Lower crustal and upper mantle xenoliths from the Sierra Nevada continental arc in California, USA reveal a complex P-T-X-t (pressure-temperature-composition-time) evolution of the deep lithosphere. Metasedimentary xenoliths, despite high quartz contents, preserve granulite-facies equilibration conditions, necessitating transport from the surface into the lower crust. U-Pb and Hf isotope data on detrital zircon in the metasediments support continental underthrusting coeval with the peak of arc magmatism. Mantle xenoliths, represented by spinel and garnet-bearing spinel peridotites, indicate that shallow (spinel-facies), high-degree melt residues underwent thickening, cooling, and metamorphism into refertilized garnet peridotites that equilibrated below 800 °C and ca. 3 GPa. The source of refertilizing melts appears to be the mantle wedge itself, rather than the subducting slab based on mass-balance models and oxygen isotopes. Textural disequilibria, particularly Al-depletion haloes between orthopyroxene and exsolved garnet in peridotites, are diagnostic features related to cooling and increasing pressure. Diffusion modeling indicate that such disequilibria formed within 6 to 8 Ma. Coupled Lu-Hf and Sm-Nd age data on garnet pyroxenites show a significant lag between closure of the two chronometers in a lower crustal pyroxenite, but negligible difference in the deepest websterites. As a whole, the Sierran xenolith data provide insights into the origin and evolution of a mature continental arc, particularly highlighting the importance of thickening in both the crust and mantle lithosphere.