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New chemical and isotopic data demonstrate that Pliocene volcanic rocks in the Kings and San Joaquin volcanic fields in the central Sierra Nevada, California, are more potassic, trend toward more mafic compositions, and have distinctly lower epsilon(Nd) values than Miocene volcanic rocks found anywhere in the southern half of the mountain range. The Pliocene magmatism apparently tapped a low-epsilon(Nd) (-6 to -8), K-metasomatized mantle not involved in Sierran magmatism at any other time in the Cenozoic. Published isotopic data from upper-mantle xenoliths entrained in Miocene and Pliocene volcanic rocks reveal that the only low-epsilon(Nd) mantle in the lithospheric column beneath the central Sierra prior to the Pliocene magmatism was shallow-level, K-metasomatized, spinel peridotite. Melting at such shallow levels in the mantle lithosphere could have been triggered by delamination of deeper parts of the mantle lithosphere, followed by uplift of the remaining continental lithosphere and its heating by upwelling asthenosphere.