Tertiary rocks at Alvord Mountain record an important part of the Miocene volcanic record associated with large-magnitude crustal extension. Sparse age data and field relations indicate that the volcanic rocks are syn- to post-extensional. Syn-extensional volcanism initiated with emplacement of thin (<3m) tuffs found locally near the top of the early Miocene Clews formation. The tuffs are overlain conformably by a section of intermediate lava flows of variable thickness. The Spanish Canyon Formation, comprising 5-6m of lacustrine tuffs and sandstones capped by 2 thin (<5m) basalt flows, rests conformably on the intermediate lava flows, and locally contains the 18.5 Ma Peach Springs Tuff. Post-extensional, predominantly sedimentary rocks of the Barstow Formation overlie the Spanish Canyon Formation, but at least 2 basalts are found within the Barstow Formation, over 100 meters above its base.

The lower section of intermediate flows are sparsely plagioclase-phyric and alkalic, ranging from basaltic andesite (59 wt.% SiO2) to trachydacite (65 wt.% SiO2). Lavas of the Spanish Canyon and Barstow formations are olivine-phyric basalts (<49 wt% SiO2) and have remarkably similar petrographic and geochemical characteristics. All Alvord Mountain volcanic rocks show modest LREE enrichment but LREE enrichment is >100x chondrite in the intermediate flows compared with 50x chondrite in the Spanish Canyon and Barstow Formation basalts. eNd(t) for the intermediate flows ranges from –2.5 to +3. eNd(t) for the Spanish Canyon and Barstow basalts are identical within error (+5).

In contrast to many Miocene volcanic rocks in the Mojave Desert, Alvord Mountain volcanic rocks lack petrographic evidence of contamination, and eNd(t) correlates weakly with SiO2. High eNd(t) values for the mafic lavas indicate that the mantle source of the lavas during and after extension was young (asthenospheric?) mantle. Trace element signatures show OIB-type enrichment of LREEs, similar to late Cenozoic alkali trachybasalts from Cima volcanic field, approximately 65 km east of Alvord Mountain. High eNd(t) values (>0) for the most felsic rocks (trachydacites) in the lower section of intermediate flows indicate that these rocks also have a large fraction of young mantle component.