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Ultrahigh‐temperature (UHT; >900°C) metamorphism drives crustal differentiation and is widely recognized in the rock record, but its geodynamic causes are debated. Previous work on granulite‐facies metapelite xenoliths from San Luis Potosí, 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 major‐element, trace‐element, and U‐Pb isotopic systematics of quartz, rutile, feldspar, garnet, and zircon by in situ electron microprobe (EPMA) and laser‐ablation inductively coupled‐plasma mass spectrometry (LA‐ICP‐MS), 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 900°C. Zircon petrochronology confirms >15 Myr of UHT conditions since its onset at ∼30 Ma. A small population of zircon record elevated temperatures following transition from backarc compression to extension during the waning stages of orogenesis (60–37 Ma). Garnet preserves trace‐element zoning and mineral inclusions consistent with suprasolidus garnet growth and subsequent compositional modification by intracrystalline rare‐earth element diffusion during protracted heating, with diffusion chronometry timescales in agreement with zircon data, followed by fluid‐driven remobilization of trace elements along now‐healed fractures within ∼1 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.

Earth’s silica-rich continental crust is unique among the terrestrial planets and is critical for planetary habitability. Cratons represent the most imperishable continental fragments and form about 50% of the continental crust of the Earth, yet the mechanisms responsible for craton stabilization remain enigmatic¹. Large tracts of strongly differentiated crust formed between 3 and 2.5 billion years ago, during the late Mesoarchaean and Neoarchaean time periods². This crust contains abundant granitoid rocks with elevated concentrations of U, Th and K; the formation of these igneous rocks represents the final stage of stabilization of the continental crust^2,3. Here, we show that subaerial weathering, triggered by the emergence of continental landmasses above sea level, facilitated intracrustal melting and the generation of peraluminous granitoid magmas. This resulted in reorganization of the compositional architecture of continental crust in the Neoarchaean period. Subaerial weathering concentrated heat-producing elements into terrigenous sediments that were incorporated into the deep crust, where they drove crustal melting and the chemical stratification required to stabilize the cratonic lithosphere. The chain of causality between subaerial weathering and the final differentiation of Earth’s crust implies that craton stabilization was an inevitable consequence of continental emergence. Generation of sedimentary rocks enriched in heat-producing elements, at a time in the history of the Earth when the rate of radiogenic heat production was on average twice the present-day rate, resolves a long-standing question of why many cratons were stabilized in the Neoarchaean period.

Metasediments are common constituents of exhumed lower‐to‐mid‐crustal granulite terranes; understanding their emplacement is significant for the assembly and tectonic evolution of deep continental crust. Here, we report a monazite U‐Th‐Pb petrochronological investigation of the Variscan Ivrea‐Verbano Zone (IVZ) (Val Strona di Omegna section)—an archetypal section of lower crust. Monazite Th‐Pb dates from 11 metapelitic samples decrease with structural depth from 310 to 285 Ma for amphibolite‐facies samples to <290 Ma for granulite‐facies samples. These dates exhibit a time‐resolved variation in monazite trace‐element composition, dominated by the effects of plagioclase and garnet partitioning. Monazite growth under prograde to peak metamorphic conditions began as early as 316 ± 2 Ma. Amphibolite‐facies monazite defines a trend consistent with progressively decreasing garnet modal abundances during decompression and cooling starting at ∼310 Ma; the timing of the onset of exhumation decreases to ∼290 Ma at the base of the amphibolite‐facies portion of the section. Structurally lower, granulite‐facies monazite equilibrated under garnet‐present pressure‐temperature conditions at <290 Ma, with monazite (re)crystallization persisting until at least ∼260 Ma. Combined with existing detrital zircon U‐Pb dates, the monazite data define a <30 Myr duration between deposition of clastic sediments and their burial and heating, potentially to peak amphibolite‐to‐granulite‐facies conditions. Similarly brief timescales for deposition, burial and prograde metamorphism of lower crustal sediments have been reported from continental magmatic arc terranes—supporting the interpretation that the IVZ represents sediments accreted to the base of a Variscan arc magmatic system >5 Myr prior to the onset of regional extension and mafic magmatism.

High‐precision dating of the metamorphic sole of ophiolites can provide insight into the tectonic evolution of ophiolites and subduction zone processes. To understand subduction initiation beneath a young, well‐preserved and well‐characterized ophiolite, we performed coupled zircon laser‐ablation inductively coupled mass spectrometry trace element analyses and high‐precision isotope dilution‐thermal ionization mass spectrometry U–Pb dating on 25 samples from the metamorphic sole of the Samail ophiolite (Oman‐United Arab Emirates). Zircon grains from amphibolite‐ to granulite‐facies (0.8–1.3 GPa, ~700–900°C), garnet‐ and clinopyroxene‐bearing amphibolite samples (n = 18) show systematic trends of decreasing heavy rare earth element slope (HREE; Yb/Dy) with decreasing Yb concentration, reflecting progressive depletion of the HREE during prograde garnet growth. For half of the garnet‐clinopyroxene amphibolite samples, Ti‐in‐zircon temperatures increase, and U–Pb dates young with decreasing HREE slope, consistent with coupled zircon and garnet growth during prograde metamorphism. In the remaining samples, there is no apparent variation in Ti‐in‐zircon temperature with decreasing HREE slope, and the combined U–Pb and geochemical data suggest zircon crystallization along either the prograde to peak or prograde to initial retrograde portions of the metamorphicP–T–tpath. The new data bracket the timing of prograde garnet and zircon growth in the highest grade rocks of the metamorphic sole between 96.698 ± 0.094 and 95.161 ± 0.064 Ma, in contrast with previously published geochronology suggesting prograde metamorphism at ~104 Ma. Garnet‐free amphibolites and leucocratic pods from lower grade (but still upper amphibolite facies) portions of the sole are uniformly HREE enriched (Yb/Dy > 5) and are ~0.5–1.3 Myr younger than the higher grade rocks from the same localities, constraining the temporal offset between the metamorphism and juxtaposition of the higher and lower grade units. Positive zircon ε_Hf(+6.5 to +14.6) for all but one of the dated amphibolites are consistent with an oceanic basalt protolith for the sole. Our new data indicate that prograde sole metamorphism (96.7–95.2 Ma) immediately predated and overlapped growth of the overlying ophiolite crust (96.1–95.2 Ma). The ~600 ky offset between the onset of sole metamorphism in the northern portion of the ophiolite versus the start of ophiolite magmatism is an order of magnitude shorter than previously proposed (~8 Ma) and is consistent with either spontaneous subduction initiation or an abbreviated period of initial thrusting during induced subduction initiation. Taken together, the sole and ophiolite crust preserve a record of the first ~1.5 Myr of subduction. A gradient in the initiation of high‐grade metamorphism from the northwest (96.7 Ma) to southeast (96.0–95.7 Ma) may record propagation of the nascent subduction zone and/or variations in subduction rate along the length of the ophiolite.