AbstractUltrahighātemperature (UHT; >900Ā°C) metamorphism drives crustal differentiation and is widely recognized in the rock record, but its geodynamic causes are debated. Previous work on granuliteāfacies metapelite xenoliths from San Luis PotosĆ, Mexico suggests the lower crust experienced a protracted UHT metamorphic event that coincided with the onset of regional extension. To determine the duration, conditions, and heat sources of UHT metamorphism recorded by these xenoliths, this study characterizes the majorāelement, traceāelement, and UāPb isotopic systematics of quartz, rutile, feldspar, garnet, and zircon by in situ electron microprobe (EPMA) and laserāablation inductively coupledāplasma mass spectrometry (LAāICPāMS), and augments these data with detailed petrography, thermobarometry, phase equilibria modeling, and diffusion modeling. Thermobarometry and phase equilibria modeling suggest peak metamorphic conditions exceeded 0.7 GPa and 900Ā°C. Zircon petrochronology confirms >15 Myr of UHT conditions since its onset at ā¼30 Ma. A small population of zircon record elevated temperatures following transition from backarc compression to extension during the waning stages of orogenesis (60ā37 Ma). Garnet preserves traceāelement zoning and mineral inclusions consistent with suprasolidus garnet growth and subsequent compositional modification by intracrystalline rareāearth element diffusion during protracted heating, with diffusion chronometry timescales in agreement with zircon data, followed by fluidādriven remobilization of trace elements along nowāhealed fractures within ā¼1 Myr of eruption. In sum, these data are most compatible with lithospheric mantle attenuation or removal as the dominant heat transport mechanism driving synextensional UHT metamorphism and crustal melting, which has bearing on models for crustal differentiation and formation of modern and ancient granulite terranes globally.