ABSTRACT Recent studies have suggested various mechanisms to generate melt during exhumation of deeply subducted crust, including dehydroxylation of omphacite, fluid‐absent omphacite‐ and/or phengite‐breakdown melting and fluid‐present melting. However, the space–time relationships among these mechanisms remain uncertain. Here, we report microstructural evidence of the reactions responsible for, and compositions of, low‐volume melts frozen in situ (as leucosome pockets) within weakly deformed granitic veins that cross‐cut foliated UHP eclogite boudins in gneisses from the central Sulu belt, China. Phengite in the granitic veins records crystallization pressures of 3.4–2.7 GPa, and the granitic veins have whole‐rock Sr–Nd isotope compositions and trace element patterns consistent with derivation from the eclogite. The granitic veins likely crystallized from a solute‐rich supercritical fluid or hydrous melt generated by dehydroxylation of nominally anhydrous minerals during the early stage of exhumation (decompression). Subsequently, leucosome pockets and grain‐boundary films of melt formed in the granitic veins by the successive breakdown of omphacite and phengite. Based on the mineral modes and chemical compositions of 55 leucosome pockets (cf. microgranitoids), we distinguish those with (1) high Na/K ratios mainly composed of plagioclase and euhedral amphibole with skeletal omphacite and (2) low Na/K ratios predominantly composed of K‐feldspar and plagioclase and which contain phengite with corroded margins and fine‐grained biotite. High Na/K leucosome pockets (HLPs) are consistent with a melting reaction involving mostly omphacite‐breakdown, whereas low Na/K leucosome pockets (LLPs) are inferred to have formed by a reaction consuming variable proportions of phengite and omphacite. We argue that the reactions to form HHP then LLP initiated at approximately 1.5 and 1.1 GPa, respectively, and document two different mechanisms to generate melt during exhumation of deeply subducted crust in which breakdown of omphacite occurs before (deeper than) phengite. At higher temperatures, these melt‐producing reactions would generate a larger volume of melt that potentially could facilitate exhumation and increase crust–mantle interactions, thereby increasing the compositional heterogeneity of orogenic mantle.

Contains the Microsoft Word file “Preservation and survival of eclogite” and the Microsoft Excel file “TP dataset 08-18-2025 with references.”

Understanding how deeply subducted dense mafic rocks, such as ultra-high pressure (UHP) eclogites, return to the surface is a critical issue in understanding orogenesis. Here, we report results from the Sulu belt, China, where felsic continental crust with eclogite boudins was deeply subducted then returned to the surface from >100 km depth during late-orogenic exhumation. Through quantitative field-based estimates of leucosome (a proxy for crustally-derived magma) in migmatized eclogites, we determine that leucosome forms 20–30 vol.% of the migmatized mafic crust. This leucosome was mostly sourced from eclogite (>80 vol.%) with only a minor proportion sourced from the host gneiss. Retrogression during magma migration and polybaric crystallization reduced eclogite density by 6–20% compared to representative unmigmatized UHP eclogite. Overall, the presence of leucosome leads to a ∼17% reduction in density of the mafic crust. Density modeling (with mafic crust constituting ∼5 vol.% of the UHP terrane) reveals that migmatization is critical to lowering the density of the mafic crust sufficiently relative to gneissic crust to provide a positive contribution to the overall buoyancy of the exhuming continental crust. This work represents the first large-scale field-based quantitative study documenting the reduction in density of deeply subducted UHP eclogite due to the presence of melt during exhumation. Such a reduction promotes exhumation and likely helps to explain the return of deeply subducted eclogites to the surface in orogens. We also provide critical parameterized constraints for use in geodynamic models of exhumation of partially melted eclogite-dominated tectonic units in continental subduction zones.

On Earth, atmospheric oxygen is inferred to have risen over three major intervals before reaching modern levels, with each interval having a profound impact on the evolution of the biosphere. However, the principal driver behind these stepwise increases remains elusive. Here, we compile metamorphic thermobaric ratios (T/P) through time and use them as a first-order, probabilistic proxy for the likelihood of "cold" subduction (i.e., with T/P < 375 °C GPa-1) during secular cooling of Earth's mantle. Then, we couple this tectonic forcing to biogeochemical modeling to test whether more efficient cold subduction may have enhanced the net transfer of reduced organic carbon and pyrite to Earth's deep interior, thereby diminishing oxygen sinks and allowing surface oxygen levels to increase at geological timescales. Modeling results indicate that the progressive emergence of cold subduction could plausibly have contributed to the long-term oxygenation trajectory and associated secular trends in atmospheric carbon dioxide, seawater sulfate, sedimentary phosphorus, and marine redox conditions. Although the absolute magnitudes remain uncertain, the predicted trajectory of surface oxygenation is qualitatively consistent with the broad three-step pattern inferred from geochemical proxies. We propose that the progressive evolution of subduction may have been a key driver of long-term surface oxygenation, linking mantle cooling to the rise of conditions favorable for aerobic lifeforms.

Eclogites are generally divided into two types, xenolithic (mantle) and orogenic (crustal). Xenolithic eclogites are late Archean (ca. 3.0-2.5 Ga) or Paleoproterozoic (ca. 2.1-1.7 Ga) in age and are widely interpreted to have been entrained in younger carbonated magmas as they rose through the lithospheric mantle roots of cratons. By contrast, all reliably dated orogenic eclogites are post-Archean, occurring in three periods ca. 2.2-1.7 Ga, 1.2-0.85 Ga, and <0.7 Ga and are generally found in sutures or accretionary complexes. Although the absence of orogenic eclogites from Archean crust may be due to a tectonic mode dominated by plumes, paleomagnetic data from several Archean cratons indicate periods of lithospheric mobility interspersed with periods of stasis, demonstrating differential motions that require active tectonic boundaries between them. The apparent contradiction between a dominantly plume origin for cratonic crust and periods of lithospheric mobility can be reconciled if tectonic units were larger than the preserved cratonic nuclei in a tectonic mode with episodic subduction. The presence of xenolithic eclogites in the mantle roots of cratons suggests that moderate late-stage thickening was driven by convergence during a transition to global plate tectonics. By contrast, the earliest orogenic eclogites occur in sutures between cratons that form the composite continental fragments familiar to the supercontinent cycle.

ABSTRACT Whether nominally anhydrous garnet pyroxenite can melt during exhumation from eclogite‐facies P–T conditions and, if so, what chemical composition such melts would have is largely unexplored. Here, we document petrographic, chemical, geochronological and thermodynamic modelling evidence for the initiation of in situ partial melting of nominally anhydrous eclogite‐facies garnet pyroxenite (garnet + omphacite/sodian augite + rutile) from the northern Sulu belt, China. The garnet pyroxenite forms the cores of amphibolitized boudins hosted in strongly deformed migmatitic felsic gneiss. The garnet pyroxenite is strongly foliated and comprises interlayered garnet‐rich and clinopyroxene‐rich layers. The latter contain abundant pockets (0.2–1 mm across) comprising volumetrically dominant plagioclase with euhedral grains of either clinopyroxene (Cpx + Pl leucosome) or amphibole (Amp + Pl leucosome), and locally grain‐boundary films of K‐feldspar (K‐bearing leucosome). These leucosome pockets are preferentially located at triple junctions between clinopyroxene grains and have microstructural features consistent with their crystallization from locally derived melt, including plagioclase seams extending along clinopyroxene grain boundaries in the garnet pyroxenite host. The calculated bulk composition of the leucosome pockets is consistent with melting through a process combining breakdown of clinopyroxene ± garnet, but requiring a contribution of mobile components (including H 2 O and K) from the host gneiss, yielding melts of gabbroic to monzonitic composition. U–Pb analysis of euhedral zircon cores yield Neoproterozoic ages of c. 740 Ma, whereas zircon rims yield Triassic metamorphic ages of 244–221 Ma, consistent with the late prograde to early retrograde evolution of the garnet pyroxenite during subduction of the Yangtze craton. Thermodynamic modelling shows that gradients in μ H 2 O and μ K 2 O (where μ is chemical potential) generated during decompression from eclogite‐facies P–T conditions drove diffusive transfer of material from the gneiss to the garnet pyroxenite. This interpretation is consistent with the elevated contents of large ion lithophile and light rare earth elements in most of the leucosome pockets relative to host clinopyroxene. Thirteen leucosomes are nepheline normative and five are hypersthene normative, suggesting that partial melting of nominally anhydrous garnet pyroxenite produces alkaline to subalkaline basaltic melts. Our findings indicate that clinopyroxene‐breakdown melting is an intrinsic consequence of suprasolidus decompression from eclogite‐facies P–T conditions. Although the volume of melt generated in the garnet pyroxenite was minor, our study may have wider implications for the genesis of alkalic OIB magmas.

A newly identified tectonic sliver of low-grade Neoproterozoic continental crust comprising hybrid granitoids is exposed between Triassic ultrahigh-pressure (UHP) metamorphic rocks of the Sulu belt and the Cretaceous Laoshan granite. Outcrop evidence of magma mingling combined with the granitic mineral assemblages and primary igneous microstructures suggest >100 km difference in likely depth of subduction compared to the proximal UHP metamorphic rocks. The granitoids are cut by a brittle-to-ductile shear zone and extensional fractures that focussed fluid flow and allowed low-temperature fluid-rock alteration. Integrated accessory mineral geochronology from zircon, allanite and apatite records crystallization ages of c. 835-700 Ma and alteration ages of c.120-90 Ma. There is no evidence of the Triassic UHP metamorphic event recorded in the Neoproterozoic granitoids. The hybrid granitoids were likely generated during Neoproterozoic rifting of Rodinia, forming part of the northern margin of the Yangtze craton, but they did not experience deep subduction like other Neoproterozoic continental crust within the Sulu belt. The adjacent Cretaceous Laoshan granite was emplaced when all units were at shallow crustal depths, after >100 km of exhumation of the UHP eclogites. The Cretaceous ages retrieved from the hybrid granitoids date the brittle deformation and fluid alteration of these rocks in the Laoshan granite aureole. Cretaceous subduction retreat (slab roll back) of the paleo-Pacific plate caused extension and lithosphere thinning of the upper plate (eastern China), forming extensional shear zones and core complexes, associated with numerous granitoid intrusions, including the Laoshan granite. The extensional geological setting also facilitated preservation of this thin sliver of hybrid granitoids at the margin of the Laoshan granite adjacent to the UHP metamorphic terrane, and likely contributed to the final exhumation of the UHP metamorphic rocks of the Sulu belt. Our study shows that post-collisional extensional could be a common mechanism promoting final exhumation and exposure of deeply subducted terrains in orogens worldwide.

Continental crust is fundamental to planetary habitability, providing the geochemical reservoirs and physical interfaces that drive and regulate exchanges among the atmosphere, hydrosphere and biosphere. However, the evolution of Earth’s crust is uncertain owing to debate regarding the competing roles of internal versus external energetic drivers. In this Review, we examine the interplay between internal and external drivers of the production, modification and destruction of crust on the early Earth using geochemical, geological and geophysical data. Internal drivers are potentially linked to plate tectonics and processes such as subduction (dripping) or delamination. External drivers from large meteorite impacts likely influenced crust formation by inducing rapid decompression melting of the mantle to form basaltic protocratons, the early, mantle-derived crustal nuclei that preceded stable continental crust. On a planet covered by water, protocratons might have been transformed by intracrustal differentiation into evolved (continental) crust. Future research into the processes driving Earth’s early evolution and habitability should consider a wide range of temporal and spatial scales from seconds to millions of years and the subgrain to the galactic, to uncover the long-wavelength patterns, in mantle overturn rates and impact flux preserved in deep-time records. Continental crust is important for Earth’s habitability. This Review explores how the formation and stabilization of Earth’s early continental crust was modulated by internal and external factors such as subduction and bolide impacts, respectively.

Providing spatio‐temporal constraints on what influences the rheology of deeply subducted continental crust during subduction–exhumation remains elusive but crucial for understanding the exhumation dynamics of ultrahigh pressure (UHP) terranes. Here, we report results of a systematic study of microstructures, crystallographic preferred orientations (CPOs) and seismic properties of four UHP–HP eclogites formed along a common P – T path from Yangkou Bay, Sulu belt, China. The eclogites have different bulk compositions and record heterogeneous strain patterns. Peak metamorphic conditions (800°C–900°C and >5.5 GPa) were retrieved from early F1 isoclinal fold hinges. Subsequent overprinting by F2 tight folds occurred during the transition to quartz‐eclogite facies. Localized shear zones exhibit amphibolite‐facies retrogression, indicative of enhanced fluid activity. Omphacite exhibits crystal plasticity, while garnet displays a brittle–plastic transition during exhumation. A change from S‐ to L‐type CPO in omphacite was controlled by folding geometry during subduction–exhumation. Strain localization controlled intergranular fluid connectivity and redistribution, correlating with increasing strain from F1 folds to localized shear zones. This process led to progressive dynamic recrystallization, and changes in deformation mechanisms and seismic properties. Dynamic recrystallization resulted in significant grain refinement, thereby triggering diffusion creep assisted grain boundary sliding in the presence of fluid. Seismic anisotropy is linked to the omphacite fabric and the presence of phengite, with modal phengite as the primary determinant in UHP–HP eclogites. Fluid migration controlled by strain localization led to heterogeneous weakening of eclogite, which enabled exhumation of tectonic slices of UHP crustal rocks from mantle depths.

We present new JWST-NIRSpec IFS data for the luminous infrared galaxy NGC7469: a nearby (70.6Mpc) active galaxy with a Sy 1.5 nucleus that drives a highly ionized gas outflow and a prominent nuclear star-forming ring. Using the superb sensitivity and high spatial resolution of the JWST instrument NIRSpec-IFS, we investigate the role of the Seyfert nucleus in the excitation and dynamics of the circumnuclear gas. Our analysis focuses on the [Fe ii], H2, and hydrogen recombination lines that trace the radiation/shocked-excited molecular and ionized ISM around the AGN. We investigate the gas excitation through H2/Br{\gamma} and [Fe ii]/Pa\b{eta} emission line ratios and find that photoionization by the AGN dominates within the central 300 pc of the galaxy and together with a small region show ing signatures of shock-heated gas; these shock-heated regions are likely associated with a compact radio jet. In addition, the velocity field and velocity dispersion maps reveal complex gas kinematics. Rotation is the dominant feature, but we also identify non-circular motions consistent with gas inflows as traced by the velocity residuals and the spiral pattern in the Pa{\alpha} velocity dispersion map. The inflow is consistent with the mass outflow rate and two orders of magnitude higher than the AGN accretion rate. The compact nuclear radio jet has enough power to drive the highly ionized outflow. This scenario suggests that the inflow and outflow are in a self-regulating feeding-feedback process, with a contribution from the radio jet helping to drive the outflow.

To constrain the rate of cooling of lower-crustal rocks from an ultrahot orogen, we determined both the age and equilibration temperature of metamorphic zircon from six widely spaced samples of metasedimentary garnet-sillimanite gneiss from the Eastern Ghats Province in eastern India. For the combined data set of metamorphic zircon, concordant dates decrease continuously within 2 sigma uncertainty from around 950 Ma to 800 Ma, consistent with similar to 150 m.y. of zircon crystallization. Ti-in-zircon temperatures for each dated spot during this period decrease with age, corresponding to linear cooling rates ranging from 0.26 to 0.90 degrees C/m.y. We propose that retention of heat-producing elements in the lower crust of the Eastern Ghats Province and a low net erosion rate were responsible for similar to 150 m.y. of ultraslow cooling.

Using petrography, in situ garnet Lu-Hf geochronology, garnet rare-earth element (REE) analysis, zircon U-Pb geochronology and phase equilibrium modelling, we provide unambiguous evidence for Eoarchean granulite-facies metamorphism in the northern Itsaq Gneiss Complex (IGC), southwest Greenland. In situ garnet Lu-Hf geochronology from two samples of variably migmatitic metabasic rocks least affected by subsequent (Neoarchean) reworking yield Lu-Hf isochron ages of 3641 +/- 62 Ma (MSWD = 1.7, n = 45/67; all age uncertainties at 2 sigma level) and 3652 +/- 69 Ma (MSWD = 1.8, n = 83/84) from garnet with REE patterns typical of single-stage prograde growth. From the same two samples, zircon grains with textures consistent with metamorphic growth give weighted-mean Pb-207/Pb-206 ages of 3620 +/- 8 Ma (MSWD = 1.2, n = 45) and 3630 +/- 8 Ma (MSWD = 0.6, n = 44), respectively. Phase equilibrium modelling constrains peak P-T conditions of Eoarchean (3640-3630 Ma) metamorphism to 8.3-9.0 kbar and 730-820 degrees C. The thermobaric ratios (T/P) of 800-1000 degrees C/GPa recorded by the investigated samples are considerably higher (warmer) than previously proposed for granulite-facies metamorphism in the northern IGC, and broadly similar to Archean metamorphic P-T data globally, with no evidence for the bimodality in T/P that characterizes younger metamorphism. Either subduction-driven metamorphism (and plate tectonics) did not operate in the Eoarchean, or the Eoarchean lithosphere had a rheology that prohibited exhumation of subducted rocks.

The petrogenesis of contemporary igneous and metamorphic rocks is commonly explained by plate tectonics, but how far back in time does this relationship hold? Here we investigate whether the distinctive petrological features of recent ocean crust, subduction-related magmatism and regional metamorphism can be unambiguously identified in the Archean geological record. From an igneous perspective based on geological relationships and Th-Nb systematics, it is difficult to claim that any Archean 'ophiolite' was part of a global plate system rather than deriving from a plume ascending through attenuating lithosphere. Furthermore, the rarity of subduction-related rocks, particularly their plutonic equivalents, which have good preservation potential, is consistent with the concept of local convergence and short-lived subduction. From a metamorphic perspective, the appearance of orogenic eclogites in the Paleoproterozoic, the widespread occurrence of blueschists and ultrahigh-pressure metamorphic rocks since the late Neoproterozoic, and a change from a unimodal to a bimodal distribution of metamorphic T/P during the Proterozoic, are responses to secular cooling and the evolution of global tectonics since the Archean. Our petrological perspective is that plate tectonics analogous to that on Earth today is probably a post-Archean phenomenon.

The Acasta Gneiss Complex (AGC) in northwestern Canada is home to the oldest known evolved (felsic) rocks on Earth, dating back to around 4.03 billion years (Ga). These rocks preserve evidence for multiple episodes of magmatism, metamorphism, and deformation, offering insights into the geological processes that shaped the Earth's crust throughout the Archean and late Hadean. However, the metamorphic pressure–temperature (P–T) conditions of this complex remain poorly constrained. In this study, we use phase equilibria modelling and in situ garnet Lu-Hf geochronology to analyse two garnet-bearing tonalitic gneisses in the AGC, providing the first quantitative P–T constraints for a late Paleoarchean tectono-metamorphic event in the AGC. Our results indicate metamorphic peak conditions of approximately 725-780°C and 4.5-6.2 kbar, with limited partial melting (<7 vol.%) of the felsic gneisses at these crustal levels. In situ Lu-Hf garnet geochronology suggests that this metamorphic event occurred between 3.3-3.2 Ga, consistent with previous findings of high-grade metamorphism at that time. Isotopic disturbance of garnet at approximately 1.9 Ga is interpreted to reflect partial resetting of the Lu-Hf systematics in response to fluid-present re-equilibration during the Paleoproterozoic Wopmay orogeny. Our study extends the limited dataset of published P–T data for Mesoarchean and older metamorphic rocks and shows that tonalitic gneisses in the AGC evolved along a high apparent thermal gradient of 125-150°C/kbar.

Melt/fluid evolution processes are important in determining the rheological behavior of lithosphere subducted to and exhumed from ultrahigh-pressure (UHP), controlling the flow of lithospheric material, the generation of melts from the upper mantle, and potentially contributing to magmatism and growth of the continental crust. In these circumstances, determining the controls on deformation mechanisms of deeply subducted eclogite is crucial to understanding exhumation dynamics of UHP terranes. However, few studies are reported on the multi-stage evolution of microstructures in natural eclogites exhumed along the same retrograde P-T-t path, particularly in the presence of supercritical fluid or melt. Recently, evidence of melt-fluid events within UHP eclogite has been reported from the Sulu belt, China. The eclogite was deformed by multiple stages of folding associated with two phases of melt/fluid evolution during exhumation: Dehydroxylation of nominally anhydrous minerals produced a supercritical fluid in the early stage of exhumation from UHP conditions, and then, omphacite-breakdown melting during the late stage of exhumation from HP conditions. Here, we present a microstructural analysis of UHP eclogite from different structural zones in single outcrops from Yangkou and Taohang, in the Sulu belt. These eclogites preserve assemblages formed at different metamorphic stages controlled by strain partitioning or located in same structural position but affected by different degrees of melt/fluid modification. Results show that deformation mechanisms switch from overall brittle to ductile with increased melt/fluid mobility. Omphacite fabrics change from S to L type, corresponding to progressive enhancement of strain associated with melt/fluid migration from early fold hinges, to fold limbs and then to strain-localized shear zones. We conclude that strain partitioning and inherited HO content of UHP rocks control fluid distribution, and influence subsequent microfabrics, deformation mechanisms, partial melting, and retrogression at different structural locations during exhumation. These variations can play critical roles in reducing strength of eclogite and thus contribute to rheological heterogeneities in convergent settings at UHP conditions, potentially enhancing exhumation.

The past 40 years have been a golden age for eclogite studies, supported by an ever wider range of instrumentation and enhanced computational capabilities, linked with ongoing developments in thermobarometry and geochronology. During this time, we have made robust estimates of pressure-temperature (P-T) conditions; determined ages related to the prograde, metamorphic peak and retrograde stages; and calculated time-integrated rates of cooling and exhumation for eclogites and related rocks, including blueschists, from orogenic belts worldwide. Improvements to single mineral thermometers and new developments in elastic barometry using inclusions of one mineral in another (e.g. quartz and/or zircon in garnet), coupled with ongoing innovations in petrochronology and diffusion modelling, presage a new age for eclogite studies in which detailed quantification of metamorphic conditions and timescales will be linked to an improved understanding of processes at all scales. Since the turn of the century, numerical modelling of subduction zone and rock exhumation processes has become increasingly important. As a result, subduction and exhumation are quite well understood, but the volume of continental crust subducted to and returned from mantle conditions and the amount lost to the mantle are largely unknown. We have generated sufficient data to investigate the spatiotemporal distribution of metamorphism and secular change but not without controversy in relation to the rare occurrence of orogenic eclogites and the absence of blueschists prior to the late Neoproterozoic and the emergence of plate tectonics on Earth. Since the turn of the century, the assumption that metamorphic pressure is lithostatic has come under increasing scrutiny. Whether local variations in stress extrapolate to the crustal scale and, if so, whether the magnitude of the calculated deviations from lithostatic pressure can be generated and sustained in mechanically heterogeneous rock units remains contentious. Could the paradigm of subduction of continental lithosphere to mantle depths be simply an artefact of the lithostatic assumption? Fluid cycling in subduction zones and understanding the role of fluids in the generation of intermediate-depth earthquakes remain important topics of current research. Dry (H2O-absent) conditions are unlikely around the peak of ultrahigh-pressure (UHP) metamorphism or during exhumation, due to dehydroxylation of nominally anhydrous minerals and breakdown of hydrous minerals at P-T conditions in the realm of supercritical fluid and hydrous melt. Indeed, the presence of melt may be necessary to facilitate the exhumation of HP and UHP tectonometamorphic rock units. Finally, our ability to interrogate inclusions in superdeep diamonds should lead to a better understanding of how the deep interior and surface are linked in the context of Earth as a fully coupled system.

The rates and mechanisms by which deeply subducted continental crust was ex-humed back to the surface are not well un-derstood, but can be better characterized using multimineral petrochronology. Here, we combine zircon, titanite, and apatite U-Pb ages from leucogranite and phengite gneiss with a pressure-temperature (P-T) path from eclogite to provide robust quantitative constraints on cooling and exhumation of the Sulu belt, a large ultrahigh-pressure meta-morphic terrane in eastern China. The leuco-granite, which formed during exhumation, is enriched in light rare earth elements (REE) relative to heavy REE and in large ion litho-phile elements relative to high field strength elements, similar to hydrous crustal melts. Whole-rock Sr-Nd isotope compositions indi-cate that the leucogranite was not directly de-rived from the host phengite gneiss, but was more likely sourced from deeper in the ex-huming crust. For the gneiss, mantles on in-herited zircon yield an age of 230 +/- 2 Ma and a temperature of 802 +/- 36 degrees C based on a min-imum pressure of 2.9 GPa, which records the minimum timing and P-T of initial decom-pression. Overgrowths on inherited zircon from the leucogranite constrain crystalliza-tion to 224 +/- 1 Ma, coeval with the growth of zircon rims in the gneiss, at a temperature of 764 +/- 42 degrees C and a pressure within the quartz- eclogite facies. Titanite and apatite define single populations with lower concordia in-tercept ages of 222 +/- 3 Ma and 198 +/- 7 Ma, at temperatures of 720 +/- 30 degrees C and-450 +/- 100 degrees C, respectively, recording the timing of pas-sage through the quartz-eclogite to the am-phibolite facies and then the transition to the upper greenschist facies. Although the data yield a nearly constant cooling rate of 10.9+4.5 -3.6 degrees C/m.y., exhumation was completed in two stages. The first stage from coesite-eclogite facies to-1.2 GPa, corresponding to the depth of the Moho, occurred at a rate of 7.5+5.8 -2.6 km/m.y. Thereafter, exhumation into the mid-crust occurred at a much slower rate of 0.87+0.86 -0.71 km/m.y. The first stage of faster exhumation was accompanied by migration of leucogranite melt along foliation in the gneiss, which would have decreased the aver-age density and weakened the crust, enhanc-ing the rate of return flow.

The thermal state of the mantle impacts lithospheric dynamics and Earth's surface evolution. Geodynamic simulations predict that aggregated continents may act as an effective thermal insulator leading to the warming of the underlying mantle (mantle warming hypothesis). Mantle warming weakens the continental lithosphere, which should lead to more distributed strain at plate margins, and result in lower-relief orogens, with lower metamorphic peak pressures (peak-P), and slower cooling and exhumation of metamorphic rocks. To test the mantle warming hypothesis, we determine the secular change of metamorphic cooling and exhumation rates based on a new dataset compiled from Neoarchean and younger orogens. We find that sluggish cooling and exhumation during the mid-Proterozoic (1.85–0.85 Ga; aka the boring billion) correlate with lower peak-P and higher thermobaric (temperature/pressure, T/P) ratios of metamorphic rocks. This result is consistent with a weakened continental lithosphere and the development of hotter lower-relief orogens under warmer mantle conditions during the mid-Proterozoic. We posit that the long-lived Columbia–Rodinia supercontinents may have acted as a thermal insulator that led to the mantle warming beneath the continental lithosphere and thus a distinctive style of mid-Proterozoic orogenesis. Low-relief mid-Proterozoic orogens with sluggish exhumation, in turn, reduced continental erosion and restricted delivery of bio-essential nutrients to the oceans, and decreased rates of organic carbon burial and thereby hindered oxygen accumulation in the atmosphere. The persistent nutrient deficiency in the oceans and suppressed atmospheric oxygenation may have stalled the surface evolution during the boring billion.