Implication of corona formation in a metatroctolite to the granulite facies overprint of HP–UHP rocks in the Moldanubian Zone (Bohemian Massif)

Corona and inclusion textures of a metatroctolite at the contact between felsic granulite and migmatites of the Gföhl Unit from the Moldanubian Zone provide evidence of the magmatic and metamorphic evolution of the rocks. Numerous diopside inclusions (1–10 μm, maximum 20 μm in size) in plagioclase of anorthite composition represent primary magmatic textures. Triple junctions between the plagioclase grains in the matrix are occupied by amphibole, probably pseudomorphs after clinopyroxene. The coronae consist of a core of orthopyroxene, with two or three zones (layers); the innermost is characterized by calcic amphibole with minor spinel and relicts of clinopyroxene, the next zone consists of symplectite of amphibole with spinel, sapphirine and accessory corundum, and the outermost is formed by garnet and amphibole with relicts of spinel. The orthopyroxene forms a monomineralic aggregate that may contain a cluster of serpentine in the core, suggesting its formation after olivine. Based on mineral textures and thermobarometric calculations, the troctolite crystallized in the middle to lower crust and the coronae were formed during three different metamorphic stages. The first stage relates to a subsolidus reaction between olivine and anorthite to form orthopyroxene. The second stage involving amphibole formation suggests the presence of a fluid that resulted in the replacement of igneous orthopyroxene and governed the reaction orthopyroxene + anorthite = amphibole + spinel. The last stage of corona formation with amphibole + spinel + sapphirine indicates granulite facies conditions. Garnet enclosing spinel, and its occurrence along the rim of the coronae in contact with anorthite, suggests that its formation occurred either during cooling or both cooling and compression but still at granulite facies conditions. The zircon U–Pb data indicate Variscan ages for both the troctolite crystallization (c. 360 Ma) and corona formation during granulite facies metamorphism (c. 340 Ma) in the Gföhl Unit. The intrusion of troctolite and other Variscan mafic and ultramafic rocks is interpreted as a potential heat source for amphibolite–granulite facies metamorphism that led to partial re‐equilibration of earlier high‐ to ultrahigh‐P metamorphic rocks in the Moldanubian Zone. These petrological and geochronological data constrain the formation of HP–UHP rocks and arc‐related plutonic complex to westward subduction of the Moldanubian plate during the Variscan orogeny. After exhumation to lower and/or middle crust, the HP–UHP rocks underwent heating due to intrusion of mafic and ultramafic magma that was generated by slab breakoff and mantle upwelling.     

Diffusion-controlled olivine corona textures in granitic rocks from Lofoten, Norway: calculation of Onsager diffusion coefficients, thermodynamic modelling and petrological implications

Reactions occurring during cooling of charnockitic intrusives on the Lofoten Islands produce characteristic diffusion-controlled textures around fayalite and Fe–Ti oxides. Thermobarometry indicates the corona textures formed at 780–840 °C and pressures of 4–10 kbar, whereas the magmatic assemblage of the charnockite (clinopyroxene–olivine–quartz) crystallized at about 850–870 °C and 4 kbar. The succession olivine|orthopyroxene+magnetite|orthopyroxene+garnet and olivine|orthopyroxene+magnetite|amphibole developed where olivine reacted with adjacent plagioclase or K-feldspar, but the modes and the thicknesses of the corona textures vary according to the feldspar type, indicating that the primary magmatic ternary feldspar was already exsolved into albitic plagioclase and alkali feldspar when the corona formation began. Simultaneously, in other parts of the rock, primary magmatic clinopyroxene reacted to amphibole and Fe–Ti oxides reacted to orthopyroxene+garnet coronas or to amphibole. Textures demonstrate significant Al diffusion in the rocks under granulite facies conditions and they suggest that no pervasive fluid influx occurred and that amphibole formation was dependant on a local source of H₂O probably related to water-release during the last stages of magmatism. Calculation of the net reaction by accounting for all observed reactions at different sites in the rock indicates that the system can be regarded as balanced on a hand-specimen scale with respect to all elements except for Na and H₂O. The larger variety of textures developed in rocks of granitic bulk composition provide more constraints than textures from gabbroic compositions, and permitted calculation of a set of relative diffusion coefficients which also reproduce textures in the gabbroic and anorthositic rocks from the Lofoten Islands. The following set of relative diffusion coefficients (L_i/L_Fe) reproduces the observed textures in the Lofoten rocks: Si=0.82, Mg=0.59, Mn=0.05, Na=0.38, K=0.39, Al=0.05 and Ca=0.07.     

Petrogenesis of sapphirine-bearing metatroctolites from the Buck Creek ultramafic body,southern Appalachians

The Buck Creek ultramafic body, North Carolina, includes aluminous lenses that have been described as troctolites. These lenses preserve mineral assemblages which record several different stages of metamorphism. The first stage is characterized by anhydrous reactions between olivine and plagioclase to produce coronas of orthopyroxene+ clinopyroxene/spinel symplectite. Thermo barometric results indicate minimum pressures of c. 6 kbar and c. 800 ^oC. Sapphirine replaces spinel in some clinopyroxene symplectites, and occurs as anhedral grains within amphibole, observations which in combination suggest peak metamorphic conditions of c. 9-10 kbar and c. 850 ^oC. Sapphirine-bearing hydrous assemblages formed at the expense of the coronas, indicating a second metamorphic episode involving deeper burial, deformation and hydration. Schistose rocks from the margins of the lenses are composed of anorthite+amphibole+margarite+corundum, and probably record a later, lower P-T event. Whole rock analyses for the Buck Creek lenses suggest an accumulate protolith of magnesian olivine and calcic plagioclase. Trace element data for the troctolites are consistent with data for adjacent amphibolites in suggesting that the Buck Creek mafic and ultramafic cumulates crystallized from magmas derived from a mantle source similar to that which produces modern intraplate or rift-related basalts. We propose that the Buck Creek ultramafics represent basal cumulates(± uppermost mantle) from ocean crust formed in a marginal basin in the latest Precambrian. Subduction-induced burial to at least 18 km under dry conditions induced corona formation. Collisional events of the Taconic orogeny thrust the Buck Creek rocks into the orogenic pile to at least 30 km depth and hydrated them along zones of weakness, locally producing P-T -P_H2O conditions appropriate for formation of sapphirine and hydrated assemblages, but still preserving some dry symplectites.     

Microstructures of olivine-plagioclase corona in meta-ultramafic rocks from Sefuri Mountains, NW Kyushu, Japan

Two types of reaction rims occur between olivine and plagioclase in ultramafic rocks from the Sefuri Mountains, NW Kyushu, Japan, which were metamorphosed under granulite-facies conditions. One occurs as a thin film of orthopyroxene along the boundary between olivine and plagioclase (orthopyroxene zone). The other is composed of two zones: symplectite of calcic amphibole and spinel on the plagioclase-side (symplectite zone) and calcic amphibole with sporadic orthopyroxene on the olivine-side (tremolite zone). In the tremolite zone, calcic amphibole shows a systematic decrease in Al content and increase in Mg/(Fe +Mg) with decreasing distance from olivine. Local equilibria maintained during the diffusion-controlled corona-forming reaction enable us to apply equilibrium thermodynamics to calcic amphibole and adjacent orthopyroxene. An integrated formulation of the Gibbs method for an Fe–Mg exchange reaction constrains the equilibrium temperature recorded in the tremolite zone to be 600–710 °C. It is significantly lower than the temperature of the granulite-facies metamorphism (800–900 °C) estimated using conventional geothermobarometry. Except for H₂O, the association of calcic amphibole and spinel in the symplectite zone is chemically equivalent to the association of olivine, plagioclase and orthopyroxene that was stable before the corona formation. This suggests that the following orthopyroxene-consuming reaction describes the paragenetic change taking place between 800–900 °C and 600–710 °C, olivine + plagioclase + orthopyroxene + aqueous fluid = calcic amphibole + spinel. In contrast, the overall reaction inferred from microstructures produces orthopyroxene as well as calcic amphibole and spinel at the expense of olivine and plagioclase. This reaction requires removal mainly of MgO that is also responsible for destabilizing the local association of olivine and plagioclase. These features suggest that the presence of orthopyroxene as a product in the corona is not always indicative of an orthopyroxene-producing reaction being responsible for the change of paragenetic relation. Microstructural features should be carefully applied to infer the reaction describing paragenetic change by which we argue the P–T path of the rocks.     

Clockwise exhumation path of granulitized eclogites from the Ama Drime range (Eastern Himalayas)

The metamorphic evolution of a granulitized eclogite from the Phung Chu Valley (Eastern Himalaya) was reconstructed combining microstructural observations, conventional thermobarometry and quantitative pseudosection analysis. The granulitized eclogite consists of clinopyroxene, plagioclase, garnet, brown amphibole, and minor orthopyroxene, biotite, ilmenite and quartz. On the basis of microstructural observations and mineral relationships, four metamorphic stages and related mineral assemblages have been recognized: (i) M1 eclogite‐facies assemblage, consisting of garnet, omphacite (now replaced by a clinopyroxene + plagioclase symplectite) and phengite (replaced by biotite +plagioclase symplectite); (ii) M2 granulite‐facies assemblage, represented by clinopyroxene, orthopyroxene, garnet, plagioclase and accessory ilmenite; (iii) M3 plagioclase + orthopyroxene corona developed around garnet, and (iv) M4 brown amphibole + plagioclase assemblage in the rock matrix. Because of the nearly complete lack of eclogitic mineral relics, M1 conditions can be only loosely constrained at >1.5 GPa and >580 °C. In contrast, assemblage M2 tightly constrains the peak granulitic stage at 0.8–1.0 GPa and >750 °C. The second granulitic assemblage M3, represented by the plagioclase + orthopyroxene corona, formed at lower pressures (～0.4 GPa and ～750 °C). During the subsequent exhumation, the granulitized eclogite experienced significant cooling to nearly 700 °C, marked by the appearance of brown amphibole and plagioclase (M4) in the rock matrix. U‐Pb SHRIMP analyses on low‐U rims of zircon from an eclogite of the same locality suggest an age of 13–14 Ma for the M3 stage. The resulting decompressional clockwise P–T path of the Ama Drime eclogite is characterized by nearly isothermal decompression from >1.5 GPa to ～0.4 GPa, followed by nearly isobaric cooling from ～775 °C to ～710 °C. Modelling of phase equilibria by a calculated petrogenetic grid and conventional thermobarometry on a biotite‐garnet‐sillimanite metapelite hosted in the country rock granitic orthogneiss extends the inferred P–T trajectory down to ～630 °C and ～0.3 GPa.     

The P–T–X(fluid) evolution of meta‐anorthosites in the Eastern Granulites,Tanzania

Meta‐anorthosite bodies are typical constituents of the Neoproterozoic Eastern Granulites in Tanzania. The mineral assemblage (and accessory components) is made up of clinopyroxene, garnet, amphibole; scapolite, epidote, biotite, rutile, titanite, ilmenite and quartz. Within the feldspar‐rich matrix (70–90% plagioclase), mafic domains with metamorphic corona textures were used for P–T calculations. Central parts of these textures constitute high‐Al clinopyroxene – which is a common magmatic mineral in anorthosites – and is therefore assumed to be a magmatic relict. The clinopyroxene rims have a diopsidic composition and are surrounded by a garnet corona. Locally the pyroxene is surrounded by amphibole and scapolite suggesting that a mixed CO₂–H₂O fluid was present during their formation. Thermobarometric calculations give the following conditions for the metamorphic peak of the individual meta‐anorthosite bodies: Mwega: 11–13 kbar, 850–900 °C; Pare Mountains: 12–14 kbar, 850–900 °C; Uluguru Mountains: 12–14 kbar, 850–900 °C. The P–T evolution of these bodies was modelled using pseudosections. The amount and composition of the metamorphic fluid and <0.5 mol.% fluid in the bulk composition is sufficient to produce fluid‐saturated assemblages at 10 kbar and 800 °C. Pseudosection analysis shows that the corona textures most likely formed under fluid undersaturated conditions or close to the boundary of fluid saturation. The stabilities of garnet and amphibole are dependent on the amount of fluid present during their formation. Mode isopleths of these minerals change their geometry drastically between fluid‐saturated and fluid‐undersaturated assemblages. The garnet coronae developed during isobaric cooling following the metamorphic peak. The cooling segment is followed by decompression as indicated by the growth of amphibole and plagioclase. The estimated of the metamorphic fluid is ～0.3–0.5. Although the meta‐anorthosites have different formation ages (Archean and Proterozoic) they experienced the same Pan‐African metamorphic overprint with a retrograde isobaric cooling path. Similar P–T evolutionary paths are known from the hosting granulites. The presented data are best explained by a tectonic model of hot fold nappes that brought the different aged anorthosites and surrounding rocks together in the deep crust followed by an isobaric cooling history.     

Formation of clinopyroxene + spinel and amphibole + spinel symplectites in coronitic gabbros from the Sierra de San Luis (Argentina): a key to post-magmatic evolution

The El Arenal metagabbros preserve coronitic shells of orthopyroxene ± Fe‐oxide around olivine, as well as three different types of symplectite consisting of amphibole + spinel, clinopyroxene + spinel and, more rarely, orthopyroxene + spinel. The textural features of the metagabbros can be explained by the breakdown of the olivine + plagioclase pair, producing orthopyroxene coronas and clinopyroxene + spinel symplectites, followed by the formation of amphibole + spinel symplectites, reflecting a decrease in temperature and, possibly, an increase in water activity with respect to the previous stage. The metagabbros underwent a complex P–T history consisting of an igneous stage followed by cooling in granulite, amphibolite and greenschist facies conditions. Although the P–T conditions of emplacement of the igneous protolith are still doubtful, the magmatic assemblage suggests that igneous crystallization occurred at a pressure lower than 6 kbar and at 900–1100 °C. Granulitic P–T conditions have been estimated at about 900 °C and 7–8 kbar combining conventional thermobarometry and pseudosection analysis. Pseudosection calculation has also shown that the formation of the amphibole + spinel symplectite could have been favoured by an increase in water activity during the amphibolite stage, as the temperature of formation of this symplectite strongly depends on aH₂O (<740 °C for aH₂O = 0.5; <790 °C for aH₂O = 1). Furthermore, but not pervasive, re‐equilibration under greenschist facies P–T conditions is documented by retrograde epidote and chlorite. The resulting counterclockwise P–T path consists of progressive, nearly isobaric cooling from the igneous stage down to the granulite, amphibolite and greenschist stage.     

P–T–t evolution and textural evidence for decompression of Pan-African high-pressure granulites, Lurio Belt, north-eastern Mozambique

Pan‐African high‐pressure granulites occur as boudins and layers in the Lurio Belt in north‐eastern Mozambique, eastern Africa. Mafic granulites contain the mineral assemblage garnet + clinopyroxene + plagioclase + quartz ± magnesiohastingsite. Garnet porphyroblasts are zoned with increasing almandine and spessartine contents and decreasing grossular and pyrope contents from core (Alm₄₆Prp₃₂Grs₂₁Sps₂) to rim (Alm₅₂Prp₂₆Grs₁₉Sps₃). This pattern is interpreted as a retrograde diffusion zoning with the preserved core chemistry representing the peak metamorphic composition. Mineral reaction textures occur in the form of monomineralic and composite plagioclase ± orthopyroxene ± amphibole ± biotite ± magnetite coronas around garnet porphyroblasts. Thermobarometry indicates peak metamorphic conditions of up to 1.57 ± 0.14 GPa and 949 ± 92 °C (stage I), corresponding to crustal depths of ～55 km. Zircon yielded an U–Pb age of 557 ± 16 Ma, inferred to date crystallization of zircon during peak or immediately post‐peak metamorphism. Formation of plagioclase + orthopyroxene‐bearing coronas surrounding garnet indicates a near‐isothermal decompression of the high‐pressure granulites to lower pressure granulite facies conditions (stage II). Development of plagioclase + amphibole‐coronas enclosing the same garnet porphyroblasts shows subsequent cooling into amphibolite facies conditions (stage III). Symplectitic textures of the corona assemblages indicate rapid decompression. The high‐pressure granulite facies metamorphism of the Lurio Belt, followed by near‐isothermal decompression and subsequent cooling, is in accordance with a long‐lived tectonic history accompanied by high magmatic activity in the Lurio Belt during the late Neoproterozoic–early Palaeozoic East‐African–Antarctic orogeny.     

Olivine‐bearing symplectites in fractured garnet from eclogite,Moldanubian Zone (Bohemian Massif) – a short‐lived,granulite facies event

Recent petrological studies on high‐pressure (HP)–ultrahigh‐pressure (UHP) metamorphic rocks in the Moldanubian Zone, mainly utilizing compositional zoning and solid phase inclusions in garnet from a variety of lithologies, have established a prograde history involving subduction and subsequent granulite facies metamorphism during the Variscan Orogeny. Two temporally separate metamorphic events are developed rather than a single P–T loop for the HP–UHP metamorphism and amphibolite–granulite facies overprint in the Moldanubian Zone. Here further evidence is presented that the granulite facies metamorphism occurred after the HP–UHP rocks had been exhumed to different levels of the middle or upper crust. A medium‐temperature eclogite that is part of a series of tectonic blocks and lenses within migmatites contains a well‐preserved eclogite facies assemblage with omphacite and prograde zoned garnet. Omphacite is partly replaced by a symplectite of diopside + plagioclase + amphibole. Garnet and omphacite equilibria and pseudosection calculations indicate that the HP metamorphism occurred at relatively low temperature conditions of ~600 °C at 2.0–2.2 GPa. The striking feature of the rocks is the presence of garnet porphyroblasts with veins filled by a granulite facies assemblage of olivine, spinel and Ca‐rich plagioclase. These minerals occur as a symplectite forming symmetric zones, a central zone rich in olivine that is separated from the host garnet by two marginal zones consisting of plagioclase with small amounts of spinel. Mineral textures in the veins show that they were first filled mostly by calcic amphibole, which was later transformed into granulite facies assemblages. The olivine‐spinel equilibria and pseudosection calculations indicate temperatures of ~850–900 °C at pressure below 0.7 GPa. The preservation of eclogite facies assemblages implies that the granulite facies overprint was a short‐lived process. The new results point to a geodynamic model where HP–UHP rocks are exhumed to amphibolite facies conditions with subsequent granulite facies heating by mantle‐derived magma in the middle and upper crust.     

胶东海阳所堆积辉长岩的变质反应结构、石榴石形成及P—T演化

海阳所堆积辉长岩由橄长岩、橄榄辉长岩和辉长岩组成。在橄长岩和橄榄辉长岩中发育有典型的变质反应结构：主要为橄榄石与斜长石之间形成由斜方辉石、尖晶石、角闪石和石榴石等矿物组成的多期次次变边，并有三个不同世代变质矿物，早期Cpx＋Opx＋Spl，中期Amp，晚期Grt；期次是钛铁矿与斜长石之间形成石榴石次变边，相对比较简单，只有一个世代变质矿物，为Grt＋Amp＋Rut或Grt＋Rut岩中石榴石是通过斜长石与角闪石或斜长石与钛铁矿之间的变质反应形成的，虽为峰值变质作用的产物，但变质反应的期次及类型不同导致了所形成石榴石的温度和压力有所不同。堆积辉长岩形成演化的温压计算表明，堆积辉长岩在经过近等压降温的岩浆作用之后的变质作用早期，仍为近等压降温，而晚期则表现为近等温升压。这一特殊的P－T演化可能反映了堆积辉长冷侵位与深俯冲特征。     

An Early Palaeozoic HP/HT granulite–garnet peridotite association in the south Altyn Tagh, NW China: P–T history and U-Pb geochronology

High‐pressure kyanite‐bearing felsic granulites in the Bashiwake area of the south Altyn Tagh (SAT) subduction–collision complex enclose mafic granulites and garnet peridotite‐hosted sapphirine‐bearing metabasites. The predominant felsic granulites are garnet + quartz + ternary feldspar (now perthite) rocks containing kyanite, plagioclase, biotite, rutile, spinel, corundum, and minor zircon and apatite. The quartz‐bearing mafic granulites contain a peak pressure assemblage of garnet + clinopyroxene + ternary feldspar (now mesoperthite) + quartz + rutile. The sapphirine‐bearing metabasites occur as mafic layers in garnet peridotite. Petrographical data suggest a peak assemblage of garnet + clinopyroxene + kyanite + rutile. Early kyanite is inferred from a symplectite of sapphirine + corundum + plagioclase ± spinel, interpreted to have formed during decompression. Garnet peridotite contains an assemblage of garnet + olivine + orthopyroxene + clinopyroxene. Thermobarometry indicates that all rock types experienced peak P–T conditions of 18.5–27.3 kbar and 870–1050 °C. A medium–high pressure granulite facies overprint (780–820 °C, 9.5–12 kbar) is defined by the formation of secondary clinopyroxene ± orthopyroxene + plagioclase at the expense of garnet and early clinopyroxene in the mafic granulites, as well as by growth of spinel and plagioclase at the expense of garnet and kyanite in the felsic granulite. SHRIMP II zircon U‐Pb geochronology yields ages of 493 ± 7 Ma (mean of 11) from the felsic granulite, 497 ± 11 Ma (mean of 11) from sapphirine‐bearing metabasite and 501 ± 16 Ma (mean of 10) from garnet peridotite. Rounded zircon morphology, cathodoluminescence (CL) sector zoning, and inclusions of peak metamorphic minerals indicate these ages reflect HP/HT metamorphism. Similar ages determined for eclogites from the western segment of the SAT suggest that the same continental subduction/collision event may be responsible for HP metamorphism in both areas.     

Coronitic textures in the ferrogabbroids of the Elet’ozero intrusive complex (northern Karelia,Russia) as evidence for the existence of Fe-rich melt. 1. Types of coronas

Fe–Ti oxides (magnetite, Ti-magnetite, ilmenite, and associated high-Al spinel) in the ferrogabbroids of the Middle Paleoproterozoic Elet’ozero syenite–gabbro intrusion are intercumulus minerals usually surrounded by coronitic rims of two types. The first type usually represents multilayer amphibole–biotite ± olivine coronas along contacts of Fe–Ti oxides with cumulus moderate-Ca plagioclase and more rarely, clinopyroxene. Two-layer rim is developed in contact with high-Ca plagioclase; the inner rim consists of pargasite and spinel, while the outer rim is made up of sadanagaite and spinel. The second type is represented by two-stage coronitic textures developed along boundaries of olivine and Fe–Ti oxide clusters with plagioclase. Initially, the olivine was surrounded by orthopyroxene rim, while Fe–Ti oxides were rimmed by pargasite with thin ingrowths of high-Al spinel (hercynite). At the next stage, the entire cluster was fringed by a common symplectite reaction rim, the composition of which also depended on the composition of plagioclase matrix: the spinel–sadanagaite rim was formed in contact with high-Ca plagioclase, while pargasite–muscovite–scapolite rim was formed in contact with moderate-Ca plagioclase. The formation of the outer rims occurred after hydration of the inner parts of coronas around olivine and oxides within the clusters. It is suggested that the Fe–Ti oxides and surrounding coronitic rims were microsystems formed by crystallization of drops of residual hydrous Fe-rich liquid.     

Material Transfer and Local Equilibria in a Zoned Kelyphite from a Garnet Pyroxenite, Ronda, Spain

A zoned kelyphite after garnet, from a garnet pyroxenite layer,the Ronda peridotite. Spain, has been studied and the mechanismof kelyphite formation is discussed. The kelyphite is an extremelyfinegrained symplectitic mixture of orthopyroxene, spinel, olivine,plagioclase, and ilmenite. It is concentrically zoned, formingthree mineralogical subzones. They are, from adjacent to a garnetgrain toward a clinopyroxene side, zone I (orthopyroxene+spinel+ plagioclase), zone II (olivine+spinel+plagioclase), and zoneIII (olivine+plagioclase). The analysis of phase equilibriashows that this mineralogical zonation can develop stably asa result of the presence of chemical potential gradients. Onthe basis of microprobe chemical analyses for each zone, materialtransfer across the zone that took place during the kelyphitizationwas quantitatively evaluated, and by locating the initial grainboundary between garnet and clinopyroxene grains and by writingmetasomatic reactions for each zone boundary, a simple dynamicmodel for the kelyphite formation is proposed. The kelyphiteformation probably took place when the host Ronda peridotiteascended from the upper mantle to the crust. It involved a co-operativebreakdown of the garnet and aluminous clinopyroxene, being accompaniedby a material transfer across the zone boundaries. By examiningthe Fe-Mg partitioning between olivine, spinel, and orthopyroxenein the kelyphite and by examining the Al content of the orthopyroxene,an attainment of local equilibrium has been confirmed, and thephysical conditions of the kelyphite formation have been estimatedto be 620–700C and 4–8 kbar.     

Quartz and orthopyroxene exsolution lamellae in clinopyroxene and the metamorphic P–T path of Belomorian eclogites

The Shirokaya Salma eclogite‐bearing complex is located in the Archean–Palaeoproterozoic Belomorian Province (Russia). Its eclogites and eclogitic rocks show multiple clinopyroxene breakdown textures, characterized by quartz–amphibole, orthopyroxene and plagioclase lamellae. Representative samples, a fresh eclogite, two partly retrograded eclogites, and a strongly retrograded eclogitic rock, were collected for this study. Two distinct mineral assemblages—(1) omphacite+garnet+quartz+rutile±amphibole and (2) clinopyroxene+garnet+amphibole+plagioclase+quartz+rutile+ilmenite±orthopyroxene—are described. Based on phase equilibria modelling, these assemblages correspond to the eclogite and granulite facies metamorphism that occurred at 16–18 kbar, 750–800°C and 11–15 kbar, 820–850°C, respectively. The quartz–amphibole lamellae in clinopyroxene formed during retrogression with water ingress, but do not imply UHP metamorphism. The superfine orthopyroxene lamellae developed due to breakdown of an antecedent clinopyroxene (omphacite) during retrogression that was triggered by decompression from the peak of metamorphism, while the coarser orthopyroxene grains and rods formed afterwards. The P–T path reconstructed for the Shirokaya Salma eclogites is comparable to that of the adjacent 1.9 Ga Uzkaya Salma eclogite (Belomorian Province), and those of several other Palaeoproterozoic high‐grade metamorphic terranes worldwide, facts allowing us to debate the exact timing of eclogite facies metamorphism in the Belomorian Province.     

Experimental modelling of corona textures

Formation of corona textures along olivine–plagioclase and orthopyroxene–plagioclase interfaces has been experimentally reproduced at 670 and 700 °C and 5 kbar with either a pure H₂O fluid phase or 0.1 and 37 m NaCl–H₂O solution fluid. In these experiments, we investigate the interaction of primary olivine and/or orthopyroxene and plagioclase in powders and polished crystals, and in small samples of a natural gabbro. The experiments result in the formation of corona textures with several layers of different assemblages (according to the experimental conditions) consisting of garnet (grossular), clinopyroxene, orthopyroxene, amphibole, chlorite and phlogopite. The experiments show major differences in the number of layers, the mineral assemblages and mineral composition, and in the trends of composition of plagioclase in coronas around olivine and orthopyroxene. The fluid phase composition influences the corona assemblages and the composition of the minerals in the experimental coronas; for example, garnet appears in the coronas in the second experiment where the NaCl–H₂O ratio is low. Experimental modelling of corona textures confirms a model of simultaneous growth of layers by the mechanism of diffusion metasomatism with participation of a fluid phase through which mass is transferred. Zoning in the experimental coronas shows opposing diffusion of Al and Ca from plagioclase and Mg and Fe from olivine/orthopyroxene; difference in the mobility of the components is inferred from observations in the coronas. The experimental corona textures are compared with natural coronas from the Belomorian belt (Baltic shield), developed at 670–690 °C and 7–8 kbar, and the Marun‐Keu complex (Polar Urals), developed at 670–700 °C and 14–16 kbar, where the corona textures correspond to a transitional stage of the gabbro‐to‐eclogite transformation.     

Transformation of Spinel Lherzolite to Garnet Lherzolite in Ultramafic Lenses of the Austridic Crystalline Complex, Northern Italy

Numerous lenticular bodies of ultramafic rocks occur withinthe upper amphibolite- to granulitefacies metamorphic terraneof the Austrides between the Non and Ultimo valleys (Nonsbergregion), northern Italy. The ultramafic rocks are divided intotwo textural types: (a) coarse-type; and (b) finetype. The coarse-typerocks have the protogranular texture and are predominantly spinellherzolite. Some coarse-type spinel lherzolites have partlytransformed to garnet lherzolite. The fine-types are consideredto be metamorphic derivatives of the former, and the observedmineral assemblages are: (1) olivine + orthopyroxene + clinopyroxene+ garnet + amphibole ? spinel, (2) olivine + orthopyroxene +garnet + amphibole + spinel; (3) olivine + orthopyroxene + amphibole+ spinel; and (4) olivine+ orthopyroxene + amphibole + chlorite.Based on the microprobe analyses of constituent minerals fromten representative peridotite samples, physical conditions ofthe metamorphism, particularly that of the spinel to garnetlherzolite transformation, are estimated. Applications of pyroxenegeothermometry yield temperature estimates of 1100–1300?Cfor the formation of the primary spinel lherzolite, and 700–800?Cfor that of the fine-type peridotites. A pressure range of 16–28kb is obtained for the garnet lherzolite crystallization dependingon the choice of geobarometers. Two alternative P-T paths, i.e.(1) isobaric cooling or (2) pressure-increase and temperaturedecrease are considered and their geodynamic implications discussed.     

Ultrahigh-pressure metamorphism and exhumation of garnet peridotite in Pohorje, Eastern Alps

New evidence for ultrahigh‐pressure metamorphism (UHPM) in the Eastern Alps is reported from garnet‐bearing ultramafic rocks from the Pohorje Mountains in Slovenia. The garnet peridotites are closely associated with UHP kyanite eclogites. These rocks belong to the Lower Central Austroalpine basement unit of the Eastern Alps, exposed in the proximity of the Periadriatic fault. Ultramafic rocks have experienced a complex metamorphic history. On the basis of petrochemical data, garnet peridotites could have been derived from depleted mantle rocks that were subsequently metasomatized by melts and/or fluids either in the plagioclase‐peridotite or the spinel‐peridotite field. At least four stages of recrystallization have been identified in the garnet peridotites based on an analysis of reaction textures and mineral compositions. Stage I was most probably a spinel peridotite stage, as inferred from the presence of chromian spinel and aluminous pyroxenes. Stage II is a UHPM stage defined by the assemblage garnet + olivine + low‐Al orthopyroxene + clinopyroxene + Cr‐spinel. Garnet formed as exsolutions from clinopyroxene, coronas around Cr‐spinel, and porphyroblasts. Stage III is a decompression stage, manifested by the formation of kelyphitic rims of high‐Al orthopyroxene, aluminous spinel, diopside and pargasitic hornblende replacing garnet. Stage IV is represented by the formation of tremolitic amphibole, chlorite, serpentine and talc. Geothermobarometric calculations using (i) garnet‐olivine and garnet‐orthopyroxene Fe‐Mg exchange thermometers and (ii) the Al‐in‐orthopyroxene barometer indicate that the peak of metamorphism (stage II) occurred at conditions of around 900 °C and 4 GPa. These results suggest that garnet peridotites in the Pohorje Mountains experienced UHPM during the Cretaceous orogeny. We propose that UHPM resulted from deep subduction of continental crust, which incorporated mantle peridotites from the upper plate, in an intracontinental subduction zone. Sinking of the overlying mantle and lower crustal wedge into the asthenosphere (slab extraction) caused the main stage of unroofing of the UHP rocks during the Upper Cretaceous. Final exhumation was achieved by Miocene extensional core complex formation.     

Ultrahigh-pressure metamorphism and exhumation of garnet-bearing ultramafic rocks from the Lanterman Range (northern Victoria Land, Antarctica)

Northern Victoria Land is a key area for the Ross Orogen – a Palaeozoic foldbelt formed at the palaeo‐Pacific margin of Gondwana. A narrow and discontinuous high‐ to ultrahigh‐pressure (UHP) belt, consisting of mafic and ultramafic rocks (including garnet‐bearing types) within a metasedimentary sequence of gneisses and quartzites, is exposed at the Lanterman Range (northern Victoria Land). Garnet‐bearing ultramafic rocks evolved through at least six metamorphic stages. Stage 1 is defined by medium‐grained garnet + olivine + low‐Al orthopyroxene + clinopyroxene, whereas finer‐grained garnet + olivine + orthopyroxene + clinopyroxene + amphibole constitutes the stage 2 assemblage. Stage 3 is defined by kelyphites of orthopyroxene + clinopyroxene + spinel ± amphibole around garnet. Porphyroblasts of amphibole replacing garnet and clinopyroxene characterize stage 4. Retrograde stages 5 and 6 consist of tremolite + Mg‐chlorite ± serpentine ± talc. A high‐temperature (～950 °C), spinel‐bearing protolith (stage 0), is identified on the basis of orthopyroxene + clinopyroxene + olivine + spinel + amphibole inclusions within stage 1 garnet. The P–T estimates for stage 1 are indicative of UHP conditions (3.2–3.3 GPa and 764–820 °C), whereas stage 2 is constrained between 726–788 °C and 2.6–2.9 GPa. Stage 3 records a decompression up to 1.1–1.3 GPa at 705–776 °C. Stages 4, 5 and 6 reflect uplift and cooling, the final estimates yielding values below 0.5 GPa at 300–400 °C. The retrograde P–T path is nearly isothermal from UHP conditions up to deep crustal levels, and becomes a cooling–unloading path from intermediate to shallow levels. The garnet‐bearing ultramafic rocks originated in the mantle wedge and were probably incorporated into the subduction zone with felsic and mafic rocks with which they shared the subsequent metamorphic and geodynamic evolution. The density and rheology of the subducted rocks are compatible with detachment of slices along the subduction channel and gravity‐driven exhumation.     

Details of the gabbro‐to‐eclogite transition determined from microtextures and calculated chemical potential relationships

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 (0.15) to H₂O‐saturation, and therefore does not imply that the rocks were ever fluid‐saturated. Calculations in the CMAS and NCFMAS systems show that the gabbro‐to‐eclogite transition is characterized by the growth of garnet, diopsidic clinopyroxene and kyanite due to diffusion of Ca (+ Na) and Mg (+ Fe) along a μCaO (+ Na₂O)–μMgO (+ FeO) chemical potential gradient developed between orthopyroxene and plagioclase compositional domains. The anhydrous nature of the textures indicate that the gabbro‐to‐eclogite transition is not driven by hydration; however, increased μH₂O acts as a catalyst that increases diffusivity of all elements and rates of dissolution–precipitation, allowing the overstepped metamorphic reactions to occur. Our results show that crustal eclogite formation requires low H₂O content, confirming that true eclogites are dry rocks.     

Magmatic versus metamorphic origin of olivine-plagioclase coronas

An SEM, REE, and mineral Sm-Nd isotope study on olivine-plagioclase coronas in the Vestre Dale gabbro, Norway, was carried out in order to solve the temporal relationships within these coronas. It is demonstrated that in contrast to common models, corona formation is a multistage, late-magmatic process, starting with the formation of orthopyroxene by (partial) dissolution of olivine, followed by the nucleation and rapid outward growth of orthopyroxene+spinel symplectites at its outer margin, and concluded by replacement of this precursory outer shell by calcic amphibole. Isotopic equilibrium between inner shell orthopyroxene and the magmatic assemblage, and the REE contents of orthopyroxene and outer shell amphibole also provide strong arguments for a magmatic origin, with the local availability of fractionated magma acting as a limiting factor. The absence of replacement of intercumulus clinopyroxene by amphibole also favours a late stage magmatic rather than a metamorphic origin for corona amphibole.