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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]

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]

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]

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]

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]