calc-alkaline and vary in composition from rhyolite to basalt. Intermediate compositions (andesite and dacite) prevail, while basalt and rhyolite are rare. The trace-element signature (low Nb, Ti, Zr, and P compared to primitive mantle) is an indication of either a magma source in the continental lithosphere or lithospheric contamination. Rhyolite and dacite probably formed by partial melting of crust, while mafic magmas (basalt and andesite) either originated by melting of lithospheric mantle or reflect asthenospheric magmas contaminated in the lithosphere.

Abstract This field trip will visit the River Mountains volcanic section (12.17 ± 0.02 to 13.45 ± 0.02 Ma) and Wilson Ridge pluton (13.10 ± 0.11 Ma) in southern Nevada and northwestern Arizona. Although this volcanic-plutonic system was disrupted by the Saddle Island detachment fault during Miocene crustal extension, there are convincing lithological, mineralogical, geochemical and geochronological indicators that suggest a cogenetic relationship. The trip consists of 17 stops that emphasize evidence that links the volcanic and plutonic sections. In addition we will visit the Saddle Island detachment fault at its type locality on Saddle Island.