The Underdown Caldera Complex is a 25.0 Ma caldera in the Western Nevada Volcanic Field formed during the mid-Cenozoic Ignimbrite Flare-up, one of the largest expressions of continental volcanism in geological time. Intracaldera and associated outflow tuffs are characteristically crystal-poor, except for one crystal-rich intracaldera tuff representing a later stage of explosive volcanism. Outflow tuffs extend to the east and west of the caldera, representing multiple phases of intracaldera tuffs. Post-deposition alteration of ignimbrites has altered Sr isotopic ratios, and caused rare earth element enrichment in the Tuff of Clipper Gap. Sr and Nd isotopic compositions of the ignimbrites show a relation to lavas collected from the margins of the caldera, and trace element ratios suggest similar origins of the lavas and ignimbrites from a mixture of mantle and crustal sources. Pb isotopes plot on mixing lines formed by a mantle-derived source and either a local sedimentary or granitoid unit.