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.

Middle Tertiary volcanic rocks of the Lake Mead field are calc-alkalic to alkalic-calcic and vary continuously in composition from basalt to rhyolite. These volcanic rocks formed during Basin-and-Range extension and are spatially and genetically associated with diorite-to-granite intrusions of the Wilson Ridge pluton. Locally, igneous rocks were subjected to potassium metasomatism. Field relations and petrography provide evidence of disequilibrium mineral assemblages and liquid-liquid mixing of basalt and granite magmas to form the intermediate rock types of the Lake Mead volcanic field. Evidence of mixing includes incompatible phase assemblages of euhedral olivine, embayed quartz, and sodic plagioclase within andesite flows. Plagioclase occurs in rounded and partially resorbed clusters of equant crystals and commonly displays oscillatory zoning and outer glass-charged zones (fretted texture). Quartz phenocrysts are commonly surrounded by rims of prismatic augite and glass. Fine-grained spheroidal to ellipsoidal inclusions of basalt are common in dacite flows and dikes. Thus, various mixing ratios of olivine basalt and granite end members may be responsible for the textural variations observed in volcanic rocks of the Lake Mead field. The evolution of the igneous rocks of the Lake Mead field was evaluated by petrogenetic models involving both crystal fractionation and magma mixing. These processes may have operated together to produce the compositional range in volcanic and plutonic rocks of the Lake Mead area. Open-system models provide estimates of the relative importance of the two processes and suggest that mixing was more important in the derivation of andesite and diorite (mass mixed component/mass crystallizing phase R = 0.8 to 2.2) than dacite, quartz monzonite, or granite (R = 0.1 to 0.65). The calc-alkaline-alkalic-calcic nature of the igneous rocks of the Lake Mead field, and possibly other similar rock suites that formed in the Great Basin during regional extension, may result from magma mixing. Basalts and rhyolites may mechanically mix in various proportions to produce intermediate rock types. The classic bimodal assemblages may only occur where mixing is incomplete or in structural situations where different magma types cannot mix.

A group of plutons were emplaced in the western Klamath Mountains province during the waning stages of the Late Jurassic Nevadan orogeny. Published U-Pb (zircon) ages indicate that the “western Klamath plutonic suite” was emplaced in the age range of 151–144 Ma. Crosscutting relationships, development of contact metamorphic aureoles, and the presence of distinctive inherited zircon populations indicate that the magmas intruded the footwall and hanging-wall rocks of the principal Nevadan thrust fault. The plutons are chiefly gabbroic to dioritic in composition, but commonly include ultramafic rocks and contain smaller volumes of tonalite and granodiorite. Hornblende is the most common mafic phase, except for some ultramafic rocks in which clinopyroxene ± olivine are locally distinctive, the two-pyroxene dioritic to monzodioritic rocks of the Buck Lake unit of the Bear Mountain pluton, and the most felsic rocks in which biotite is the most abundant mafic phase. Compositions of fine-grained mafic dikes suggest the presence of two principal parental, H 2 O-rich magmas: primitive basalt and evolved basalt/basaltic andesite. The former was parental to the ultramafic rocks of this suite. It was also parental to the basalt/basaltic andesite magmas by deep-seated fractional crystallization processes. The latter magmas were parental to the gabbroic and dioritic units. Many of the felsic rocks show evidence of origins by partial melting of metabasaltic crustal rocks, particularly their low heavy rare-earth element concentrations and high Sr/Y ratios. Mixing of crustal melts with primitive basaltic magmas was locally important (e.g., Pony Peak pluton). The mafic parental magmas show trace element features typical of an origin by partial melting of a subduction-modified mantle wedge. It is unclear whether subduction was coeval with western Klamath magmatism or whether the subduction signature developed as the result of Middle Jurassic subduction.