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The Malawi Rift is localized within Precambrian amphibolite‐granulite facies metamorphic belts, bounded by up to 150 km long border faults, and generates earthquakes throughout ∼40 km thick crust. Rift‐related faults are inferred to exploit pre‐existing weaknesses that allow rifting of otherwise dry and strong crust. It is unclear what these weaknesses are, and how localization into weak zones can be reconciled with strength required for lower crustal seismicity. We present results of mineral equilibria modeling, which indicate that Proterozoic metamorphism generated dry crust dominated by a quartz‐feldspar assemblage that is metastable at current conditions. For rift propagation to be possible at current cool thermal gradients and in mechanically strong, dry quartzofeldspathic rocks, mid‐ to lower‐crustal strain must be localized into relatively weak, inherited shear zones that deform primarily by aseismic, viscous creep. These shear zones are embedded within high‐strength crust, and interaction between creeping shear zones and enveloped or surrounding rocks may locally increase stress and trigger frictional, seismic slip at mid‐ to lower‐crustal depths. Over time, this interaction may produce a fracture network that allows infiltration of fluids. We therefore suggest that during rifting of previously deformed and metamorphosed crust, major faults are most likely to grow from below, with their location and orientation prescribed by underlying inherited viscous shear zones. In this case, fluids may infiltrate and locally weaken metastable lower crust, including allowing time‐dependent fluid‐driven seismicity and local partial melting, but length‐scales of this weakening is limited by the scale of the permeability network. Plain Language Summary: In Malawi, East Africa, the Earth's crust is slowly splitting apart. This "rifting" has two unusual characteristics: (a) the crust is thick and strong and therefore difficult to split apart, and (b) there are earthquakes at greater depth and temperature than typical for the continents. We propose that the reason rifting occurs with these two characteristics, in this location, is that the crust is inherited from past periods of mountain building. Based on thermodynamic principles, specifically what minerals are stable under what conditions, we model the composition of Malawi crust in the geological past. We then compare those results to current conditions. We find that the likely inherited composition of Malawi crust is unstable under current conditions, but needs an addition of water to undergo reactions to new, stable minerals. We also note that deformation is only possible in weak zones inherited from past deformation, and these zones will deform by high‐temperature flow of rock, not generating earthquakes. However, flow of local weak zones may induce fracturing in surrounding harder rocks, generating local earthquakes and a fracture system that water can flow along. Over time, this water infiltration can weaken the crust and ultimately allow continental rifting as seen further north in Africa. Key Points: The Malawi crust is mostly dry, metastable and frictionally strong, and rifting requires viscous flow in inherited lower crustal shear zonesMajor faults are likely to grow from below, with their location and orientation prescribed by underlying inherited viscous shear zonesViscous flow in heterogeneous crust may trigger lower crustal seismicity generating permeability that allows infiltration of external fluids [ABSTRACT FROM AUTHOR]

Geophysical observations indicate that patches of localized fracturing occur within otherwise viscous regions of subduction plate boundaries. These observations place uncertainty on the possible down‐dip extent of the seismogenic zone, and as a result the maximum magnitude of subduction thrust earthquakes. However, the processes controlling where and how localized fracturing occurs within otherwise viscous shear zones are unclear. We examined three exposures of exhumed plate boundary on Kyushu, Japan, which contain subducted sediments and hydrated oceanic crust deformed at ∼300 to ∼500°C. These exposures preserve subduction‐related viscous deformation, which in two of the studied exposures has a mutually overprinting relationship with quartz veins, indicating localized cyclical embrittlement. Where observed, fractures are commonly near lithological contacts that form viscosity contrasts. Mineral equilibrium calculations for a metabasalt composition indicate that exposures showing cyclical embrittlement deformed at pressure‐temperature conditions near dehydration reactions that consume prehnite and chlorite. In contrast, dominantly viscous deformation occurred at intervening pressure‐temperature conditions. We infer that at conditions close to metamorphic dehydration reactions, only small stress perturbations are required for transient embrittlement, driven by localized dehydration reactions reducing effective stress, and/or locally increased shear stresses along rheological contrasts. Our results show that the protolith composition of the subducting oceanic lithosphere controls the locations and magnitudes of dehydration reactions, and the viscosity of metamorphosed oceanic crust. Therefore, compositional variations might drive substantial variations in slip style. Plain Language Summary: Along tectonic plate boundaries, with increasing depth, pressure and temperature, plate movement by brittle fracturing is suppressed, and mechanisms allowing steady slip become more efficient. Along subduction plate boundaries, where one plate sinks beneath another, observations indicate that a proportion of slip sometimes occurs by fracturing at pressure‐temperature conditions where steady slip typically dominates. We use outcrops of an ancient, inactive plate boundary exposed on Kyushu, Japan to investigate the reason for this behavior. We found that chemical reactions, triggered by increasing temperature in sinking oceanic crust, produce water, with the effect of locally raising the fluid pressure within the plate boundary. We suggest that locally raised fluid pressures assist fracturing at pressure‐temperature conditions where steady slip typically dominates. In some outcrops, fractures are especially common along contacts between different rock types, suggesting that mixing of different materials along the plate boundary might also favor fracturing. Key Points: Brittle fracturing occurred locally within viscously deforming hydrated oceanic crust and subducted sedimentsFracturing is localized at viscosity contrasts and P‐T conditions of metamorphic dehydration reactionsRocks that deformed at P‐T conditions away from dehydration reactions record dominantly viscous behavior [ABSTRACT FROM AUTHOR]

Metamorphism in the lower Witwatersrand Supergroup exposed in the Vredefort Dome has previously been proposed to be related to elevated heat flow linked to the 2.06 Ga Bushveld magmatic event; however, there are no unambiguous chronological data to confirm this timing. Microtextural and mineral compositional analysis of a garnet-bearing metapelite in the northwestern collar of the Vredefort Dome suggests at least two metamorphic events with distinctly different P-T conditions preceding the Dome-forming meteorite impact at 2.02 Ga. THERMOCALC mineral equilibrium calculations yield P-T conditions of 500°C, 3.1 kbar for the M1 mineral assemblage garnet1-plagioclase-muscovitebiotite-chlorite-ilmenite. Thin, discontinuous, garnet2 overgrowths on the garnet1 porphyroblasts define a subsequent, M2, event with P-T conditions of 530°C, 5 kbar. Garnet Sm-Nd chronology yields an isochron age of 2 796.0 ± 1.5 Ma, indicating an early Ventersdorp (Klipriviersberg) timing of M1 metamorphism. Although the M2 garnet overgrowths are volumetrically too small to date, the calculated M2 pressure is consistent with the predicted overburden thickness above the lower Witwatersrand Supergroup during emplacement of the Bushveld Complex. While elevated, the M2 apparent geotherm (30°C/km) is significantly lower than the M1 apparent geotherm (46°C/km); however, thermal modelling suggests both events benefitted from local perturbations caused by contemporaneous sill emplacement. Our results thus show that the initial garnet-forming, mid-amphibolite facies metamorphism in the collar of the Vredefort Dome is not related to the emplacement of the Bushveld Complex, but rather to the early stages of magmatism associated with the Ventersdorp Large Igneous Province (LIP). Nonetheless, elevated heat flow associated with the Bushveld LIP also reached comparable amphibolite facies conditions. [ABSTRACT FROM AUTHOR]

The Stolzburg domain to the south of the Barberton Greenstone Belt preserves evidence for a 3.23 Ga subduction- collision tectonic event. Garnet amphibolite greenstone remnants have previously yielded conventional thermobarometric P-T estimates of 12 to 15 kbar at 600 to 650°C, 8 to 11 kbar at 650 to 700°C and 7.5 to 8.5 kbar at 560 to 640°C from, respectively, the Inyoni shear zone along the western margin of the Stolzburg domain, the central part of the domain and from the Tjakastad schist belt on the boundary with the main body of the Barberton Greenstone Belt. Pseudosection calculations constrain the stability conditions of the peak metamorphic assemblages at the three localities to be 10 kbar at 675 to 690°C, ~10 kbar at 700°C and ~7 and 10 kbar at 660°C respectively. Although it is possible that the peak metamorphic assemblages may be displaced to somewhat lower conditions if Mn is considered in the calculations, these estimates are generally in good agreement with existing estimates, and confirm that the Stolzburg domain exposes an intact mid- to lower-crustal section that was metamorphosed in a relatively cool environment at 3.23 Ga. Our results do not support previously documented higher-pressure conditions, and we contend that the mineral assemblages used to derive these estimates can equally reflect the conditions determined here. The presence of albite-epidote inclusion assemblages in garnet indicates that the likely prograde path involved a component of heating at depth, which is typical of subduction-collision environments and markedly different from the heating-burial paths expected for sinking greenstones in a vertical tectonic model. [ABSTRACT FROM AUTHOR]

Midcrustal rocks in retrograde metamorphic settings are typically H2O-undersaturated and fluid-absent and have low permeability. Exhumed continental retrograde faults, nonetheless, show evidence for the operation of fluid-mediated weakening mechanisms during deformation at midcrustal conditions. To explore the origin and effects of fluids in retrograde faults, we study the Kuckaus Mylonite Zone (KMZ), an exhumed crustal-scale, strike-slip shear zone in the southern Namibian Namaqua Metamorphic Complex. The KMZ deformed quartzofeldspathic migmatised gneisses at midcrustal retrograde conditions (450-480°C, 270-420 MPa) in the Mesoproterozoic, 40 Ma after granulite facies peak metamorphism at 825°C and 550 MPa. The mylonites contain fully hydrated retrograde mineral assemblages, predominantly adjacent to anastomosing high-strain zones, providing evidence of local H2O saturation and fluid presence during deformation. Whole rock and quartz vein δ18O values suggest that at least some of the fluids were meteoric in origin. The rocks across the shear zone retain the effect of different protoliths, implying little effect of fluid-rock interaction on whole rock major element chemistry. Together with a general scarcity of quartz veins, this suggests that fluid/rock ratios remained low in the KMZ. However, even small amounts of H2O allowed reaction weakening and diffusion-precipitation, followed by growth and alignment of phyllosilicates. In the ultramylonites, a fine grain size in the presence of fluids allowed for grain size sensitive creep. We conclude that the influx of even small volumes of fluids into retrograde shear zones can induce drastic weakening by facilitating grain size sensitive creep and retrograde reactions. In retrograde settings, these reactions consume fluids, and therefore elevated fluid pressures will only be possible after considerable weakening has already occurred. Our findings imply that the range of seismic styles recently documented at active retrograde transform faults may not require high fluid pressures but could also arise from other local weakening mechanisms. [ABSTRACT FROM AUTHOR]

Permian-aged metagabbros from the eclogite type-locality in the eastern European Alps were partially to completely transformed to eclogite during Eoalpine intracontinental subduction. Microtextures developed along a preserved fluid infiltration and reaction front in the gabbros record the incipient gabbro-to-eclogite transition, allowing the details of the eclogitization process to be investigated. Original, anorthite-rich igneous plagioclase is pervasively replaced by fine-grained intergrowths of clinozoisite, kyanite and Na-rich plagioclase. Where plagioclase was in contact with igneous orthopyroxene, 100-200 μm thick bimineralic coronae of symplectic kyanite and diopsidic clinopyroxene form along the edges of the grains. The rims of igneous orthopyroxene develop a complementary bimineralic corona of diopsidic clinopyroxene and garnet. Igneous clinopyroxene does not show any breakdown textures; however, jadeite content gradually increases towards the rims. In addition, exsolution lamellae inherited from the igneous clinopyroxene become progressively more jadeitic as eclogitization proceeds. Given that the igneous plagioclase is pervasively replaced by clinozoisite, kyanite and Na-rich plagioclase, whereas kyanite-diopside symplectites are confined to narrow rim zones, we suggest that the development of these textures was controlled by the (im)mobility of different elements on different length scales. The presence of hydrous minerals in the core of anhydrous plagioclase indicates that H2O diffusivity occurred on a mm-scale. By contrast, the size of the anhydrous diopside-kyanite and diopside-garnet symplectites indicate that Fe-Mg-Ca-Na diffusivity was limited to a 10s of μm scale. Chemical potential relations calculated in the idealized NCASH chemical system show that the clinozoisite-kyanite-albite intergrowths formed due to an increase of μH2O to plagioclase, whereas all other elements remained effectively immobile on the scale of this texture. Fluid conditions indicated by this texture span from virtually dry conditions ( [ABSTRACT FROM AUTHOR]

Rocks of basic and intermediate bulk composition occur in orogenic terranes from all geological time periods and are thought to represent significant petrological components of the middle and lower continental crust. However, the former lack of appropriate thermodynamic models for silicate melt, amphibole and clinopyroxene that can be applied to such lithologies at high temperature has inhibited effective phase equilibrium modelling of their petrological evolution during amphibolite- and granulite facies metamorphism. In this work, we present phase diagrams calculated in the Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-O2 (NCKFMASHTO) compositional system for a range of natural basic and intermediate bulk compositions for conditions of 2-12 kbar and 600-1050 ∘C using newly parameterized activity-composition relationships detailed in a companion paper by Green et al. in this issue. Particular attention is given to mid-ocean ridge basalt (MORB) and diorite protolith bulk compositions. Calculated subsolidus mineral assemblages in all basic and intermediate rock types are modally dominated by hornblende and plagioclase, with variable proportions of epidote, clinopyroxene, garnet, biotite, muscovite, quartz, titanite or ilmenite present at different pressures. The H2O-saturated (wet) solidus has a negative P− T slope and occurs between ∼620-690 ∘C at mid- to lower-crustal pressures of 5-10 kbar. The lowest- T melts generated close to the wet solidus are calculated to have granitic major-element oxide compositions. Melting at higher temperature is attributed primarily to multivariate hydrate-breakdown reactions involving biotite and/or hornblende. Partial melt compositions calculated at 800-1050 ∘C for MORB show good correlation with analysed compositions of experimental glasses produced via hydrate-breakdown melting of natural and synthetic basic protoliths, with Niggli norms indicating that they would crystallize to trondhjemite or tonalite. Diorite is shown to be significantly more fertile than MORB and is calculated to produce high- T melts (>800 ∘C) of granodioritic composition. Subsolidus and suprasolidus mineral assemblages show no significant variation between different members of the basalt family, although the P− T conditions at which orthopyroxene stabilizes, thus defining the prograde amphibolite-granulite transition, is strongly dependent on bulk-rock oxidation state and water content. The petrological effects of open- and closed-system processes on the mineral assemblages produced during prograde metamorphism and preserved during retrograde metamorphism are also examined via a case-study analysis of a natural Archean amphibolite from the Lewisian Complex, northwest Scotland. [ABSTRACT FROM AUTHOR]

A set of thermodynamic models is presented that, for the first time, allows partial melting equilibria to be calculated for metabasic rocks. The models consist of new activity-composition relations combined with end-member thermodynamic properties from the Holland & Powell dataset, version 6. They allow for forward modelling in the system Na [ABSTRACT FROM AUTHOR]

Cordierite–orthoamphibole gneisses and rocks of similar composition commonly contain low-variance mineral assemblages that can provide useful information about the metamorphic evolution of a terrane. New calculated petrogenetic grids and pseudosections are presented in the FeO–MgO–Al2O3–SiO2–H2O (FMASH), Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O (NCKFMASH) and Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3 (NCKFMASHTO) chemical systems to investigate quantitatively the phase relations in these rocks. Although the bulk compositions of cordierite–orthoamphibole gneisses are close to FMASH, calculations in this system do not adequately account for the observed range of mineral assemblages. Calculations in NCKFMASH and NCKFMASHTO highlight the role of minor constituents such as Ca, Na and Fe3+ in the mineral assemblage evolution of such rocks and these systems are more appropriate for interpreting the evolution of natural examples. [ABSTRACT FROM AUTHOR]

Orthopyroxene-rich quartz-saturated granulites of the Strangways Range, Arunta Block, central Australia, record evidence of two high-grade metamorphic events. Initial granulite facies metamorphism (M1, at c. 1.7 Ga) involved partial melting and migmatization culminating in conditions of 8.5 kbar and 850 °C. Preservation of the peak M1 mineral assemblages from these conditions indicates that most of the generated melt was lost from these rocks at or near peak metamorphic conditions. Subsequent reworking (M2, at c. 1.65 Ga) is characterized by intense deformation, the absence of partial melting and the development of orthopyroxene–sillimanite ± gedrite-bearing mineral assemblages. Gedrite is only present in cordierite-rich lithologies where it preferentially replaces M1 cordierite porphyroblasts. Pseudosection calculations indicate that M2 occurred at subsolidus fluid-absent conditions ( aH2o ∼ 0.2) at 6–7.5 kbar and 670–720 °C. The mineral assemblages in the reworked rocks are consistent with closed system behaviour with respect to H2O subsequent to M1 melt loss. M2 reworking was primarily driven by increased temperature from the stable geotherm reached after cooling from M1 and deformation-induced recrystallization and re-equilibration, rather than rehydration from an externally derived fluid. The development of the M2 assemblages is strongly dependent on the intensity of deformation, not only for promoting equilibration, but also for equalizing the volume changes that result from metamorphic reactions. Calculations suggest that the protoliths of the orthopyroxene-rich granulites were cordierite–orthoamphibole gneisses, rather than pelites, and that the unusual bulk compositions of these rocks were inherited from the protoliths. Melt loss is insufficient to account for the genesis of these rocks from more typical pelitic compositions. In quartz-rich gneisses, however, melt loss along the M1 prograde path was able to modify the bulk rock composition sufficiently to stabilize peak metamorphic assemblages different from those that would have otherwise developed. [ABSTRACT FROM AUTHOR]

Ridge subduction is an inescapable plate tectonic process, but has only been documented in modern circum-Pacific environments and not yet been recognized from suture zones associated with supercontinent assembly, likely because its imprint is obliterated by later collision. The formation of the Pan-African Damara Belt of central Namibia involved northward subduction of the Khomas Sea underneath the Congo Craton, prior to final suturing of the Congo and Kalahari Cratons. The accretionary history of the Belt is preserved in the Southern and Southern Marginal Zones, which consist of turbiditic metasedimentary and intercalcated mafic rocks with MORB affinity. Two localities in the Kuiseb and Gaub canyons reveal that aluminous metapelites contain a fabric-defining assemblage of fine-grained muscovite, chlorite, biotite, quartz and graphite that is overprinted by randomly oriented porphyroblasts and poikiloblasts of garnet, staurolite, kyanite and biotite. Associated metamafic rocks consist of hornblende, chlorite, epidote, rutile and quartz, with actinolite cores preserved in amphibole porphyroblasts. Metamorphic conditions for the fabric-defining assemblage are estimated at ∼10 kbar and 540-560 [ABSTRACT FROM AUTHOR]

The Aggeneys terrane is a central part of the western Namaqua Metamorphic Complex (NMC) that is most renowned for hosting world-class massive sulphide mineralisation. Peak metamorphism reached upper amphibolite facies conditions, in contrast to the granulite facies conditions experienced by many of the surrounding terranes in the western NMC. Pseudosection modelling of metapelitic and metamafic lithologies constrains peak P-T conditions at 650 ± 20°C and 5 ± 1 kbar, corresponding to an apparent geotherm of 130°C/kbar. Prograde metamorphism involved heating and concomitant burial from conditions of andalusite stability, and initial retrograde metamorphism appears to have involved cooling, rather than decompression, resulting in an anticlockwise path. Whereas the absolute P-T conditions for the Aggeneys terrane are lower than that of the granulite terranes, metamorphism of the entire area occurred under similar, highly elevated apparent thermal gradients and was dominated by heating and cooling along similar P-T paths. However, peak metamorphism in the Aggeneys terrane is inferred to have occurred during the c. 1.17 to 1.13 Ga O'okiepian Orogeny whereas in other terranes it occurred during the younger, c. 1.07 to 1.03 Ga Namaquan Orogeny. The style of metamorphism experienced by the western NMC requires higher than average lithospheric heat flow generally attributed to mafic underplating, or elevated crustal heat production that can be caused by the slow burial of highly radiogenic crust. Both processes occur over protracted periods of time and affect large areas of the crust, such that the apparent diachronicity of peak metamorphism in the NMC, and the mechanisms by which it might have occurred require further investigation. [ABSTRACT FROM AUTHOR]

The Mesoarchaean Tasiusarsuaq terrane of southern West Greenland consists of Tonalite-trondhjemite-granodiorite gneisses and, locally, polymetamorphic mafic and ultramafic rocks. The terrane experienced medium-pressure granulite facies conditions during M1A in the Neoarchean, resulting in the development of two-pyroxene melanosome assemblages in mafic granulites containing garnet-bearing leucosome. Reworking of these rocks during retrogression introduced garnet to the melanosome in the form of overgrowths, coronas and grain necklaces that separate the mafic minerals from plagioclase. NCFMASHTO pseudosection modelling constrains the peak metamorphism during M1A to ∼850 °C and 7.5 kbar at fluid-saturated conditions. Following M1A, the rocks retained their M1A H2O content and became fluid-undersaturated as they underwent near-isobaric cooling to ∼700 °C and 6.5-7 kbar, prior to reworking during M1B. These low H2O contents allowed for the formation of garnet overgrowths and coronas during M1B. The stability of garnet is greatly increased to lower pressure and temperature in fluid-absent, fluid-undersaturated mafic rocks, indicating that fluid and melt loss during initial granulite facies metamorphism is essential for the introduction of garnet, and the formation of garnet coronas, during retrogression. The occurrence of garnet coronas is consistent with, but not unique to, near-isobaric cooling paths. [ABSTRACT FROM AUTHOR]

Recent activity-composition models for clinopyroxene and amphibole are revised to provide better consistency with observed phase relations in natural rocks. For clinopyroxene, the calibration in NCFMAS is retained, but the incorporation of acmite is revised to improve the partitioning of ferric iron between coexisting clinopyroxenes. For amphibole, the NCFMASH calibration is retained, but the addition of ferric iron is changed to provide consistency with the clinopyroxenes. The thermodynamics of orthoamphibole (gedrite) is also adjusted to resolve an unrelated inconsistency. The effects of these improvements are illustrated through comparison of calculated pseudosections produced with the existing and new models with natural data from lawsonite eclogites. [ABSTRACT FROM AUTHOR]

Ferric iron is present in all metamorphic rocks and has the ability to significantly affect their phase relations. However, the influence of ferric iron has commonly been ignored, or at least not been considered quantitatively, mainly because its abundance in rocks and minerals is not determined by routine analytical techniques. Mineral equilibria calculations that explicitly account for ferric iron can be used to examine its effect on the phase relations in rocks and, in principle, allow the estimation of the oxidation state of rocks. This is illustrated with calculated pseudosections in NCKFMASHTO for mafic and pelitic rock compositions. In addition, it is shown that ferric iron has the capacity to significantly increase the stability of the corundum + quartz assemblage, making it possible for this assemblage to exist at crustal P– T conditions in oxidized rocks of appropriate composition. [ABSTRACT FROM AUTHOR]

Lower temperature eclogite (with T = 600 °C) represents a significant part of the occurrences of eclogite in orogenic belts. ‘True’ eclogite, with, for example, garnet + omphacite >70%, is well represented in such an occurrence. Calculated phase equilibria in Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O (NCKFMASHTO), for just one rock composition – that of a representative mid-ocean ridge basalt,morb– are used to see under what circumstances ‘true’ eclogite is predicted to occur. The variables considered are not only pressure ( P) and temperature ( T) but also water content and oxidation state. The latter two variables are known to exert a significant control on mineral assemblage but are difficult to establish retrospectively from the observed rocks themselves. It is found that whereas oxidation state does have a strong effect on mineral assemblage, the key control on developing ‘true’ eclogite is shown to be temperature and water content. If temperature is established to be <600 °C, water content has to be low (less or much less than that for H2O saturation) in order for ‘true’ eclogite to form. Moreover, unless pressure is at the high end in the range considered, lawsonite eclogite and ‘true’ eclogite will tend to be mutually exclusive, with the former requiring high water content at the lower temperature where it occurs, but the latter requiring low water content. [ABSTRACT FROM AUTHOR]

A recent thermodynamic model for the Na–Ca clinoamphiboles in the system Na2O–CaO–FeO–MgO–Al2O3–SiO2–H2O–O (NCFMASHO), is improved, and extended to include cummingtonite–grunerite and the orthoamphiboles, anthophyllite and gedrite. The clinoamphibole model in NCMASH is adopted, but the extension into the FeO- and Fe2O3-bearing systems is revised to provide thermodynamic consistency and better agreement with natural assemblage data. The new model involves order–disorder of Fe–Mg between the M2, M13 and M4 sites in the amphibole structure, calibrated using the experimental data on site distributions in cummingtonite–grunerite. In the independent set of end-members used to represent the thermodynamics, grunerite (rather than ferroactinolite) is used for FeO, with two ordered Fe–Mg end-members, and magnesioriebeckite (rather than ferritschermakite) is used for Fe2O3. Natural assemblage data for coexisting clinoamphiboles are used to constrain the interaction energies between the various amphibole end-members. For orthamphibole, the assumption is made that the site distributions and the non-ideal formulation is the same as for clinoamphibole. The data set end-members anthophyllite, ferroanthophyllite and gedrite, are used; for the others, they are based on the clinoamphibole end-members, with the necessary adjustments to their enthalpies constrained by natural assemblage data for coexisting clino- and orthoamphiboles. The efficacy of the models is illustrated with P– T grids and various pseudosections, with a particular emphasis on the prediction of mineral assemblages in ferric-bearing systems. [ABSTRACT FROM AUTHOR]

Subduction interfaces exhibit various slip styles, including slow slip events (SSEs). We use a micromechanics‐based approach to calculate the effective rheology of a shear zone containing ellipsoidal amphibolite clasts deforming by dislocation creep within an interconnected linear‐viscous phyllosilicate‐dominated matrix. Frictional failure occurs if local stress exceeds Mohr‐Coulomb yield strength. At moderate fluid overpressure, mixed‐frictional‐viscous behavior emerges at ∼ ${\sim} $350–560° ${}^{\circ}$C, consistent with a broad zone of mixed fault slip behavior without requiring extreme fluid overpressures. Increasing stress in this transition zone promotes local frictional failure and raises bulk strain rate. If, however, the bulk strain rate increases by more than one order of magnitude, system‐wide frictional sliding becomes preferable. This strain rate increase is insufficient to explain the slip rates observed in geophysically detectable SSEs. Therefore, viscous matrix flow as modeled here cannot explain SSEs without either invoking dynamic weakening within a frictional‐viscous flow or a mechanism switch to dominantly frictional sliding. Plain Language Summary: Subduction plate boundaries are locked near the Earth's surface and will release the stored energy as earthquakes. Subduction zones creep steadily and viscously at deeper depths where temperatures and pressures are high. At the depth of the transition from earthquakes to steady creep, episodic aseismic slip is often observed. Rocks from this region are mixtures of strong, fractured clasts surrounded by a weak matrix. The observations of exhumed rocks suggest that the episodic, aseismic slip may nucleate when local frictional failure occurs in strong clasts, but the surrounding weak matrix stops this failure from generating major earthquakes. However, it is unclear how much the small‐scale rock behavior could be linked to the large‐scale slip. We use a numerical model to simulate the interplay between frictional and viscous creep and calculate the overall behavior of the subduction zone plate boundary. We explore how the slip style changes with depth and determine the transition zone's depth/temperature range. In the transition zone, a small increase in stress or decrease in strength can lead to a change from pure viscous flow to frictional sliding. This study overcomes the scale challenge between the small‐scale features preserved on outcrops and the large‐scale geophysical observations. Key Points: Frictional‐viscous flow in a two‐phase shear zone modeled by a multiscale approach occurs at ∼350–560°C with moderate fluid overpressureStress loading and/or fluid pressure weakening can cause a switch from steady viscous creep to transient frictional slipViscous creep modeled here can accommodate tectonic strain rates but not slow slip events without invoking a switch to frictional sliding [ABSTRACT FROM AUTHOR]

The present continental crust is characterized by a felsic upper crust and a mafic lower crust, resulting from significant geochemical differentiation over geological time. While various processes have been proposed to explain this differentiation, subduction zones remain pivotal regions for understanding the compositional evolution of continental crust. This study focuses on the South Altyn (SA) continental subduction‐collision belt in western China, a unique setting that experienced ultra‐deep (>300 km) continental subduction followed by multi‐stage exhumation. We present a comprehensive study of four granitoid suites from Tatelekebulake (TTLK) area in SA: biotite granite (BG), monzogranite (MG), K‐feldspar granite (KG), and leucogranite (LG). Comprehensive studies on petrology, geochemistry and zircon U‐Pb dating show that these granitoids formed at 494, 451, 414, and 418 Ma, respectively, and originated from protoliths with affinity to the subducted continental crust in SA. Phase equilibrium modeling suggests that BG formed at ∼800°C and 0.6 GPa, while the MG, KG, and LG formed by differentiation crystallization of the BG magma under progressively decreasing temperature and pressure conditions (750°C, 0.5 GPa; 740–700°C, 0.2 GPa; and 700–640°C, 0.1 GPa, respectively). These results, combined with previous studies, allow us to reconstruct the tectonic processes of continental exhumation and subsequent orogenic collapse in SA during the Early Paleozoic. Importantly, our findings reveal that magmatism derived from partial melting of subducted continental crust can promote the geochemical evolution of continental crust toward more felsic compositions, even in the absence of significant crustal growth or mantle‐derived magmatism. This study provides a valuable case for understanding the compositional evolution of continental crust in deep subduction zones and challenges conventional models that rely heavily on arc magmatism for crustal differentiation. Moreover, our results contribute to a broader understanding of crustal evolution processes in collisional orogens worldwide and highlight the importance of recycling and differentiation of subducted continental material in shaping crustal compositions. Plain Language Summary: The geochemical evolution of the continental crust, from mafic composition 3 billion years ago to current composition stratified structure, is commonly believed to be driven by crustal growth through arc magmatism and undergoes a series of subsequent complex geodynamic processes. The study of the Early Paleozoic multi‐stage magmatism related to continental crust subduction and exhumation in the Tatelekebulake region of South Altyn western China reveals the differentiation, evolution and material cycling occurring in the absence of prominent arc magmatism mechanisms within the upper to middle crust range and provides a new scenario of the evolution of the continental crust, and crustal growth. This process could drive the upper continental crust toward a more felsic composition, while the middle to lower continental crust becomes more mafic characteristics. Key Points: The geochemistry characteristics of the four samples suggest derivation from the subducted continental crustThe study revealed multi‐stage magmatism associated with continental crust subduction and subsequent exhumationPartial melting of subducted continental crust can promote the geochemical evolution of continental crust toward more felsic compositions [ABSTRACT FROM AUTHOR]

Understanding the underlying mechanisms of strain localization in the Earth's lithosphere is crucial for explaining the mechanics of tectonic plate boundaries and various failure-assisted geophysical phenomena, such as earthquakes. Geological field observations suggest that shear zones are the most important lithospheric structures demonstrating intense shear localization at plate boundaries, accommodating a major portion of tectonic deformations. Despite extensive studies over the past several decades, the factors governing how shear zones accommodate bulk shear, whether via distributed strain (i.e. the development of macroscopic S (schistosity) foliations normal to the principal shortening strain axis) or via localized shearing (i.e. the formation of shear-parallel C bands, where C refers to the French "cisaillement" (shear)), remain largely unexplored. This study aims to address this gap in knowledge by providing observational evidence of varying S and C development in crustal shear zones from two geological terrains in eastern India. These field observations are complemented by 2D viscoplastic numerical simulations within a strain-softening rheological framework to constrain the factors controlling two competing shear accommodation mechanisms: distributed strain accumulation and shear band formation. The model-based analysis recognizes the bulk shear rate (γ˙b) , initial viscosity (ηv), and initial cohesion (Ci) of a shear zone as the most critical factors determining the dominance of one mechanism over the other. For a given Ci value, low γ˙b and ηv values facilitate the formation of S foliation (uniformly distributed strain), which transitions to a C-dominated shear accommodation mechanism as ηv increases. However, increasing γ˙b facilitates shear accommodation through a combination of the two mechanisms, leading to S–C structures. The article finally discusses the conditions under which shear zones can significantly intensify rates of localized shear, producing rapid slip events, such as frictional melting and seismic activities. [ABSTRACT FROM AUTHOR]

The Central lapetus Magmatic Province (CIMP) is a large igneous province (LIP) emplaced in the Baltican and Laurentian paleocontinents that marks the onset of the Caledonian Wilson Cycle. Paleozoic magmatism of the CIMP is preserved in both northeastern America and northern Europe. This study investigates rocks belonging to the hyper-extended margin of Baltica currently found in the Seve Nappe Complex of the Scandinavian Caledonides. Specifically, U-Pb zircon geochronology and whole-rock geochemistry are applied to a migmatitic variety of the Vierrucohkka amphibolite of the Marma Terrane, to the Aurek gabbro, and amphibolite of the Aurek Assemblage exposed in the Seve Nappe Complex in the Kebnekaise region, northern Swedish Caledonides. U-Pb zircon geochronology yields crystallization ages of 626 ±7 Ma for the protolith of the Vierrucohkka amphibolite, and 614 ±2 Ma and 609 ±1 Ma for the emplacement of the Aurek gabbro and amphibolite protolith, respectively. A younger age of 599 ±3 Ma is recorded in the Vierrucohkka amphibolite and is interpreted as the age of partial melting and migmatization. The geochemical signatures of the rocks demonstrate crustal assimilation during the emplacement of their protoliths and modification due to prograde metamorphic processes during Caledonian subduction. The Vierrucohkka amphibolite and the Aurek Assemblage samples display upper and lower crustal assimilation, respectively. Trace elements (Dy, Sm, Lu, and Y) record the growth of metamorphic garnet, while elevated TiO2contents record the crystallization of metamorphic rutile. Nevertheless, high field strength elements (HSFE) and ANb suggesta depleted mantle source forthe magmas of the protoliths of the investigated rocks. Altogether, geochemical data indicate that the igneous activity recorded in the Vierrucohkka amphibolite and the Aurek Assemblage between c. 626-609 Ma is related to continental rifting processes associated with the opening of the lapetus Ocean. [ABSTRACT FROM AUTHOR]

In the Parktown Formation of the lower West Rand Group (Witwatersrand Supergroup), remarkably well-preserved soft-sedimentary deformation features (SSDs) occur in a number of locations in three stratigraphic horizons across the Vredefort Mountainland and in key sites in Johannesburg. The SSDs were the inevitable products of static liquefaction during early deposition in the Witwatersrand Basin. The trigger to static liquefaction is interpreted to be autogenic and related to cyclic fluctuations in pore fluid pressure(s) during capped dewatering. Allogenic triggers are not indicated. We suggest that static liquefaction was an integral physical process to sediment deposition on the Parktown Formation offshore platform, but a process not previously recognised as important to the modification of the West Rand Group stratigraphy across the Witwatersrand Basin. [ABSTRACT FROM AUTHOR]

The study of rock chemistry is a milestone in understanding fluid–rock interactions and fluid migration in subduction zones. When combined with thermodynamic models, it can provide direct insight into fluid composition, metasomatic reactions, and pressure–temperature (P – T) conditions, as well as their role in rock deformation. Here, a shear zone – located in the Mont Avic area of the Zermatt-Saas zone (Western Alps) – is analyzed. This shear zone consists of several blocks of different lithotypes, including a Ca-rich metasomatite block embedded in a serpentinite mylonitic matrix, and structurally underlies a coherent eclogitic mafic unit. This work aims to estimate the pressure–temperature conditions of the Ca-rich metasomatism and the amount of fluid involved. The brecciation exhibits mosaic breccia textures with clasts comprising ∼80 vol % of garnet, together with omphacite, epidote, titanite, rutile, and apatite hosted in an omphacite matrix. Quantitative chemical mapping of the garnet reveals primary garnet cores with embayment and lobate edges with a chemical composition similar to unaltered reference eclogite garnet. These primary garnet cores are overlain by Ca-rich metasomatic garnet rims with oscillatory chemical zoning. The oscillatory chemical zoning, together with the morphology of the primary garnet cores, suggests repeated influxes of external Ca-rich fluid that destabilized the primary garnet cores and promoted the growth of Ca-rich rims. Mass balance calculations between precursor metabasite and Ca-metasomatite indicate multiple fluid sources involving dehydrated serpentinite, calcic metasediments, and metabasites with time-integrated fluid fluxes calculated between 11.5×103 and 5.5×104 m fluid3 m rock-2 , consistent with channelized fluid flow in an open system. Thermodynamic modeling of garnet from unbrecciated and non-metasomatized metabasites – from the Savoney eclogitic mafic unit – indicates peak metamorphic conditions of 2.5±0.1 GPa and 535±40 °C, consistent with regional estimates. Pressure–temperature conditions of metasomatism were constrained using P – X and T – X phase modeling (where X represents changes in bulk CaO and Na 2 O composition) between 2.6–2.2 GPa and 570–500 °C, showing that Ca-rich fluid percolation occurred close to the metamorphic peak (i.e., prograde to the peak or early exhumation path). [ABSTRACT FROM AUTHOR]

Drawing on a broader study on perceptions of time and well-being during Covid-19, we show how governments can use social media platforms, such as Twitter, to acquire knowledge for policy learning and design. We argue social knowledge, which includes personal storytelling, emotion, and use of hashtags and emojis, can contribute to policy learning. Using a qualitative approach, we examine citizens' pandemic-related experiences, including changing work routines, mental health and self-care, sleep patterns, domestic violence, and feelings of solidarity. Such data could be useful to policymakers as they provide insights into the impact of the pandemic on citizens' everyday lives. [ABSTRACT FROM AUTHOR]

Purpose: The study aims to investigate how pregnant and nursing mothers' stories have been neglected in writing about gender, security and spaces. Design/methodology/approach: The study chronicles Agogo Traditional Area's pregnant and nursing mothers' resistance and survival in this conflicted environment. The author used photo voices in a participatory photography design to give conflict-area women a voice. Interviews and observations supported this. Pregnant and nursing mothers were sampled using the purposive and snowball sampling techniques. The data analysis considered narrative analysis, photographic and inductive approaches. Findings: The findings highlighted how these mothers in vicious settings experienced healthcare access and problems, societal issues including gender dynamics, food insecurity, and emotional and psychological well-being. Originality/value: The findings in this study expand the socio-cultural narratives of pregnant and nursing mothers in violent spaces. [ABSTRACT FROM AUTHOR]

Oxygen and hydrogen stable isotope analyses of quartz and muscovite veins from the footwall of the Raft River detachment shear zone (Utah) provide insight into the hydrology and fluid‐rock interactions during ductile deformation. Samples were collected from veins containing 90%–100% quartz with orientations either at a high angle or sub‐parallel to the surrounding quartzite mylonite foliation. Stable isotope analysis was performed on 10 samples and compared with previous quartzite mylonite isotope data sets. The results indicate that the fluid present during deformation of the shear zone was meteoric in origin, with a δ2H value of approximately −100‰ and a δ18O value of approximately −13.7‰. Oxygen stable isotope O18O depletion correlates with the muscovite content of the analyzed rocks. Many of the analyzed samples in this and other studies show an apparent lack of equilibrium between the oxygen and hydrogen isotope systems, which can be explained by hydrogen and oxygen isotope exchange at varying fluid‐rock ratios. Our results suggest that the Raft River detachment shear zone had a low static fluid‐rock ratio (<0.1), yet experienced episodic influxes of fluids through semi‐brittle structures. This fluid was then expelled out into the surrounding mylonite following progressive shearing, causing further 18O‐depletion and fluid‐related embrittlement. Key Points: Hydrogen isotope analyses revealed that the fluid that filled and precipitated in the quartz veins was meteoric in originDifferences in stable isotope depletion correlate with muscovite content, which may be related to the original porosity of the myloniteFluid‐rock ratio depletion modeling suggests that the DSZ was rock‐dominated with the highest fluid‐rock ratio of ∼0.1 [ABSTRACT FROM AUTHOR]

Despite the fact that rock textures depend on the 3D spatial distribution of minerals, our tectono-metamorphic reconstructions are mostly based on a 2D visualisation (i.e. thin sections). This work compares 2D and 3D investigations of petrography and microstructures, modal abundances, and local bulk rock composition and their implication for P – T estimates, showing the pros and cons and reliability of 2D analysis. For this purpose, a chloritoid–garnet-bearing mica schist from the Dora-Maira Massif in the Western Alps has been chosen. In particular, for 2D a thin section scan has been combined with chemical X-ray maps, whereas for 3D the X-ray computerised axial microtomography (µ CT) has been applied. Two-dimensional investigations are readily accessible and straightforward but do not consider the entire rock volume features. Conversely, the rise of 3D techniques offers a more comprehensive and realistic representation of metamorphic features in the 3D space. However, they are computationally intensive, requiring specialised tools and expertise. The choice between these approaches should be based on the research aims, available resources, and the level of detail needed to address specific scientific questions. Nevertheless, despite differences in the modal distribution, the estimated bulk rock compositions and relative thermodynamic modelled phase fields show similarities when comparing the 2D and 3D results. Also, since different thin section cut orientations may influence the results and consequent interpretations, three different cuts from the 3D model have been extrapolated and discussed (i.e. XZ, YZ, and XY planes of the finite-strain ellipsoid). This study quantitatively corroborates the reliability of the thin section approach for tectono-metamorphic reconstructions, still emphasising that 3D visualisation can help understand rock textures. [ABSTRACT FROM AUTHOR]

The brittle‐ductile rheological behavior in subduction zones is commonly proposed to explain deep transient slips. Generally observed at large scales in tectonic "mélanges", here we show that it is also observed at the grain scale in exhumed blueschist metagabbros. In these rocks, petrologic and microstructural observations show a bi‐phase material constituted by strong microfractured magmatic pyroxene clasts located in a weak and ductile lawsonite‐rich metamorphic matrix. To constrain the mechanical conditions allowing the brittle deformation of a clast in a ductile matrix, we used two‐dimensional simple shear numerical experiments. Results show four behaviors: (a) entirely brittle; (b) brittle‐ductile with clast fracturing in a ductile matrix; (c) ductile‐dominant with limited plastic deformation at clast edges; and (d) entirely ductile. We propose that the conditions of the brittle‐ductile behavior, commonly associated with deep transient slips, are controlled by the strength ratio between the strong brittle phase and the weak ductile phase. Plain Language Summary: In subduction zones, brittle‐ductile behavior is commonly proposed to explain deep transient slips at the subduction interface. This particular behavior is generally characterized by the fracturing of strong pods located in a weak fluid‐like material. In this study, we observe this mixed rheological behavior at the mineral‐scale in oceanic rocks under deep transient slip conditions. We carry out petrological and microstructural observations that show micro‐fracturing of strong magmatic clasts and ductile deformation of a weak metamorphic hydrated matrix. Numerical experiments, inspired by these observations, are used to constrain the physical conditions for this brittle‐ductile behavior. Numerical results show four types of behavior: (a) both matrix and clast are brittle and fractured; (b) the clast is brittle and fractured and the matrix is ductilely deformed; (c) only the clast is brittle and fractures are localized at clast edges; and (d) both matrix and clast are ductile. This study demonstrates that the behavior of this bi‐phase material is controlled by the strength ratio between the brittle strong clast and the ductile weak matrix. These physical conditions significantly differ from the theoretical rheological prediction and may be the key to a better understanding of the mechanics of deep transient slips. Key Points: The modeled brittle‐ductile rheological behavior occurs in a more limited temperature range than theoretically predictedThe strength ratio between the brittle yield stress and the dislocation creep stress predicts conditions for the brittle‐ductile behaviorThe temperature, pressure and pore‐fluid pressure conditions for the brittle‐ductile behavior are those of the deep transient slips [ABSTRACT FROM AUTHOR]

The increase in initial Hf isotopes identified in early Mesoarchean detrital zircon is commonly interpreted as a reflection of the geodynamic transition from stagnant‐lid to mobile‐lid tectonics. However, given the lack of petrogenetic context, interpreting detrital zircon may lead to spurious conclusions. In this contribution, we use zircon U‐Pb‐Hf‐O isotopic and bulk rock compositions of newly identified 3.05–2.9 Ga granitoids from the SW Yangtze Block to posit petrogenesis within an isotopically juvenile magmatic system. A statistical analysis of these data with a global igneous zircon Lu‐Hf isotopic compilation reveals an increase in average initial radiogenic Hf isotopes during the Paleoarchean to Mesoarchean transition. We posit that the Earth's continental crust underwent a global rejuvenation across the Paleo‐Mesoarchean transition. This rejuvenation can be explained by an independently observed increase in mantle temperatures resulting from mantle thermal evolution and does not require a change in tectonic style. Plain Language Summary: In this study, we explore the Earth's continental crust evolution before 3.0 billion years ago, a period characterized by limited preserved ancient rocks. We focus on zircon from igneous rocks, which provides a more reliable petrogenetic record than detrital zircon. Our study challenges the widely observed increase in detrital zircon initial Hf isotopes during the Paleo‐to Mesoarchean transition, often interpreted as a shift in tectonic regimes. We present new data from 3.05 to 2.9 billion‐year‐old granitoids in the Yangtze Block, suggesting isotopically juvenile magmatic activity. Statistical analysis of a global database of igneous zircon Hf isotopes reveals a state‐shift increase in εHf(t) across the Paleo‐to Mesoarchean transition, indicating global crustal rejuvenation at that time. Contrary to prevailing theories, we propose that this rejuvenation can be attributed to elevated mantle temperatures, challenging the notion of a significant geodynamic change during this period. Our findings emphasize the need to consider mantle thermal evolution in understanding continental crust growth on early Earth. Key Points: 3.05 and 2.9 Ga granitoids derived from juvenile crust are identified in the SW Yangtze BlockGlobal crust rejuvenation occurred across the Paleo‐to Mesoarchean transitionThis period of rejuvenation resulted from the effects of peak mantle temperatures [ABSTRACT FROM AUTHOR]

The study of metapelitic sillimanite- and garnet-bearing granulite xenoliths brought to the surface by the basanite of the 650 ka Tafraoute maar shed new light on the lower crust of the Tabular Middle Atlas (Morocco). Two main types of granulites are distinguished: (1) layered quartzo-feldspathic and (2) unlayered restitic. Mineralogy, petrology, P-T estimates and EBSD data support that these granulites underwent two successive tectono-metamorphic events, before their entrapment in lava. During the first event, probably the Hercynian orogeny, the Tafraoute lower crust acquired its foliation and primary paragenesis likely including kyanite: it yields P, T conditions of 1.1 ± 0.1 GPa and 850–880 °C. The second event corresponds to a reheating up to ultrahigh temperatures (1050 ± 50 °C) under slightly lower pressure conditions (0.9 ± 0.1 GPa). This led first to the transformation of kyanite into large prismatic sillimanite. The latter displays uncommon evidence of dislocation-creep deformation of moderate intensity that points to a tectonic episode occurring after their formation. After deformation has stopped, a reaction between sillimanite and garnet resulted in the crystallization of orthopyroxene and spinel deformation-free coronas around garnets. Approaching the peak of temperature, anhydrous partial melting of quartzo-feldspathic layers likely occurred and the resulting felsic melt spread into the rocks. This reheating event might be the consequence of the Late Permian to Mid-Jurassic rifting that preceded the formation of the Middle Atlas range, possibly associated with underplating of hot gabbroic magma. This event was followed by gradual cooling down to ~800 °C, leading to static crystallization of the felsic melt in the quartzo-feldspathic granulites. The last event susceptible to have affected the lower crust is the alkali magmatism active in the Middle Atlas during the Mio-Plio-Quaternary. In this context, the origin of restitic granulites is questionable. It may result either from the thermal event associated to the pre-alpine rifting, or from the emplacement of basaltic dykes in the lower crust before the quaternary eruption of the Tafraoute volcano. During this eruption, the studied granulites were entrapped in the ascending lava and very quickly transferred up to the surface, triggering the formation of small vesicular glass pockets. This study highlights the contrasted post-Hercynian evolution of the lower crust in the northern coastal alpine orogen (Rif) and the Tabular Middle Atlas: the first one underwent a tectonic exhumation without reheating during the Alpine orogeny, while the second one is characterized by a reheating to ultra-high temperature, probably during the pre-alpine rifting, but was probably not or very little affected by the alpine events. [ABSTRACT FROM AUTHOR]

The Haib porphyry copper deposit is situated in the Richtersveld Subprovince and is host to unique Palaeoproterozoic porphyry copper mineralisation in Namibia. Several lines of evidence, including machine-learning geothermobarometry, indicate that the deposit is exposed at mid- to upper-crustal levels, as constrained from average pressure and temperature estimates of 4 kbar and 870°C, respectively. The Haib porphyry copper deposit is associated with, and is in close proximity to, a mafic-ultramafic intrusion named the Kokerboom Intrusion (KI) in this study. Together with several other mafic intrusions in this region, these intrusions are known collectively as the Vuurdood Subsuite of the Richtersveld Subprovince. The purpose of this study is to demonstrate a genetic link between the KI and the Vuurdood Subsuite using lithological characteristics, mineral and alteration assemblages, major and trace element geochemistry and U-Pb geochronology. Pyroxenites from the KI contain magmatic sulphides and have a geochemical affinity with shallow plutonic and volcanic rocks of the Haib porphyry, providing a unique mid-crustal perspective on porphyry copper deposit metallogenesis. [ABSTRACT FROM AUTHOR]

The Pampean flat slab in central Chile and Argentina is characterized by the inland migration and subsequent cessation of arc volcanism since the mid‐Miocene. Slab flattening also affects the distribution and number of intermediate‐depth earthquakes and the evolution of the overlying continental thermal structure. In this study, we combine thermal‐mechanical models with petrological models to examine temporal changes in pressure, temperature, and composition during flat‐slab subduction and estimate water carrying capacity, predicted melt distributions and predicted changes in melt composition. Model results indicate that the present‐day flattened Nazca plate carries water to ∼700 km inland from the trench and could cause flux melting if the material above the slab remains fertile. Observed slab seismicity matches areas where hydrated materials have ∼>3 wt% H2O in the oceanic crust and mantle lithosphere. Seismicity increases as slab water carrying capacity decreases (slab dehydration). As P‐T conditions and compositions of the rock trapped above the slab change during slab flattening, flux melting switches from a peridotite‐dominated early phase to a combined mid‐ocean ridge basalt/eclogite and peridotite melting at ∼8 Ma. The results provide broad consistency with known earthquake distributions, seismic velocities, and observed temporal and spatial changes in volcanic patterns above the Pampean flat slab and point toward the role of melt depletion in the decrease and ultimate cessation of arc volcanism in this region. Key Points: We estimate the water carrying capacities, melt distributions, and composition during the Pampean flat‐slab subductionThe predicted hydrated areas match the observed slab seismicity in the Pampean regionFlux melting above the flat slab is predicted to migrate inland, providing constraints on the causes of the spatiotemporal changes in magmatism [ABSTRACT FROM AUTHOR]

Exhumed serpentinites are fragments of ancient oceanic lithosphere or mantle wedge that record deep fluid‐rock interactions and metasomatic processes. While common in suture zones after closure of ocean basins, in non‐collisional orogens their origin and tectonic significance are not fully understood. We study serpentinite samples from five river basins in a segment of the non‐collisional Andean orogen in Ecuador (Cordillera Real). All samples are fully serpentinized with antigorite as the main polymorph, while spinel is the only relic phase. Watershed delineation analysis and in‐situ B isotope data suggest four serpentinite sources, linked to mantle wedge (δ11B = ∼−10.6 to −0.03‰) and obducted ophiolite (δ11B = −2.51 to +5.73‰) bodies, likely associated with Triassic, Jurassic‐Early Cretaceous, and potentially Late Cretaceous‐Paleocene high‐pressure (HP)–low‐temperature metamorphic sequences. Whole‐rock trace element data and in‐situ B isotopes favor serpentinization by a crust‐derived metamorphic fluid. Thermodynamic modeling in two samples suggests serpentinization at ∼550–500°C and pressures from 2.5 to 2.2 GPa and 1.0–0.6 GPa for two localities. Both samples record a subsequent overprint at ∼1.5–0.5 GPa and 680–660°C. In the Andes, regional phases of slab rollback have been reported since the mid‐Paleozoic to Late Cretaceous. This tectonic scenario favors the extrusion of HP rocks into the forearc and the opening of back‐arc basins. Subsequent compressional phases trigger short‐lived subduction in the back‐arc that culminates with ophiolite obduction and associated metamorphic rock exhumation. Thus, we propose that serpentinites in non‐collisional orogens are sourced from extruded slivers of mantle wedge in the forearc or obducted ophiolite sequences associated with regional back‐arc basins. Plain Language Summary: Serpentinites are metamorphic rock products of fluid‐mediated alteration of the mantle. They occur in the ocean floor and the core of mountain belts resulting from continental collisions after the closure of ancient oceanic basins. However, their origin in non‐collisional mountain belts, such as the Andes, remains unclear. To address this conundrum, we studied serpentinite boulders from five river basins in the Ecuadorian northern Cordillera Real. We found that rocks are composed of the high‐temperature serpentine mineral, while spinel is the only original mineral preserved. River basin analysis and boron stable isotopes indicate four potential sources for the studied rocks, juxtaposed to rocks ranging in age from ∼240 to 55 million. Bulk‐rock chemistry and boron isotopes suggest that the serpentinization was triggered by crustal fluids at depths between 80 and 30 km in a subduction zone environment. Through time, the Andes have been characterized by extensional and compressional tectonic phases. These tectonic scenarios enhance the extraction of rocks at deep sections of the Earth along major faults. We propose that Andean serpentinites are fragments of the Earth's mantle sourced from ancient subduction zones and back‐arc basins. Key Points: Serpentinites associated with HP–LT rocks are common in the Andes, but their origin and tectonic significance are not fully understoodOur results in Cordillera Real serpentinites suggest four sources derived from the mantle wedge and obducted ophiolitesSerpentinites in non‐collisional orogens are exhumed during slab rollback and back‐arc basin closure phases [ABSTRACT FROM AUTHOR]

The 3D modelling of basic bodies of the Koperberg Suite (1060 to 1030 Ma) and their wall rocks from Narrap Mine illustrates the distribution, geometries and, by implication, processes that determined the ascent and emplacement of the mantle-derived mafic magmas into the partially-molten, mid-crustal granite-gneiss sequence of the Okiep Copper District in Namaqualand. The lens-like, discontinuous geometry of basic bodies suggests the transfer of the mafic magmas as self-contained, buoyancy-driven hydrofractures. The presence of both shallowly-dipping, foliation-parallel sills and subvertical lenses in zones of steep foliation development, so-called steep structures, indicates an emplacement of the mafic magmas under low deviatoric stresses and irrespective of the regional stress field. Instead, the emplacement of the mafic magmas parallel to pre-existing anisotropies (tectonic fabrics or lithological contacts) highlights those differences in tensile strength and fracture toughness parallel to or across anisotropies determined the propagation of the magmas. This also accounts for the common occurrence of basic bodies in steep structures in which the vertical gneissosity promoted the buoyancy-driven ascent of the mafic magmas. On a regional scale, the mechanical stratification of the subhorizontal, sheet-like granite gneisses and interlayered metasediments exerted important controls on the ascent of Koperberg Suite magmas. The preferential emplacement of basic bodies in schist and gneiss units suggests that the lower rigidity of the ductile wall rocks facilitated magma emplacement through a combination of viscous wall-rock deformation and fracture blunting that led to the arrest of the magma-filled hydrofractures. Multiple intrusive relationships of successively emplaced magma batches suggest that later magmas reutilised earlier established magma pathways, particularly in steep structures. High-rigidity lithologies, such as the massive Springbok Quartzite, in contrast, only allowed for smaller fracture apertures and limited dilation, resulting in the pinching of basic bodies and rather stringer-like geometries. It is conceivable that the higher fracture toughness of the quartzite may also have prevented propagation of the mafic magmas through the Springbok Quartzite and, instead, led to the ponding of basic bodies below the metasediments. The geometry and structural and lithological controls of basic bodies at Narrap Mine are similar to Koperberg Suite intrusions documented from many of the other mine workings in the Okiep Copper District. This suggests similar underlying emplacement controls of the cupriferous rocks, which can be extrapolated on a regional scale and that may guide exploration. [ABSTRACT FROM AUTHOR]

Due to the inherently fluid‐mobile nature of W, the 182W record of the early Earth may have been obscured by fluid‐induced mobilization of W. To investigate W mobilization in Archean greenstone sequences, we analyzed 182W isotope systematics and major and trace element concentrations in samples from the 3.53 Ga old Onverwacht Group of the Kaapvaal Craton (South Africa) and the >3.51 Ga old Badampahar Group of the Singhbhum Craton (India). Our results for mafic and ultramafic metavolcanic rocks show W/Th ratios significantly higher than primary magmatic values, which suggests fluid‐induced W enrichment. Samples least affected by secondary W enrichment (W/Th < 0.26) show no resolvable W isotope anomalies from modern mantle values in both cratons. Samples from the Kaapvaal Craton with elevated W/Th exhibit deficits in 182W as low as −8.1 ± 4.3 ppm compared to the modern mantle. Covariations of μ182W with W/Th, and Ce/Pb suggest that negative isotope signatures were introduced during secondary fluid‐mediated processes. The enrichment of W is most evident in altered ultramafic rocks comprising serpentine, resulting in additional covariations between MgO, LOI, and W/Th. The W isotope composition of serpentinized komatiites reflects the composition of younger intruding granitoids. We therefore infer the latter as a possible source of W‐rich fluids. The Badampahar Group samples exhibit little W isotope variability. A well‐resolved 182W deficit of −6.2 ± 2.9 ppm was determined in a single komatiite sample, which indicates an unknown fluid source, currently not represented in any other unit of the Singhbhum Craton. Plain Language Summary: The tungsten (W) isotope composition of ancient rocks can be used to trace processes that occurred during Earth's early evolution. However, interactions between rocks and fluids may alter the W concentration and therefore influence the interpretation of W isotope data. To identify the source of such fluids and the processes by which they affect the W isotope composition of rocks, we analyzed ancient rock samples from South Africa and India. The isotope composition of rocks with a low W concentration reflects that of the modern Earth. Therefore, they do not trace the processes that occurred during Earth's early evolution. Samples from South Africa with untypically high W concentrations show a different isotopic composition. The variation in the W isotope signature correlates with other chemical indices that are susceptible to modification by fluid‐related processes. This shows that the W within the rocks is derived from an external fluid source and not from their original magmatic source. Samples with the highest W enrichment have a similar isotope composition as spatially associated intrusive rocks. By inference, the latter likely represent the source of W‐rich fluids. The samples from India show similar enrichment in W, indicating similar fluid‐related processes and W sources at both localities. Key Points: The magmatic sources of metavolcanic rocks from the Onverwacht Group and the Badampahar Group do not exhibit W isotope anomaliesNegative W isotope signatures in the Onverwacht Group are likely derived from fluids sourced from younger intrusive granitoidsFelsic intrusive rocks are a major source of W‐rich fluids in Paleoarchean greenstone units [ABSTRACT FROM AUTHOR]

Recent geophysical campaigns in the Alps produce images with seismic property variations along the slab of sufficiently fine resolution to be interpreted as rock transformations. Since the reacting European lower crust is presumed responsible for the variations of velocities at the top of the Alpine slab, we sampled local analogs of the lower crustal lithologies in the field and modeled the evolution of equilibrium seismic properties during burial, along possible pressure‐temperature paths for the crustal portion of the slab. The results are then compared to the range of the S‐wave velocities obtained from the S‐wave velocity tomography model along the CIFALPS transect. The velocity increase from 25 to 45 km within the slab, in the tomographic model is best reproduced by the transformation of specific lithologies in the high‐pressure granulite facies along a collisional gradient (30°C/km). Although the crust is certainly not completely homogeneous, the best candidates for the rocks that make up the top of the Alpine dip crustal panel are a kinzigite from Monte San Petrone, a gneiss from the Insubric line, and blueschist mylonite from Canavese. While they may not represent the entirety of the crust, they are sufficient to explain the tomographic velocity of the Alpine slab. A lateral lithological contrast inherited from the Variscan orogeny is not required. Eclogitization, suggested as the first‐order transformation in convergence zones, could be a second‐order transformation in collisional wedges. These results also imply a partially re‐equilibrated thermal gradient, consistent with the Alpine thermal state data at depth. Plain Language Summary: Tomography, that is, imaging of deep geological structures based on seismic wave travel to time anomalies, is now so sensitive that it allows us to see changes in the properties of rocks buried at depth beneath mountain belts. In the Alps, the European plate is imaged down to 80 km and shows a sharp velocity increase close to 30 km depth. By calculating the bulk seismic wave velocities on exhumed analogs sampled throughout the Western Alps, the present study proposes to interpret the velocity jump as the consequence of the transformation of the European lower crust from amphibolite to granulite (the high‐temperature metamorphic rocks produced during a collision) rather than the usually admitted transformation to eclogite (the higher pressure metamorphic rocks produced during subduction). This has implications regarding the present‐day thermal structure of the Alps: the Western Alps are not a frozen subduction zone but a collision zone exposing subduction‐related rocks and structures at the surface only. Key Points: The lower crustal top of the European Alpine slab is mostly composed of felsic to intermediate rocksThe transformation of hydrated rocks into HP granulites along a collision gradient reproduces the slab tomographic velocity increase at 30 kmThe often‐supposed eclogitization produces velocities that are significantly higher than the crustal top of the European Alpine slab [ABSTRACT FROM AUTHOR]

Changing thermal regime is one of the key mechanisms driving seismogenic behaviors at cold megathrusts, but it is difficult to interpret warm subduction zones such as Vanuatu for the temperatures are higher than that accommodates shallow brittle failures. We construct a 3-D thermomechanical model to clarify the thermal structure that controls tectonic seismicity in Vanuatu and predict a warm circumstance associated with abundant seismicity. Results reveal a heterogeneous slab ranging from 300 °C to over 900 °C from the Moho to subvolcanic depth. The subduction seismicity corresponds well to the plate interface where dynamic thermal dehydration is focused. The transformation from hydrated basalts to eclogites along the slab facilitates the occurrence of intense earthquakes and slips. Multistage mineralogical metamorphism affects the dynamic stability of megathrusts and favors the generation of active interplate large events. Therefore, slab thermal dehydration plays a greater role than slab temperature condition in influencing the subduction earthquake distribution in warm subduction systems. [ABSTRACT FROM AUTHOR]

The presence of eclogite marks high‐pressure metamorphism and deep, cold burial of crust, and is therefore a widely accepted indicator for the onset of modern‐style plate tectonics. However, eclogite is rarely preserved in many heavily granulite‐overprinted orogens, which are particularly common across ancient metamorphic terranes (i.e., Archean and Paleoproterozoic terranes). Here, we show that eclogite melting processes may be principally responsible for the poor preservation of high‐pressure records. We demonstrate eclogite melting via detailed petrological, geochronological, and geochemical analyses on eclogites and separated centimetric leucosomes from the central Himalaya. The central Himalayan eclogites were overprinted by strong granulite‐facies metamorphism, such that omphacite is only sparsely preserved. Thermodynamic modeling results indicate the eclogite experienced two types of anatectic reactions: phengite dehydration melting at high‐pressure in the presence of omphacite, and subsequent omphacite dominated melting during exhumation. Diagnostic features for these melting reactions include (a) the occurrence of Kfs‐Pl‐Qz multiphase solid inclusions and Cpx‐Pl‐Qz polymineralic inclusions; (b) the high Na2O/K2O melts and Na‐rich plagioclase in leucosomes; (c) the consistent trace elements patterns between omphacite and leucosomes; and (d) obvious Na2O zonation in clinopyroxene. Omphacite dominated melting is characterized by the jadeite breakdown, releasing Na and Al into the melts. This melting mechanism subsequently forms a less sodic clinopyroxene and high Na2O/K2O melts. These Himalayan findings appear relevant to early Earth explorations because of the high thermal gradients and intense granulite‐facies overprints, implying that partial melting of eclogite dominated by omphacite breakdown could have erased early high‐pressure records of modern‐style plate tectonics. Plain Language Summary: Eclogites are high‐pressure metamorphic rocks mainly composed of garnet and omphacite. The occurrence of high‐pressure and low‐temperature eclogite is representative of modern‐style tectonics. Eclogite partial melting is an important process for material transfer in subduction zones and has been regarded as an important mode of continental growth. In this paper, we found that the central Himalayan eclogites underwent two types of melting reactions: phengite breakdown in the presence of omphacite in the high‐pressure eclogite‐facies domain and subsequent omphacite dominated melting on the exhumation. These melting reactions make the breakdown of jadeite in omphacite and form a less sodic clinopyroxene. Considering the similar high thermal gradients and intense granulite‐facies overprint, we propose that eclogite melting by omphacite breakdown could be a viable mechanism for the rare preservation of high‐pressure records on early earth (Archean and Paleoproterozoic). Key Points: The granulitized eclogite has experienced omphacite dominated melting with phengite contributions at eclogite‐facies stageMelts from omphacite breakdown have low Nb/Ta and high Zr/Sm, similar to tonalite‐trondhjemite‐granodioritesOmphacite dominated melting could erase early high‐pressure records of modern‐style tectonics [ABSTRACT FROM AUTHOR]

Serpentinites are polymineralic rocks distributed almost ubiquitously across the globe in active tectonic regions. Magnetite-rich serpentinites are found in the low-strain domains of serpentinite shear zones, which act as potential sites of nucleation of unstable slip. To assess the potential of earthquake nucleation in these materials, we investigate the link between mechanical properties and fabric of these rocks through a suite of laboratory shear experiments. Our experiments were done at room temperature and cover a range of normal stress and slip velocity from 25 to 100 MPa and 0.3 to 300 µm s−1, respectively. We show that magnetite-rich serpentinites are ideal materials since they display strong sensitivity to the loading rate and are susceptible to nucleation of unstable slip, especially at low forcing slip velocities. We also aim at the integration of mechanical and microstructural results to describe the underlying mechanisms that produce the macroscopic behaviour. We show that mineralogical composition and mineral structure dictates the coexistence of two deformation mechanisms leading to stable and unstable slip. The weakness of phyllosilicates allows for creep during the interseismic phase of the laboratory seismic cycle while favouring the restoration of a load-bearing granular framework, responsible of the nucleation of unstable events. During dynamic slip, fault zone shear fabric determines the mode of slip, producing either asymmetric or Gaussian slip time functions for either fast or slow events. We report rate/state friction parameters and integrate our mechanical data with microstructural observations to shed light on the mechanisms dictating the complexity of laboratory earthquakes. We show that mineralogical and fabric heterogeneities control fault slip behaviour. [ABSTRACT FROM AUTHOR]

The use of NanoSIMS on primary melt inclusions in partially melted rocks is a powerful approach to clarify the budget of volatiles at depth during crust formation and its reworking. Anatectic melt inclusions are indeed gateways to quantify H 2 O, halogens and other species (e.g. CO 2 , N) partitioned into the deep partial melts generated during metamorphism of the continental crust. Here we present new datasets of NanoSIMS measurements of H 2 O and Cl in preserved melt inclusions from metamorphic rocks with different protoliths – magmatic or sedimentary – which underwent partial melting at different pressure–temperature–fluid conditions. These new datasets are then compared with similar data on natural anatectic melts available in the literature to date. Our study provides novel, precise constraints for the H 2 O content in natural melts formed at high pressure, a field previously investigated mostly via experiments. We also show that H 2 O heterogeneities in partial melts at the microscale are common, regardless of the rock protolith. Correlations between H 2 O contents and P – T values can be identified merging new and old data on anatectic inclusions via NanoSIMS. Overall, the data acquired so far indicate that silicate melt generation in nature always requires H 2 O, even for the hottest melts found so far (>1000 ∘ C). Moreover, in agreement with previous work, preserved glassy inclusions always appear to be poorer in H 2 O than crystallized ones, regardless of their chemical system and/or P – T conditions of formation. Finally, this study reports the very first NanoSIMS data on Cl (often in amounts >1000 ppm) acquired in situ on natural anatectic melts, showing how anatectic melt inclusions – additionally to magmatic ones – may become a powerful tool to clarify the role of halogens in many geological processes, not only in crustal evolution but also in ore deposit formation. [ABSTRACT FROM AUTHOR]

Surface deformation plays a key role in illuminating magma transport at active volcanoes, however, unambiguous separation of deep and shallow transport remains elusive. The Socorro Magma Body (SMB) lacks an upper crustal magma transport system, allowing us to link geodetic measurements with predictions of numerical models investigating rheologic heterogeneities and magma‐mush interaction in the mid‐/lower crust. New InSAR observations confirm that a pattern of central surface uplift surrounded by a region of subsidence (previously coined "sombrero" deformation) has persisted over >100 years at the SMB. Our models suggest this pattern may reflect the presence of a large (>100 km width), weaker‐than‐ambient, compliant region (CR) surrounding the mid‐crustal magma body. Interactions between a pressurizing (e.g., due to melt injection and/or volatile exsolution) sill‐like magma body and CR drive the sombrero pattern, depending on both viscoelastic relaxation and pressurization timescales, explaining its rare observation and transient nature. Plain Language Summary: Magma in the crust is transported and stored within magma bodies (regions that are mostly liquid magma) and "mush" (mostly solid crystals and some liquid magma). Mush zones are thought to be too viscous to be erupted but are likely to be weaker than the surrounding rock. To understand volcanic eruptions, it is important to understand the distribution of magma and mush, and their mutual interactions. Here we study these interactions in a mid‐crustal magma body, the Soccorro Magma Body (SMB), that does not have a surface volcano. Surface deformation at the SMB helps us study magma‐mush interaction, especially in the middle or lower crust. Previous surface deformation measurements at the SMB show "sombrero" deformation: a central area of uplift surrounded by a ring of subsidence. New satellite radar measurements are consistent with the previously reported pattern, confirming that this deformation remained remarkably constant through nearly 100 years. We suggest this is due to a large weak, mush region surrounding the SMB. Our computer models reproduce a long‐lasting, consistent sombrero deformation pattern depending on mush properties as well as pressurization history of the magma body, and we suggest these factors may explain why this pattern is relatively rare. Key Points: InSAR confirms coeval subsidence and uplift (a so‐called "sombrero" deformation pattern) persisted for >100 years at the Socorro Magma Body (SMB)A compliant region, modeled as a viscoelastic body surrounding a sill, is able to reproduce both the pattern and duration of deformationViscoelastic deformation within a broad compliant region supports the presence of mush zones at SMB and other mid‐crustal magma bodies [ABSTRACT FROM AUTHOR]

Slow slip and tremor is observed along many subduction margins and is commonly linked to fluid pressure variations and migration. Accurate estimates of porosity and permeability around subduction megathrust shear zones are vital for understanding fluid‐seismicity interactions. We use high‐resolution digital outcrop data and (micro)structural analysis to assess transient permeability and porosity of a deep‐seated subduction interface exposed on Syros Island, Greece. We document the orientations, relative timing, and opening aperture (based on crack‐seal textures) of veins that were emplaced synkinematically with ductile deformation during early exhumation within the subduction channel. Our findings indicate high permeability through vein‐filled fractures amidst a lower permeability matrix, with transient, fracture‐controlled permeabilities ranging from 10−14 to 10−15 m2 and fracture porosities of 1%–10%. These estimates align with low‐end values from seismological/geodetic observations in active subduction zones, and are also consistent with fault‐valve‐like numerical models that suggest high background‐to‐transient permeability contrasts favor unstable slip. Plain Language Summary: Understanding the physical mechanisms of fast and slow slip in subduction zones requires knowledge on how rocks respond when they are squeezed, fractured and deformed (rheology). Recent numerical studies show that fluid flow through rocks (permeability) is important in controlling their slip response. Geophysical and geological observations also suggest that fluids can cause mechanical instabilities, but quantitative data from natural observations is extremely scarce. To address this gap, we developed a new methodology to quantify the permeability of these rocks from natural observations. We study in detail the geometry, distribution and microstructures of vein systems (former paleo‐fractures filled with fluids) that opened and allowed for transient fluid circulation. From these data, we quantify the paleo‐permeability associated with these former fractures and compare our results to previous investigations and explore their implications in the frame of plate interface rheology and slip. Key Points: We present a new methodology to estimate subduction zone paleo‐permeabilityHigh permeability is associated with fracture‐controlled, channelized fluid flow at the base of the forearcStrong permeability contrast between fracture‐controlled (transient) and matrix‐related (background) fluid flow [ABSTRACT FROM AUTHOR]

The integration of garnet‐based petrologic constraints with multimineral geochronologic data in eclogites and blueschists allows the timing and rate of subduction zone metamorphism to be constrained. We present a combined garnet Lu–Hf/Sm–Nd and zircon/rutile U–Pb geochronology study on three eclogites, a garnet‐bearing blueschist, and a micaschist from the Changning–Menglian orogenic belt, a newly discovered ultrahigh‐pressure metamorphic belt in southeast Tibet, in order to characterize tectono‐metamorphic events and determine the duration of Paleo‐Tethys oceanic subduction. Integration of phase equilibrium modelling and conventional thermobarometry for the eclogites defines a clockwise P–T path evolving from blueschist facies conditions at ~1.4 GPa and ~505–530°C to peak eclogite facies conditions at ~2.8 GPa and ~630–640°C, followed by isothermal decompression to amphibolite facies at ~1.0 GPa and ~630–650°C. The Lu–Hf ages of c. 239–236 Ma obtained for the eclogites and the blueschist are indistinguishable from the rutile U–Pb age of c. 239 Ma obtained for the eclogites and, combined with the observation of well‐preserved Rayleigh‐fractionation‐style Mn and Lu zoning profiles in garnet, reflect the timing of early prograde garnet growth. The Sm–Nd ages of c. 242–236 Ma reflect a later period of garnet growth, evidenced by flat and/or M‐shaped Sm zoning profiles. Each of the Sm–Nd ages overlaps, within uncertainty, with its corresponding Lu–Hf age (i.e., from the same garnet fraction). The consistency of the Lu–Hf and Sm–Nd ages indicates a short overall duration of garnet growth from blueschist to eclogite facies metamorphism, reflecting rapid subduction of the oceanic slab. The magmatic zircon U–Pb dates of c. 247 Ma constrain the protolith age of these metabasaltic rocks. The close protolith and the high‐pressure metamorphic ages, together with the consistent garnet Lu–Hf and Sm–Nd ages and the overlapping youngest and oldest metamorphic ages of the oceanic‐type and continental‐type eclogites, respectively, suggest a fast tectonic transition from divergence to convergence highlighted by rapid oceanic subduction, continuous transition from oceanic to continental subduction, and a rapid cooling of the subduction interface. [ABSTRACT FROM AUTHOR]

High‐pressure rocks from the island of Ios in the Greek Cyclades were examined to resolve the P–T conditions reached during subduction of the two distinct lithotectonic units that are separated by the South Cycladic Shear Zone (SCSZ)—the footwall complex composed of Hercynian basement gneisses, schists and amphibolites, and the hangingwall complex composed of blueschists and eclogites. A combination of elastic tensor quartz inclusion in garnet (QuiG) barometry and Zr‐in‐rutile (ZiR) trace element thermometry was used to constrain minimum garnet growth conditions. Garnet from the hangingwall (blueschist) unit record formation pressures that range from 1.5 to 1.9 GPa and garnet from the footwall basement complex record garnet formation pressures of 1.65–2.05 GPa. ZiR thermometry on rutile inclusions within garnet establishes the minimum temperature for garnet formation to be ~480–500°C. That is, there is no evidence in the QuiG and ZiR results that the rocks of the blueschist hangingwall and basement experienced different metamorphic histories during subduction. This is the first reported observation of blueschist facies metamorphism in the Hercynian basement complex. A model is proposed in which initial subduction occurred along a relatively shallow P–T trajectory of ~11°C/km and then transitioned to a steeper, nearly isothermal trajectory at a depth of ~45 km reaching similar peak metamorphic conditions of ~500–525°C at 2.0 GPa for all samples. Such a change in the subduction path could be accomplished by either an increase in the rate of subduction or an increase in the angle of the subduction zone. The present juxtaposition of samples with contrasting mineral assemblages and garnet growth histories is interpreted to have arisen from differences in bulk compositions and variations in the preservation of high‐pressure prograde mineral assemblages during exhumation. The existence of similar P–T conditions and prograde paths in the two units does not require that the rocks were all metamorphosed at the same time and that the SCSZ experienced little movement. Rather, it is suggested that the two units experienced prograde and peak metamorphism at different times and were subsequently juxtaposed along the SCSZ. [ABSTRACT FROM AUTHOR]

High‐grade metamorphic rocks are widely exposed along the SE–NW‐ to E–W‐trending shear zones in the Truong Son Belt, Central Vietnam, but few petrological studies have been conducted in this area. Herein, we report the occurrence of mylonitized granulites that crop out along the Dai Loc shear zone in the southernmost Truong Son Belt. Detailed petrographic analysis, geochemistry and P–T–t estimates of the evolution of two granulite samples are presented to elucidate the formation processes of these high‐grade metamorphic rocks. The results indicate that the rocks underwent two distinct metamorphic cycles. The first cycle (M1) is characterized by coarse‐grained granulite mineral assemblages, defining a tight clockwise P–T path with near‐isobaric heating to a near ultrahigh‐temperature peak at low pressure, followed by cooling. The prograde mineral assemblage (M1a) is indicated by inclusions of cordierite + sillimanite + biotite + quartz + spinel ± plagioclase in coarse‐grained garnet, orthopyroxene and cordierite. The mineral assemblage of garnet + orthopyroxene + cordierite + plagioclase + K‐feldspar + ilmenite + melt ± biotite (M1b) defines the peak P–T conditions of 5.3–6.3 kbar and 850–920°C. Post‐peak cooling (M1c) is marked by the formation of quartz + biotite symplectites around garnet and orthopyroxene. The second cycle involved medium‐pressure amphibolite facies metamorphism (M2), characterized by domainal development of fine‐grained kyanite‐bearing mineral associations. Petrographic observations indicate that these fine‐grained associations were formed during mylonitization. Zircon U–Pb dating reveals that the timing of granulite facies metamorphism appears to be coeval with the intrusion of a post‐collisional granitoid at 430–410 Ma. Granulite facies metamorphism and crustal melting were probably driven by asthenospheric mantle upwelling triggered by slab breakoff during the Early Palaeozoic. Considering previous structural and geochronological studies, the second metamorphic event likely occurred during the Triassic Indosinian orogeny. [ABSTRACT FROM AUTHOR]

The melting behavior of subducted ophicarbonate is experimentally investigated under conditions relevant for slabs at subarc depths (2.5–6 GPa and 650–1000°C). Hydrous carbonatitic liquids appear between 900 and 950°C at 2.5 GPa, between 800 and 850°C at 4–5 GPa, and at ∼800°C over 6 GPa. Water significantly depresses the thermal stability of carbonate, which drives the formation of hydrous carbonatitic liquid at low temperatures realistic for most subduction zones. Ophicarbonate fully dehydrates prior to wet melting. From an experimental point of view, the melting of ophicarbonate is essentially equal to the fluid–present melting of magnesite–bearing wehrlite lithology (i.e., meta–ophicarbonate). In the natural subduction process, the melting of meta–ophicarbonate occurs only when external water is available assuming dehydration fluid by ophicarbonate has been efficiently expelled at shallower depths. In the framework of the previously constructed dehydration history of subducting slab, the flux melting of meta–ophicarbonate at the slab–mantle interface is examined to be feasible under majority of geothermal conditions, even in cold subduction zones. Thus, hydrous carbonatitic liquid acts as a pervasive agent to transfer carbon from the slab surface to the mantle wedge at shallow depths beneath arcs. In contrast, ophicarbonate within the slab–serpentinized mantle just undergoes dehydration, and the vast majority of carbon retained in these lithologies is likely transported deeper into the mantle. Plain Language Summary: Subduction zones are critical sites for carbon cycling between Earth's surface and interior. Dissolution of carbonate and metamorphic decarbonation are thought to be common decarbonation mechanisms for most subducting slabs, whereas melting is restricted to occur at relatively hot subduction zones. Ophicarbonate (i.e., carbonated serpentinite) may be a dominant carrier of both carbon and water in subduction zones where slow‐spreading oceanic crust is subducted, however, phase relations of subducted ophicarbonate are rarely studied under subarc conditions. Here we experimentally determine the melting behavior of ophicarbonate 2.5–6 GPa and 650–1000°C. Our results show that hydrous carbonatitic liquids appear between 900 and 950°C at 2.5 GPa, between 800 and 850°C at 4–5 GPa, and ∼800°C at 6 GPa. Water is essential for the formation of hydrous carbonatitic liquids by lowering the melting points of meta‐ophicarbonate. In the framework of the dehydration history of subducting slab, pervasive hydrous carbonatitic liquids could form by water‐induced melting of meta‐ophicarbonate at slab surface at most subduction zones. Therefore, carbonatitic liquids could play a much more important role than previously thought in transferring carbon from the slab surface to the subarc mantle. Key Points: Phase relations of subducted ophicarbonates were experimentally studied at 2.5–6 GPa and 650–1000°CHydrous carbonatitic liquids are observed between 900 and 950°C at 2.5 GPa, between 800 and 850°C at 4–5 GPa, and at ∼800°C over 6 GPaHydrous carbonatitic liquids act as a pervasive agent to transfer carbon from slab surface to the subarc mantle [ABSTRACT FROM AUTHOR]

The north‐dipping Hout River Shear Zone (HRSZ), which marks the boundary between the high‐grade Limpopo Complex in its hanging wall and the low‐grade granite‐greenstone terrane of the Kaapvaal Craton in South Africa in its footwall, is a deep‐crustal shear zone that controlled emplacement of hot Limpopo Complex granulites over and against low‐grade granite‐greenstones during exhumation at ~2.72–2.62 Ga. This major shear zone controlled migration of large volumes of hydrous fluids released during devolatilization of underthrusted greenstones that infiltrated into hot overlying granulites, establishing a retrograde anthophyllite‐in isograd and associated zone of retrograde rehydrated granulite that bounds the Limpopo Complex in the south. Here, we report new petrological data based on mineral equilibrium modelling of amphibolites from the Rhenosterkoppies Greenstone Belt (RGB) located in the immediate footwall of the HRSZ and discuss evidence that these rocks also interacted with high‐temperature fluids that infiltrated along the HRSZ. This study provides an important perspective of the interaction of hot granulites with underthrusted relatively cold greenschist‐facies rocks along the steeply north‐dipping section of the HRSZ, and shows that evidence for such interaction is restricted to a relatively narrow zone in the footwall of the HRSZ termed a hot‐iron‐zone. The peak mineral assemblage of a garnet‐free amphibolite from the RGB (magnesio‐hornblende + plagioclase + quartz + clinopyroxene + titanite + ilmenite) yielded the peak P–T condition of 7–8 kbar and 640–680°C with H2O content of 3.5–3.8 mol%. Clinopyroxene is replaced by actinolite + quartz and epidote + quartz symplectites by a post‐peak hydration event at <5 kbar and <590°C caused by elevated H2O content in the rock (>4.0 mol%), possibly related to fluid infiltration along the HRSZ. Mineral equilibria in a garnet‐bearing amphibolite records a prograde path from 5 to 7 kbar and 540–630°C to the peak conditions at 7.6–8.7 kbar and 620–650°C. A clockwise P–T path suggesting rapid compression and decompression has been inferred for the amphibolites from the RGB, possibly because of pressure increase related to overthrusting of the high‐grade terrane (Southern Marginal Zone of the Limpopo Complex) onto the low‐grade granite‐greenstone terrane, followed by rapid exhumation. The quantity of H2O associated with the amphibolite‐facies metamorphism in the RGB was elevated from the peak stage (M(H2O) = 3.5–3.8 mol%) to the retrograde stage (M(H2O) = 4.0–4.5 mol%) of metamorphism in the RGB, possibly suggesting fluid infiltration not only from internal sources but also from external sources. The results of this study indicate that a H2O‐bearing fluid, which infiltrated along the HRSZ, affected both the footwall and the hanging wall sections of the shear zone, although the effect on the footwall section was limited. [ABSTRACT FROM AUTHOR]