Eclogite-facies Mineral Research Articles

Magmatic genesis, hydration, and subduction of the tholeiitic eclogite-facies Allalin gabbro (Western Alps, Switzerland)

The Allalin gabbro of the Zermatt-Saas meta-ophiolite consists of variably metamorphosed Mg- to Fe-Ti-gabbros, troctolites, and anorthosites, which are crosscut by basaltic dykes. Field relationships of the various rock types and petrographic studies together with bulk rock and mineral chemical composition data allow the reconstruction of the complete geological history of the Allalin gabbro. With increasing magmatic differentiation, the incompatible element content in clinopyroxene increases (e.g., REEs and Zr by a factor of 5), whereas the Mg# decreases (from 86.4 to 74.6) as do the compatible element contents (e.g., Cr and Ni by factors of 3.5 and 5, respectively). Exhumation to shallower depths led to subsolidus ductile deformation and cooling of the gabbro followed by the intrusion of fine-grained basaltic dykes, which display chilled margins. Bulk rock data of these dykes reveal strong similarities in fluid-immobile trace element patterns to tholeiitic pillow basalts of the Zermatt-Saas and nearby meta-ophiolites. The recalculated REE patterns of the melt in equilibrium with igneous clinopyroxene is very similar to the REE patterns of the mafic dykes, indicating a cogenetic origin of pillow basalts, dykes, and gabbros. Together with the previously determined Jurassic intrusion age of the gabbro, our observations demonstrate that the Allalin gabbro intruded as a tholeiitic magma in a slow spreading MOR environment of the Piemonte-Ligurian ocean of the Alpine Tethys. Subduction of the Allalin gabbro resulted in different eclogitization extents of the Mg-gabbros as a function of variable hydration degrees. Metagabbros with low extents of hydration record incomplete eclogitization; the magmatic mineralogy (olivine + clinopyroxene + plagioclase) is preserved together with disequilibrium textures in the form of reaction coronae surrounding mineral boundaries. Metagabbros with high extents of hydration are completely eclogitized and display pseudomorphic replacement textures of magmatic minerals by eclogite-facies mineral assemblages, which required significant major to trace element transport across mineral domains. The locally variable extents of hydration took place near the sea floor, as recorded by the presence of Cl-apatite (6.28 wt% Cl), and an increase in B concentrations of minerals pseudomorphically replacing olivine (e.g., chlorite with 0.20–0.31 µg/g B and omphacite with 0.22–0.25 µg/g B) compared to magmatic olivine (0.12–0.16 µg/g B). Moreover, the chemical zonation pattern of metamorphic garnet coronae is different in completely eclogitized gabbros and gabbros with relic igneous minerals, in agreement with a main hydration event prior to subduction. The Allalin gabbro therefore represents a classical example of an oceanic gabbro formed in a slow spreading setting in the mid Jurassic that experienced heterogeneous hydration near the sea floor. Paleogene subduction of the gabbro to 70–80 km depth produced variably equilibrated gabbroic eclogites. In eclogite-facies Mg-gabbros, the water-rich minerals chlorite, talc, and chloritoid pseudomorphing magmatic olivine remained stable to these depths, revealing the potential relevance of hydrated Mg-gabbros as a fluid source at subarc depths in subduction zones.

On the occurrence of a Variscan eclogite in the Argentera-Mercantour Massif, Western Alps: Implications for the evolution of the southern Variscan belt

The newfound Bois de Sélasse eclogite in the eastern Argentera-Mercantour Massif (External Crystalline Massifs, Western Alps) is crucial for better constraining the tectonic evolution of the southern part of the European Variscan belt. The whole-rock composition of this eclogite aligns with that of a basaltic protolith with a normal mid-oceanic-ridge affinity, and U-Pb dating on igneous zircon cores reveals an emplacement age of 524 ± 5 Ma. The emplacement may have occurred either in the oceanic lithosphere to the north of the active Gondwana margin or within a back-arc basin during the subduction beneath Gondwana. Exceptionally preserved prehnite−pumpellyite to eclogite facies minerals provide evidence of prograde metamorphism along a standard oceanic subduction geotherm (≤10 °C/km). Peak eclogite facies conditions are constrained at 610−660 °C and 1.9−2.3 GPa by thermodynamic modeling combined with Ti-in-zircon and Zr-in-rutile thermometry. A minimum age for eclogite facies metamorphism is established at 339 ± 6 Ma by U-Pb dating on metamorphic zircon rims. The protolith of the Bois de Sélasse eclogite is indeed older than the Variscan oceans, but it was similarly affected by Variscan subduction. We discuss the implications of this new finding in the context of the European Variscan belt.

Reactive fluid flow guided by grain-scale equilibrium reactions during eclogitization of dry crustal rocks

Fluid flow in crystalline rocks in the absence of fractures or ductile shear zones dominantly occurs by grain boundary diffusion, as it is faster than volume diffusion. It is, however, unclear how reactive fluid flow is guided through such pathways. We present a microstructural, mineral chemical, and thermodynamic analysis of a static fluid-driven reaction from dry granulite to ‘wet’ eclogite. Fluid infiltration resulted in re-equilibration at eclogite-facies conditions, indicating that the granulitic protolith was out of equilibrium, but unable to adjust to changing P–T conditions. The transformation occurred in three steps: (1) initial hydration along plagioclase grain boundaries, (2) complete breakdown of plagioclase and hydration along phase boundaries between plagioclase and garnet/clinopyroxene, and (3) re-equilibration of the rock to an eclogite-facies mineral assemblage. Thermodynamic modelling of local compositions reveals that this reaction sequence is proportional to the local decrease of the Gibbs free energy calculated for ‘dry’ and ‘wet’ cases. These energy differences result in increased net reaction rates and the reactions that result in the largest decrease of the Gibbs free energy occur first. In addition, these reactions result in a local volume decrease leading to porosity formation; i.e., pathways for new fluid to enter the reaction site thus controlling net fluid flow. Element transport to and from the reaction sites only occurs if it is energetically beneficial, and enough transport agent is available. Reactive fluid flow during static re-equilibration of nominally impermeable rocks is thus guided by differences in the energy budget of the local equilibrium domains.

Neoproterozoic Eclogite-to Granulite-Facies Transition in the Ubendian Belt, Tanzania, and the Timescale of Continental Collision

AbstractIn collision-type orogens, where high-pressure and ultrahigh-pressure (HP–UHP) metamorphism usually occurs, deeply subducted continental slabs with eclogitized mafic rocks often undergo recrystallization/overprinting with various geothermal gradients after the peak conditions at lower-to-middle-crustal levels. During the crustal stabilization, the transition from eclogite-to granulite-facies is common. We conducted metamorphic petrology and zircon geochronology on (1) bimineralic and (2) partially granulitized eclogites from the Neoproterozoic Ufipa Terrane (Southwestern Tanzania). Microtextural relationships and mineral chemistry define three metamorphic stages: eclogite metamorphism (M1), HP granulite-facies overprinting (M2), and amphibolite-facies retrogression (M3). The bimineralic eclogite has a basaltic composition and lacks M2 minerals. In contrast, the kyanite eclogite is characterized by a gabbro-dioritic whole-rock composition and contains inherited magmatic zircon. Although the matrix is highly granulitized, garnet and kyanite contain eclogite-facies mineral inclusions. Phase equilibria modeling revealed P–T conditions of 2.1–2.6 GPa and 650–860°C for the M1 stage and 1.4–1.6 GPa and 750–940°C for the M2 stage. Zircon with eclogite-facies mineral inclusions from the bimineralic eclogite lacks Eu anomaly in the REE patterns and yielded the M1 eclogite metamorphic age of 588 ± 3 Ma. Zircon overgrowths surrounding the inherited Paleoproterozoic magmatic cores in kyanite eclogite yielded 562 ± 3 Ma. A weak negative Eu anomaly in the REE patterns and the absence of eclogitic mineral inclusions suggest the zircon growths at the M2 HP granulite-facies metamorphic stage. These new data indicate an eclogite-to granulite-facies transition time of 26 ± 4 million years (Myr), suggesting a rate of HP rock exhumation toward a lower crustal level of 0.7–1.5 mm/year. Furthermore, the density evolution model indicates that buoyant host orthogneiss with low-density gabbro-dioritic eclogite plays an important role in carrying high-density basaltic eclogite. Our 2D thermomechanical modeling also suggests that a slab break-off with a lower angle subduction of <20° triggers the exhumation of the HP slab sliver with 20–30 Myr eclogite-to granulite transition time of large HP–UHP terranes in major collision zones.

Petrology and Tectonic Geophysics of Massive and Foliated Eclogites in the North Qilian Orogenic Belt: Changes in Mineral Composition, Oxygen Fugacity, and Fabric during Exhumation

The North Qilian orogenic belt is a typical area of “cold” subduction of the early Paleozoic oceanic plate, forming a series of high pressure and low temperature metamorphic rock assemblages. Among them, eclogite is a kind of protolith, which is basaltic or gabbro high pressure metamorphic rock, mainly composed of garnet and chlorite which are two kinds of minerals. Eclogites record the entire history of subduction zone metamorphism and later exhumation. Due to the crystal habit and the developed joints, the strength of the pyroxene in the matrix is weak, so it is subjected to the main strain during deformation, whereas garnet tends to show only passive rotational deformation. This paper presents some new results in petrology and tectonic geophysics of eclogite block-like and planar eclogite. The massive and facial eclogite rocks contain eclogite facies mineral assemblages, and the peak temperature and pressure conditions are [Formula: see text]°C and [Formula: see text] GPa, which are consistent with the adjacent eclogite. Combined with the characteristics of in situ Lu-Hf isotopes, Ce4+/Ce3+ ratios of zircons, relative oxygen fugacity, and absolute oxygen fugacity, it is shown that the oxygen fugacity of the granodiorite porphyry (BL023, BLO31, DB048) of the folio chemical and massive eclogite deposits are all located in MH (magnetite-hematite) buffer zone. Through the calculation results of absolute oxygen fugacity of rock mass, it can be seen that the absolute oxygen fugacity of ore-bearing rock mass is significantly higher than that of non-ore-bearing rock mass. This paper systematically summarizes the research progress of the microscopic and ultrastructural deformation of eclogite minerals in high-pressure metamorphic zones, and discusses the changes of mineral composition, oxygen fugidity, and fabric of eclogite deformation characteristics during the recovery of subduction and reentry.

Halogens in Eclogite Facies Minerals from the Western Gneiss Region, Norway

Ultra-high-pressure (UHP) eclogites and ultramafites and associated fluid inclusions from the Western Gneiss Region, Norwegian Caledonides, have been analysed for F, Cl, Br and I using electron-probe micro-analysis, time-of-flight secondary ion mass spectrometry and neutron-irradiated noble gas mass spectrometry. Textures of multi-phase and fluid inclusions in the cores of silicate grains indicate formation during growth of the host crystal at UHP. Halogens are predominantly hosted by fluid inclusions with a minor component from mineral inclusions such as biotite, phengite, amphibole and apatite. The reconstructed fluid composition contains between 11.3 and 12.1 wt% Cl, 870 and 8900 ppm Br and 6 and 169 ppm I. F/Cl ratios indicate efficient fractionation of F from Cl by hydrous mineral crystallisation. Heavy halogen ratios are higher than modern seawater by up to two orders of magnitude for Br/Cl and up to three orders of magnitude for I/Cl. No correlation exists between Cl and Br or I, while Br and I show good correlation, suggesting that Cl behaved differently to Br and I during subduction. Evolution to higher Br/Cl ratios is similar to trends defined by eclogitic hydration reactions and seawater evaporation, indicating preferential removal of Cl from the fluid during UHP metamorphism. This study, by analogy, offers a field model for an alternative source (continental crust) and mechanism (metasomatism by partial melts or supercritical fluids) by which halogens may be transferred to and stored in the sub-continental lithospheric mantle during transient subduction of a continental margin.

A New HP–UHP Eclogite Belt Identified in the Southeastern Tibetan Plateau: Tracing the Extension of the Main Palaeo-Tethys Suture Zone

Abstract The Changning–Menglian orogenic belt (CMOB) in the southeastern Tibetan Plateau is an important link between the Longmu Co–Shuanghu suture (LCSS) in the northern Tibetan Plateau and the Chiang Mai–Inthanon and Bentong–Raub sutures in Thailand and Peninsular Malaysia. These belts and sutures are generally regarded as containing the remnants of the oceanic crust of the Palaeo-Tethys that formed by seafloor spreading as a result of the separation of Gondwana- and Eurasia-derived blocks during the Middle Cambrian. In this paper we report the first discovery of abundant unaltered and retrograde eclogites that occur as irregular lenses and blocks in metasedimentary rocks of the CMOB, and these eclogites form an elongate and almost north–south-trending high-pressure (HP)–ultrahigh-pressure (UHP) metamorphic belt that is ∼200 km long and ∼50 km wide. The newly discovered phengite/talc/epidote–glaucophane eclogites, lawsonite–talc–phengite eclogites, dolomite/magnesite–kyanite eclogites and phengite–kyanite-bearing retrograde eclogites have enriched (E-) and normal mid-ocean ridge basalt (N-MORB)-like affinities and mainly positive as well as some negative whole-rock εNd values (–4·34 to +7·89), which suggest an enriched and depleted oceanic lithosphere source for their protoliths. Magmatic zircons separated from the epidote–glaucophane, magnesite–kyanite and (phengite–kyanite-bearing) retrograde eclogites gave protolith ages of 317–250 Ma, which fit well within the time frame of the opening of the Palaeo-Tethys during the Middle Cambrian and its closure during the Triassic. Abundant metamorphic zircons in the eclogites indicate a Triassic metamorphic event related to the subduction of the Palaeo-Tethys oceanic crust from 235 to 227 Ma. Taking into account previous isotopic age data, we now establish the periods of Early–Middle Triassic (246–227 Ma) and Late Triassic (222–209 Ma) as the ages of subduction and exhumation of the Palaeo-Tethyan oceanic crust, respectively. Thermodynamic modelling revealed that the eclogites record distinct HP–UHP peak metamorphic conditions of 23·0–25·5 kbar and 582–610 °C for the phengite–glaucophane eclogites, 24·0–25·5 kbar and 570–586 °C for the talc–glaucophane eclogites, 29·0–31·0 kbar and 675–712 °C for the dolomite–kyanite eclogites, and 30·0–32·0 kbar and 717–754 °C for the magnesite–kyanite eclogites. These P–T estimates and geochronological data indicate that the Palaeo-Tethys oceanic slab was subducted to different mantle depths from 75 km down to 95 km, forming distinct types of eclogite with a variety of peak eclogite-facies mineral assemblages. The eclogites consistently record clockwise metamorphic P–T–t paths characterized by a heating–compression prograde loop under a low geothermal gradient of 5–10 °C km–1, indicating the rapid subduction of cold oceanic crust at a rate of 4·5–6·0 km Ma–1, followed by isothermal or cooling–decompressive retrogression and exhumation at an average rate of 3·2–4·2 km Ma–1. The newly discovered eclogites of the CMOB with their signatures of ocean-crust subduction are petrologically, geochemically and geochronologically comparable with those of the LCSS, providing powerful support for the idea that a nearly 2000 km long HP–UHP eclogite belt extends from the northern Tibetan Plateau to the southeastern Tibetan Plateau, and that it represents the main boundary suture of the Palaeo-Tethyan domain. These results have far-reaching implications for the tectonic framework and complex metamorphic evolution of the Palaeo-Tethyan domain.

Transformation weakening: Diffusion creep in eclogites as a result of interaction of mineral reactions and deformation

The deformation of eclogites and the driving forces for their fabric development are an important topic, potentially allowing to determine deformation rates and stresses in subduction zones, where the greatest number of large earthquakes occurs. Here, fabric studies of grain size and shape, texture, and chemical composition from two locations of Variscan and Alpine eclogites are presented. All samples show a well-developed crystallographic preferred orientation (CPO) of omphacite with a strong maximum of [001] in the lineation direction and a weaker maximum of poles to (010) normal to foliation. Garnet shows no systematic CPO. Anisotropic chemical zoning developed in omphacite and garnet during growth together with elongated grain shapes and can be related to a prograde (in terms of pressure change) P,T-path. The individual chemically zoned and elongated grains orientated in the stretching direction are single crystals without major internal misorientations. Chemical, microstructural, and CPO data indicate that the deformation microstructure and texture were produced by preferential crystal growth of garnet and omphacite grains in the extension direction. Dislocation creep can be excluded as a possible fabric formation process by the systematic and oriented chemical zonation of single crystals and absence of dynamic recrystallization microstructures. The dominant deformation is inferred to be diffusion creep, where dissolution of material took place in reacting mafic phases (plagioclase, pyroxene) and precipitation took place in the form of new eclogite facies minerals (omphacite, garnet, zoisite). This type of diffusion creep deformation represents a transformation process involving both, deformation and metamorphic reactions. It is emphasized that the weakening is directly connected to the transformation and therefore transient. The weakening facilitates diffusion creep deformation of otherwise strong minerals (pyroxene, garnet, zoisite) at far lower stresses than dislocation creep. The results imply low stresses during the deformation of eclogite blocks in subduction zones. These results can be applied to other rock types, too.

Thermobarometric constraints on burial and exhumation of 2-billion-year-old eclogites and their metapelitic hosts

One of the first appearances of eclogite-facies mineral assemblages in the geological record occurs in the c. 2000 Ma Palaeoproterozoic Usagaran Belt in central Tanzania, where the extended margin of the Tanzanian Craton is interpreted to have been subducted. Mafic rocks are interpreted to have contained the mineral assemblage garnet + omphacite + rutile + quartz ± amphibole. This high-pressure assemblage has been overprinted by a secondary mineral assemblage containing clinopyroxene + plagioclase + hornblende + ilmenite ± orthopyroxene. Mineral equilibria forward modelling indicates that the eclogite-facies assemblages reached minimum peak pressure–temperature (P–T) conditions of ~17 kbar and ~700 °C. Inclusions in garnet document a prograde P–T history consistent with burial through upper amphibolite-facies conditions and possible partial melting. Petrological and compositional evidence from garnet suggests that following peak metamorphism, the eclogite-facies rocks were heated while stalled at approximate peak pressures. Temperature estimates derived from Zr concentrations in interpreted texturally retrograde rutile support a near-isothermal post-peak P–T evolution for the eclogite-facies rocks – an evolution that terminates at retrograde P–T conditions of approximately 7.6–8.2 kbar and 680–790 °C. The relict eclogite domains form part of a larger assemblage with enclosing migmatitic metapelitic lithologies (the Isimani Suite). The metapelitic gneisses contain garnet + kyanite + biotite + staurolite + hornblende + plagioclase + muscovite + rutile + quartz and preserve minimal evidence of a high-pressure history, conceivably due to post-peak mineralogical recrystallisation. P–T modelling, inclusion assemblages and compositional zonation patterns in porphyroblastic garnet suggests the metapelitic gneisses — similarly to the relict eclogites — experienced burial to minimum peak pressures of approximately 16.5–17 kbar. Compositional zoning patterns in eclogitic garnet suggest the Isimani system was buried, reached peak metamorphic conditions, and was subsequently exhumed within a timeframe of up to 20 Myr. A tectonic regime involving crustal thickening and subduction, followed by extensional exhumation of the entire Isimani Suite is our preferred model for the development of the c. 2000 Ma Usagaran Belt.

Cambrian eclogite-facies metamorphism in the central Transantarctic Mountains, East Antarctica: Extending the record of early Palaeozoic high-pressure metamorphism along the eastern Gondwanan margin

The Nimrod Complex in the Miller and Geologists ranges of the central Transantarctic Mountains comprises Mesoarchean and Palaeoproterozoic crust that was incorporated into the Ross Orogen during metamorphism and deformation along the eastern Gondwanan convergent margin in the late Neoproterozoic and early Palaeozoic. In the Miller Range, boudinaged mafic domains are hosted within upper-amphibolite- to granulite-facies mylonitic rocks that were deformed during the Ross Orogeny. The mafic rocks preserve evidence of a precursor eclogite-facies mineral assemblage containing garnet, omphacite, amphibole, quartz and rutile. Mineral equilibria forward modelling and Zr-in-rutile thermometry indicate that the eclogite assemblage records peak pressure-temperature conditions of 16–18 kbar and 675–760 °C. A secondary metamorphic assemblage of clinopyroxene-plagioclase symplectite, plagioclase coronae rimming garnet, fine-grained amphibole, and interstitial ilmenite orthopyroxene overprint the high-pressure assemblage. Compositions and modal proportions of retrograde garnet, orthopyroxene and amphibole suggest near isothermal exhumation to conditions of 8–10 kbar and 660–840 °C, followed by a long-lived mid-crustal residence. In-situ LA–ICP–MS UPb dating of zircon and rutile yields concordant ages of 536 ± 14 Ma and 536 ± 20 Ma, respectively. The Cambrian-aged zircons show an absent to weakly negative Eu anomaly signature and flat heavy rare earth element (HREE) distributions that are typical of zircons crystallised in the absence of plagioclase and presence of garnet, and which is diagnostic of eclogite-facies conditions. The Cambrian-aged zircon population contrasts with older, highly discordant Palaeoproterozoic zircons that have negative Eu anomalies and are HREE enriched. These zircons are interpreted to be protolithic, and their ages are consistent with existing geochronology from host rocks in the region. The Cambrian-aged zircon and rutile are inferred to represent eclogite-facies metamorphism associated with Ross orogenesis. The early Palaeozoic eastern Gondwanan margin contains a diverse array of eclogite precursors that document contrasting geodynamic styles. Eclogites in northern Victoria Land and central Tasmania record comparatively warm thermal gradients, short-lived high-pressure conditions, and rapid exhumation within their continental crust hosts, whereas oceanic eclogites in the southern New England Fold Belt record refrigerated, long-lived convergent-margin metamorphism. The Miller Range eclogite-facies rocks in this study appear to have been derived from shallow subduction of ancient continental crust, after which they stalled at thermally elevated mid-crustal depths prior to their final exhumation. The discovery of Cambrian-aged eclogite-facies rocks in the Miller Range expands the spatial footprint of high-pressure metamorphism during the Ross Orogeny along the early Palaeozoic eastern Gondwanan margin.

Sulfur isotope compositions of pyrite from high-pressure metamorphic rocks and related veins (SW Tianshan, China): Implications for the sulfur cycle in subduction zones

A detailed petrographic, microstructural and geochemical investigation of pyrites from four eclogites / blueschists and their veins in the Tianshan (NW China) have revealed the occurrence of four types of pyrites: δ34S values of the pyrites range from −24.7 to +13.2‰, whereby three populations can be distinguished that have distinct δ34S ranges corresponding to different paragenetic stages. Py1 pyrite grains contain micro- to nanoscale inclusions of pyrrhotite, chalcopyrite, bornite, sphalerite, galena, arsenopyrite, cobalt-pentlandite, enargite, titanite, biotite, barite, and have δ34S values of −3.8 to +2.7‰ (average δ34S = −0.6 ± 0.9‰), which is inherited from a magmatic and hydrothermal fluid input during alteration of the oceanic crust. Py2 pyrite occurs as inclusions within the cores of garnet where it coexists with fine-grained inclusions of rutile, glaucophane and rarely with omphacite. Thus, these pyrite inclusions likely formed during prograde evolution. Py2 is too small to be analyzed by SIMS and thus no isotopic compositions could be determined. The Py3 pyrite grains contain eclogite-facies mineral inclusions, such as omphacite, glaucophane, epidote, rutile and lawsonite, and are inferred to have formed during the late prograde evolution close to or at peak eclogite-facies conditions. Py3 pyrite display positive δ34S values ranging from +3.6 to +13.2‰ (average δ34S = +9.7 ± 2.8‰), suggesting the source of sulfur was either anhydrite incorporated during seafloor alteration or 34S-enriched sulfide from lower sections of the oceanic lithosphere. Py4 pyrite occurs as irregular patches, isolated euhedral to subhedral grains or as rims around Py1 or Py3 pyrites, associated with magnetite and ilmenite. Py4 grains have δ34S values of −24.7 to −3.9‰ (average δ34S = −5.0 ± 2.1‰), with the most likely source of sulfur being 32S-enriched pelagic sediments that in the Tianshan are interlayered with the metavolcanic lithologies and underwent amphibolite facies metamorphism during exhumation. This study provides insight into the sulfur cycle within subduction zones. We document that during subduction and exhumation fluids with distinct sulfur isotopic compositions driven by dehydration reactions in the downgoing slab cause metasomatism in the surrounding lithologies. In particular, we suggest that at least locally the plate interface is characterized by sediment-derived fluids that are characterized by 32S-enriched sulfur and comparatively higher oxygen fugacities.

High-P amphibolite-facies metamorphism in the Adrar–Souttouf Metamafic Complex, Oulad Dlim Massif (West African Craton margin, Morocco)

The Oulad Dlim Massif represents the northern segment of the Mauritanide belt that thrusts over the western margin of the Reguibat Shield, north of the West African Craton (WAC). This belt includes various metamorphic units of Archean, Neoproterozoic and Palaeozoic ages that were stacked and thrust eastward during the Variscan orogeny. The core of the Oulad Dlim Massif comprises the Adrar–Souttouf Metamafic Complex that represents a large tectonic unit made of high-grade mafic rocks and vast exposures of amphibolites. A characterisation of the metamorphism in these amphibolites is essential to understand the relationships of the Oulad Dlim Massif with the southern segments of the Mauritanide belt and to provide constraints on the geodynamic evolution of the western margin of the WAC. Here we determine the P–T conditions of metamorphism of two samples of garnet amphibolites collected at the northernmost end of the Adrar–Souttouf Metamafic Complex. The samples show a main mineral assemblage of garnet+low-Ti pargasite+oligoclase+phengite+epidote+quartz+rutile±paragonite±K-feldspar. We calculated their P–T conditions using the amphibole–plagioclase NaSi–CaAl exchange thermometer, and the garnet–amphibole–plagioclase–quartz and the amphibole–plagioclase Si–Al partitioning barometers. The thermobarometric results indicate that this mineral assemblage was formed at high-P amphibolite-facies conditions at 650–700°C and 10–13 kbar. The observed stability of paragonite and phengite reveals fluid-absent conditions or the presence of a fluid phase with reduced H2O activity during the peak of metamorphism. We found no relicts of eclogite-facies mineral assemblage in the garnet amphibolites. This contrasts with the eclogite-facies metamorphism found due south in the Tarf Magneïna unit. This suggests that the northernmost end of the Adrar–Souttouf Metamafic Complex may have been buried to shallower depths than the units further south, probably during the Variscan orogeny. However, precise absolute radiometric dating of the high-P amphibolite-facies metamorphism is required to confirm these findings.

Mineral textural evolution and PT-path of relict eclogite-facies rocks in the Paleoproterozoic Nagssugtoqidian Orogen, South-East Greenland

The Nagssugtoqidian Orogen in South-East Greenland is a deeply eroded, Paleoproterozoic collision orogen. It consists of a variety of Archean and Paleoproterozoic rocks, most notably TTG gneiss, a variety of supracrustal rocks and basic dykes. This study aims at providing new insight into the geodynamic processes and subduction depth of this orogen by investigating the metamorphic evolution of garnet pyroxenite, retrogressed eclogite and amphibolite-facies rocks that are exposed within the Kuummiut Terrane of the Nagssugtoqidian Orogen. The garnet-pyroxenite has a dominant mineral assemblage of garnet, orthopyroxene, clinopyroxene and hornblende, while garnet-amphibolite and garnet-kyanite schist are made up of garnet, hornblende, plagioclase and quartz, and garnet, kyanite, biotite and quartz, respectively. Relicts of, and pseudomorphs after, eclogite-facies mineral assemblages are frequently found within basic metavolcanic rocks and Paleoproterozoic discordant basic dykes. In the retrogressed eclogite, the retrograde mineral reactions ceased prior to completion, resulting in the formation of two domains. A clinopyroxene domain consists of diopside-plagioclase symplectites, which are interpreted to have grown at the expense of omphacite. The symplectites are surrounded and partly replaced by hornblende and plagioclase. Omphacite (XJd 25–42) is preserved in a Na-rich sample, where it occurs in the core of large clinopyroxene and as inclusion in garnet and hornblende. In a garnet domain, garnet is variably replaced by an inner corona of plagioclase and an outer corona of amphibole +/− orthopyroxene and clinopyroxene. The degree of retrogression as well as the type of the retrograde assemblage in both domains appears to be dependent on fluid activity. Large garnet grains preserve Ca-rich cores, interpreted as prograde in origin, while Mg-rich garnet rims formed during eclogite-facies metamorphism and later re-equilibration. Pseudosection modelling combined with conventional geothermobarometry reveals a clockwise PT-evolution, involving eclogite-facies conditions of 17–19kbar and 740–810°C, followed by near-isothermal decompression to medium-pressure granulite-facies conditions (13.8–15.4kbar, 760–880°C) and subsequent decompression with minor cooling to high-pressure amphibolite-facies grades (8.8–10.9kbar, 660–840°C). These data show that rocks of the Kuummiut Terrane were exhumed from 70 to about 30km into the mid- and lower crust. The PT-path implies that exhumation initially was rapid and tectonically-controlled.

Age and P–T conditions of the Gridino‐type eclogite in the Belomorian Province, Russia

ABSTRACTAlthough eclogites in the Belomorian Province have been regarded as Archean in age and among the oldest in the world, there are also multiple studies that have proposed a Paleoproterozoic age. Here, we present new data for the Gridino‐type eclogites, which occur as boudins and metamorphosed dykes within tonalite–trondhjemite–granodiorite gneisses. Zircon from these eclogites has core and rim structures. The cores display high Th/U ratios (0.18–0.45), negative Eu anomalies and strong enrichment in HREE, and have Neoarchean U–Pb ages of c. 2.70 Ga; they are interpreted to be magmatic in origin. Zircon cores have δ18O of 5.64–6.07‰ suggesting the possibility of crystallization from evolved mantle‐derived magmas. In contrast, the rims, which include the eclogite facies minerals omphacite and garnet, are characterized by low Th/U ratios (<0.035) and flat HREE patterns, and yield U–Pb ages of c. 1.90 Ga; they are interpreted to be metamorphic in origin. Zircon rims have elevated δ18O of 6.23–6.80‰, which was acquired during eclogite facies metamorphism. Based on petrography and phase equilibria modelling, we recognize a prograde epidote amphibolite facies mineral assemblage, the peak eclogite facies mineral assemblage and a retrograde high‐P amphibolite facies mineral assemblage. The peak metamorphic conditions of 695–755°C at >18 kbar for the Gridino‐type eclogites suggest an apparent thermal gradient of <39–42°C/kbar for the Lapland–Kola collisional orogeny.

Felsic granulite with layers of eclogite facies rocks in the Bohemian Massif; did they share a common metamorphic history?

High pressure granulite and granulite gneiss from the Rychleby Mountains in the East Sudetes form an approximately 7km long and 0.8km wide body, which is enclosed by amphibolite facies orthogneiss with a steep foliation. Well preserved felsic granulite is located in the central part of the body, where several small bodies of mafic granulite are also present. In comparison to other high pressure granulites in the Bohemian Massif, which show strong mineral and textural re-equilibration under granulite facies conditions, the mafic granulite samples preserve eclogite facies minerals (garnet, omphacite, kyanite, rutile and phengite) and their field and textural relations indicate that both mafic and felsic granulites shared common metamorphic history during prograde eclogite facies and subsequent granulite facies events. Garnet from both granulite varieties shows prograde compositional zoning and contains inclusions of phengite. Yttrium and REEs in garnet show typical bell-shaped distributions with no annular peaks near the grain rims. Investigation of major and trace elements zoning, including REEs distribution in garnet, was combined with thermodynamic modelling to constrain the early eclogite facies metamorphism and to estimate pressure-temperature conditions of the subsequent granulite facies overprint. The first (U)HP metamorphism occurred along a low geothermal gradient in a subduction-related environment from its initial stage at 0.8GPa/460°C and reached pressures up to 2.5GPa at 550°C. The subsequent granulite facies overprint (1.6–1.8GPa/800–880°C) affected the rocks only partially; by replacement of omphacite into diopside+plagioclase symplectite and by compositional modification of garnet rims. The mineral textures and the preservation of the eclogite facies prograde compositional zoning in garnet cores confirm that the granulite facies overprint was either too short or too faint to cause recrystallisation and homogenisation of the eclogite facies mineral assemblage. The results of this study are compared with other granulite massifs in the Moldanubian Zone. In addition, a possible scenario for the Variscan eclogite and subsequent granulite facies metamorphism in the Bohemian Massif is discussed.

The relationship between mantle pH and the deep nitrogen cycle

Nitrogen is distributed throughout all terrestrial geological reservoirs (i.e., the crust, mantle, and core), which are in a constant state of disequilibrium due to metabolic factors at Earth’s surface, chemical weathering, diffusion, and deep N fluxes imposed by plate tectonics. However, the behavior of nitrogen during subduction is the subject of ongoing debate. There is a general consensus that during the crystallization of minerals from melts, monatomic nitrogen behaves like argon (highly incompatible) and ammonium behaves like potassium and rubidium (which are relatively less incompatible). Therefore, the behavior of nitrogen is fundamentally underpinned by its chemical speciation. In aqueous fluids, the controlling factor which determines if nitrogen is molecular (N2) or ammonic (inclusive of both NH4+ and NH30) is oxygen fugacity, whereas pH designates if ammonic nitrogen is NH4+ or NH30. Therefore, to address the speciation of nitrogen at high pressures and temperatures, one must also consider pH at the respective pressure–temperature conditions. To accomplish this goal we have used the Deep Earth Water Model (DEW) to calculate the activities of aqueous nitrogen from 1–5GPa and 600–1000°C in equilibrium with a model eclogite-facies mineral assemblage of jadeite+kyanite+quartz/coesite (metasediment), jadeite+pyrope+talc+quartz/coesite (metamorphosed mafic rocks), and carbonaceous eclogite (metamorphosed mafic rocks+elemental carbon). We then compare these data with previously published data for the speciation of aqueous nitrogen across these respective P-T conditions in equilibrium with a model peridotite mineral assemblage (Mikhail and Sverjensky, 2014). In addition, we have carried out full aqueous speciation and solubility calculations for the more complex fluids in equilibrium with jadeite+pyrope+kyanite+diamond, and for fluids in equilibrium with forsterite+enstatite+pyrope+diamond.Our results show that the pH of the fluid is controlled by mineralogy for a given pressure and temperature, and that pH can vary by several units in the pressure-temperature range of 1–5GPa and 600–1000°C. Our data show that increasing temperature stabilizes molecular nitrogen and increasing pressure stabilizes ammonic nitrogen. Our model also predicts a stark difference for the dominance of ammonic vs. molecular and ammonium vs. ammonia for aqueous nitrogen in equilibrium with eclogite-facies and peridotite mineralogies, and as a function of the total dissolved nitrogen in the aqueous fluid where lower N concentrations favor aqueous ammonic nitrogen stabilization and higher N concentrations favor aqueous N2.Overall, we present thermodynamic evidence for nitrogen to be reconsidered as an extremely dynamic (chameleon) element whose speciation and therefore behavior is determined by a combination of temperature, pressure, oxygen fugacity, chemical activity, and pH. We show that altering the mineralogy in equilibrium with the fluid can lead to a pH shift of up to 4 units at 5GPa and 1000°C. Therefore, we conclude that pH imparts a strong control on nitrogen speciation, and thus N flux, and should be considered a significant factor in high temperature geochemical modeling in the future. Finally, our modelling demonstrates that pH plays an important role in controlling speciation, and thus mass transport, of Eh-pH sensitive elements at temperatures up to at least 1000°C.

Zircon U–Pb dating of eclogite from the Qiangtang terrane, north-central Tibet: a case of metamorphic zircon with magmatic geochemical features

Zircon is probably the most important mineral used in the dating formation of high-pressure (HP) and ultrahigh-pressure (UHP) metamorphic rocks. The origin of zircon, i.e., magmatic or metamorphic, is commonly assessed by its external morphology, internal structure, mineral inclusions, Th/U ratios and trace element composition. In this study, we present an unusual case of metamorphic zircon from the Qiangtang eclogite, north-central Tibet. The zircon grains contain numerous eclogite-facies mineral inclusions, including omphacite, phengite, garnet and rutile; hence, they are clearly of metamorphic origin. However, they display features similar to common magmatic zircon, including euhedral crystal habit, high Th/U ratios and enriched heavy rare earth elements pattern. We suggest that these zircon grains formed from a different reservoir from that for garnet where no trace elements was present and trace element equilibrium between zircon and garnet was achieved. U–Pb dating of zircon gave an age of 232–237 Ma for the eclogite, and that of rutile yielded a slightly younger age of ca. 217 Ma. These ages are consistent with the reported Lu–Hf mineral isochron and phengite Ar–Ar ages. The zircon U–Pb and mineral Lu–Hf isochron ages are interpreted as the time of the peak eclogite-facies metamorphism, whereas the rutile U–Pb and phengite Ar–Ar ages represent the time of exhumation to the middle crust. Thus, the distinction between metamorphic and magmatic zircons cannot be made using only Th/U ratios and heavy REE compositions for HP–UHP metamorphic rocks of oceanic derivation.

Metamorphic PT path and zircon U–Pb dating of Archean eclogite association in Gridino complex, Belomorian province, Russia

The eclogites from the Gridino eclogite-bearing mélange complex in the Belomorian province, Fennoscandian Shield may represent one of the oldest terrain in the world. The Belomorian province is a superposition of Archean and Palaeoproterozoic collisional orogens, which experienced repeated high-grade metamorphic reworking and intense polyphase tectonic deformation in the early Precambrian. Main purpose of this study is to determine the metamorphic age(s) and evolutionary PT path of eclogite as boudin-like body within the Gridino tectonic mélange. Representative rock materials of well-preserved, plagioclase-free eclogite and associated retrogressed eclogitic rock were sampled from Stolbikha Island in the Gridino area for our research. The eclogite sample mainly comprises omphacite, garnet, amphibole, rare quartz, and some accessory minerals (rutile, magnetite), whereas the retrogressed eclogite sample is made of diopside – Na-rich plagioclase symplectites, amphibole, quartz, and accessory minerals. Thermodynamic modeling points to a clockwise metamorphic PT path, with peak conditions of P=18.5kbar, T=710°C, and a “warm” subduction is inferred thus. LA-ICP-MS zircon U–Pb dating yields diverse metamorphic episodes from the Neoarchean to the Palaeoproterozoic. While metamorphic zircon formed at eclogite-facies condition is identified by studying the trace element distribution and partitioning in and between zircon and co-existing eclogite-facies minerals (omphacite, garnet); consequently, Neoarchean eclogite-facies metamorphic event of ca. 2.70Ga can be distinguished for the Gridino eclogite complex, and a regional-scale thermal event may have occurred in the period of 2.70–2.61Ga.

First-report on Mesozoic eclogite-facies metamorphism preceding Barrovian overprint from the western Rhodope (Chalkidiki, northern Greece)

The Chalkidiki block in Northern Greece represents the southwesternmost piece of the ultrahigh-pressure Rhodope and has played an important role in the evolution of the North Aegean. The eastern part of the Chalkidiki block is a basement complex (Vertiskos Unit) that is made largely of Palaeozoic granitoids and clastic sediments that metamorphosed during the Mesozoic. This basement is traditionally considered as part of the Rhodopean hanging-wall, an assignment mainly supported by the absence of high-pressure mineral indicators and the presence of a regional medium-pressure/medium-temperature amphibolite-facies Barrovian metamorphic imprint. Toward the west, the basement is juxtaposed with meta-sedimentary (Circum-Rhodope belt) and arc units (Chortiatis Magmatic Suite) that carry evidence of a Mesozoic high-pressure/low-temperature event. In this study, garnet–staurolite-mica schists from the eastern part of the basement were examined by means of micro-textures, mineral chemistry and isochemical phase-diagram sections in the system NCKFMASHMn(Ti) [Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–MnO–(TiO2)]. The schists represent former Mesozoic sedimentary sequences deposited on the Palaeozoic basement. We document the presence of a relict eclogite-facies mineral assemblage (garnet+chloritoid+phengite+rutile) in an amphibolite-facies matrix composed of garnet+staurolite+phengite±kyanite. Model results suggest the existence of a high-pressure/medium-temperature metamorphic event (1.9GPa/520°C) that preceded regional re-equilibration at medium-pressure/medium-temperature conditions (1.2GPa/620°C). Clearly, the eastern part of the Chalkidiki block (basement complex) retains memory of an as yet unidentified Mesozoic eclogitic metamorphic event that was largely erased by the later Barrovian overprint. In light of our findings, the basement complex of the Chalkidiki block shares a common tectono-metamorphic evolution with both the high-pressure units to the west, and the high-grade Rhodopean gneisses further to the northeast. Our results are consequential for the geodynamic reconstruction of the Rhodope since they require participation of the Chalkidiki block to the well-established Mesozoic subduction system.

Metamorphic history of glaucophane‐paragonite‐zoisite eclogites from the Shanderman area, northern Iran

AbstractThe Shanderman eclogites and related metamorphosed oceanic rocks mark the site of closure of the Palaeotethys ocean in northern Iran. The protolith of the eclogites was an oceanic tholeiitic basalt with MORB composition. Eclogite occurs within a serpentinite matrix, accompanied by mafic rocks resembling a dismembered ophiolite. The eclogitic mafic rocks record different stages of metamorphism during subduction and exhumation. Minerals formed during the prograde stages are preserved as inclusions in peak metamorphic garnet and omphacite. The rocks experienced blueschist facies metamorphism on their prograde path and were metamorphosed in eclogite facies at the peak of metamorphism. The peak metamorphic mineral paragenesis of the rocks is omphacite, garnet (pyrope‐rich), glaucophane, paragonite, zoisite and rutile. Based on textural relations, post‐peak stages can be divided into amphibolite and greenschist facies. Pressure and temperature estimates for eclogite facies minerals (peak of metamorphism) indicate 15–20 kbar at ~600 °C. The pre‐peak blueschist facies assemblage yields <11 kbar and 400–460 °C. The average pressure and temperature of the post‐peak amphibolite stage was 5–6 kbar, ~470 °C. The Shanderman eclogites were formed by subduction of Palaeotethys oceanic crust to a depth of no more than 75 km. Subduction was followed by collision between the Central Iran and Turan blocks, and then exhumation of the high pressure rocks in northern Iran.