Abstract Extreme extension along the Colorado River has exposed the shallow to mid-crustal Spirit Mountain batholith and the roots of the roughly coeval Secret Pass Canyon volcanic center. Examination of the Spirit Mountain batholith reveals evidence for multiple replenishment and rejuvenation over a two million year period (ca. 17.5−15.3 Ma), with extensive coarse cumulate granites and leucogranite (high-silica rhyolites) sheets, mafic-felsic mingling and mixing, and a major dike swarm. The roots of the possibly related Secret Pass Canyon volcanic center comprise a large, very shallow, composite laccolith and smaller dikes, sills, and a volcanic neck. The volcanic sequence was emplaced within about a one million year period (ca. 18.5–17.3 Ma) and includes volcanogenic sediments, ignimbrites, domes, and block-and-ash flow deposits. An appended road log serves as a geologic guide to this magmatic region.

Abstract Ernie Anderson, Lauren Wright, and Bennie Troxel were among the first to rec-ognize the coeval nature of magmatism and crustal extension in the Colorado River and Death Valley extensional terrains, respectively. The earliest magmatic rocks in the Colorado River extensional terrain include the 15.7 Ma Aztec Wash and 15.8-16.9 Ma Searchlight plutons as well as the 15.3-17.4 Ma Spirit Mountain batholith. The Aztec Wash and Searchlight plutons are well exposed, stratified plutons that show a similar range in composition from 45 to 75 wt% SiO 2 . Homogeneous granites that compose about one-third of each intrusion are nearly identical in texture, major- and trace-element geochemistry, and isotopic composition; mafic rocks that are present in both plutons document basaltic input into felsic magma chambers. Isotopic data suggest that mafic magmas were derived from enriched lithospheric mantle with minor crust-al contamination; the more felsic rocks crystallized from hybrid melts that represent mixtures of juvenile basaltic magma + crustal melt, or products of anatexis of ancient crust + young mafic magmas that underplate the crust during extension. Although emplacement generally proceeded peak east-west extension, the Aztec Wash pluton is in part coeval with extension in the Colorado River extensional terrain, as indicated by the uniform north-south orientation of late dikes and mafic injections into the magma chamber. The Spirit Mountain batholith is more homogeneous and felsic than the Aztec Wash and Searchlight plutons; granite composition varies from 66.6 to 76.4 wt% SiO 2 across the batholith. Geochemical and isotopic data suggest that granites within the Spirit Mountain batholith may be derived by the melting of K-feldspar-dominated crust that includes both ancient and juvenile rocks, or by other processes involving fresh mantle input and Proterozoic rocks deep in the Miocene crust. The oldest and youngest plutonic rocks coeval with crustal extension in the Death Valley extensional terrain are the 12.4 Ma granite of Kingston Peak and the 9.8 Ma Shoshone pluton, respectively. Both plutons are texturally zoned and are charac-terized by rapakivi textures. Chemical and isotopic data suggest that the granite of Kingston Peak represents a partial melt of Mesozoic plutonic rocks at mid-crustal levels; the juvenile isotopic composition of this granite as well as the Shoshone plu-ton probably reflects post-subduction hybridization of mantle-derived mafic magmas with the crustal melt. Keywords: magmatism, crustal extension, Colorado River, Death Valley.