On the origin of multi‐layer coronas between olivine and plagioclase at the gabbro–granulite transition,Valle Fértil–La Huerta Ranges,San Juan Province,Argentina

Troctolitic gabbros from Valle Fértil and La Huerta Ranges, San Juan Province, NW‐Argentina exhibit multi‐layer corona textures between cumulus olivine and plagioclase. The corona mineral sequence, which varies in the total thickness from 0.5 to 1 mm, comprises either an anhydrous corona type I with olivine|orthopyroxene|clinopyroxene+spinel symplectite|plagioclase or a hydrous corona type II with olivine|orthopyroxene|amphibole|amphibole+spinel symplectite|plagioclase. The anhydrous corona type I formed by metamorphic replacement of primary olivine and plagioclase, in the absence of any fluid/melt phase at <840 °C. Diffusion controlled metamorphic solid‐state replacement is mainly governed by the chemical potential gradients at the interface of reactant olivine and plagioclase and orthopyroxene and plagioclase. Thus, the thermodynamic incompatibility of the reactant minerals at the gabbro–granulite transition and the phase equilibria of the coronitic assemblage during subsequent cooling were modelled using quantitative μMgO–μCaO phase diagrams. Mineral reaction textures of the anhydrous corona type I indicate an inward migration of orthopyroxene on the expense of olivine, while clinopyroxene+spinel symplectite grows outward to replace plagioclase. Mineral textures of the hydrous corona type II indicate the presence of an interstitial liquid trapped between cumulus olivine and plagioclase that reacts with olivine to produce a rim of peritectic orthopyroxene around olivine. Two amphibole types are distinguished: an inclusion free, brownish amphibole I is enriched in trace elements and REEs relative to green amphibole II. Amphibole I evolves from an intercumulus liquid between peritectic orthopyroxene and plagioclase. Discrete layers of green amphibole II occur as inclusion‐free rims and amphibole II+spinel symplectites. Mineral textures and geochemical patterns indicate a metamorphic origin for amphibole II, where orthopyroxene was replaced to form an inner inclusion‐free amphibole II layer, while clinopyroxene and plagioclase were replaced to form an outer amphibole+spinel symplectite layer, at <770 °C. Calculation of the possible net reactions by considering NCKFMASH components indicates that the layer bulk composition cannot be modelled as a ‘closed’ system although in all cases the gain and loss of elements within the multi‐layer coronas (except H₂O, Na₂O) is very small and the main uncertainties may arise from slight chemical zoning of the respective minerals. Local oxidizing conditions led to the formation of orthopyroxene+magnetite symplectite enveloping and/or replacing olivine. The sequence of corona reaction textures indicates a counter clockwise P–T path at the gabbro–granulite transition at 5–6.5 kbar and temperatures below 900 °C.     

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.     

Late magmatic to subsolidus coronas in gabbroic rocks from the sila massif (Calabria,Italy)

Summary Corona textures between olivine and plagioclase or orthopyroxene and plagioclase are present in Hercynian gabbroic rocks from the Calabrian Sila Massif. They have been studied through optical and SEM investigations together with EDS and WDS analyses. Textural features indicate the existence of two extreme corona types formed during late magmatic stages or during subsolidus cooling. Magmatic coronas are characterized by an inner orthopyroxene layer and an outer orange-brown amphibole layer that might be in optical continuity with orthopyroxene and amphibole poikilites respectively. Subsolidus coronas consist of an inner layer of colourless amphibole and an outer layer of amphibole ± spinel. They sometimes form a collar also around plagioclase enclosed in olivine. A large spectrum in the composition of corona amphiboles from Ti-bearing pargasite to Mg-hornblende was observed. The variation in Ti content of amphibole was interpreted as a consequence of the different conditions of crystallization from late magmatic to subsolidus with temperatures ranging from 880°C to 580°C. The significant gahnite component in spinel possibly indicates that subsolidus reactions occurred in an open system. The pressure of formation constrained by the mineral assemblage of metamorphic basement rocks and by the neighbouring diorites has been estimated at 4 kbar.

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Spätmagmatische und Subsolidus-Koronatexturen in gabbroiden Gesteinen des Sila Massives (Kalabrien, Italien)

Zusammenfassung In herzynischen gabbroiden Gesteinen des Sila Massives in Kalabrien treten Korona-texturen zwischen Olivin und Plagioklas oder Orthopyroxen und Plagioklas auf. Diese wurden mittels optischer Methoden und SEM in Verbindung mit EDS und WDS Analytik untersucht. Textureile Kriterien belegen die Existenz zweier verschiedenartiger Koronatypen die während deospätmagmatischen Stadiums oderwährend der Abkühlung lung im Subsolidus Bereich gebildet wurden. Die magmatischen Koronatexturen sind durch eine innere Othopyrozenschicht und eine äußere orange-braune Ampkibolschicht gekennzeichnet, die in optischer Kontinuatät Orthopyroxen bzw. Amphibihol-poikilitn steht. Subsolidus-Koronas bestehen aus einer inneren Lage eines farblosen Amphiboles und einer äußeren Schicht von Amphibol ± Spinell. Bisweilen umgeben sie ringförmig in Olivin eingeschlossenen Plagioklos. Die Amphibolzusammensetzung in diesen Koronas variiert stark von Ti-führendem Pargasit bis Mg-Hornblende. Die Streubreite dumTi-Gehaltes der Amphibole wird durch unterschiedliche Kristallisations-bedingungen während des spätmagmatischen bis Subsolidusstadiums (880°C bis 550°C) interpretiert. Die signifikante (Gahnitkomponente des Spinells weist auf Subsolidusreaktionen in einem offenen System hin. Der Bildungsdruck, ablegeit aus der Mineral-vergesellschaftung der metamorphen Basementgetsteine und der benachbarten Diorite, wird mit 4kb abgeschätzt.

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With 3 Figures     

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.     

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.     

Coronas from the Thessaloniki gabbros (North Greece)

Coronas which have been developed between olivine and plagioclase in Precambrian gabbroic rocks from Thessaloniki, Greece, have been studied. These consist commonly of (olivine), clinopyroxene, amphibole (plagioclase) and rarely of (olivine) orthopyroxene, clinopyroxene amphibole (plagioclase) assemblages. The results of the electron-probe microanalysis of the above constituent minerals are presented. The formation of these coronas by regional or thermal metamorphism is unlikely, but an origin by a two-way diffusion of material across the olivine and plagioclase interface is proposed.     

Origin of coronas in metagabbros of the Adirondack mts., N. Y.

Metagabbros from two widely separated areas in the Adirondacks show development of coronas. In the Southern Adirondacks, these are cored by olivine which is enclosed in a shell of orthopyroxene that is partially, or completely, rimmed by symplectites consisting of clinopyroxene and spinel. Compositions of the corona phases have been determined by electron probe and are consistent with a mechanism involving three partial reactions, thus:

1. Olivine=Orthopyroxene+(Mg, Fe)⁺⁺.
2. Plagioclase+(Mg, Fe)⁺⁺+Ca⁺⁺=Clinopyroxene+Spinel+Na⁺.
3. Plagioclase+(Mg, Fe)⁺⁺+Na⁺=Spinel+more sodic plagioclase+Ca⁺⁺.

Reaction (a) occurs in the inner shell of the corona adjacent to olivine; reaction (b) in the outer shell; and (c) in the surrounding plagioclase, giving rise to the spinel clouding which is characteristic of the plagioclase in these rocks. Alumina and silica remain relatively immobile. These reactions, when balanced, can be generalized to account for the aluminous nature of the pyroxenes and for changing plagioclase composition. Summed together, the partial reactions are equivalent to:

1. Olivine + Anorthite = Aluminous orthopyroxene + Aluminous Clinopyroxene + Spinel (Kushiro and Yoder, 1966).

In the Adirondack Highlands, coronas between olivine and plagioclase commonly have an outer shell of garnet replacing the clinopyroxene/spinel shell. The origin of the garnet can also be explained in terms of three partial reactions:

1. Orthopyroxene+Ca⁺⁺=Clinopyroxene+(Mg, Fe)⁺⁺.
2. Clinopyroxene+Spinel+Plagioclase+(Mg, Fe)⁺⁺=Garnet+Ca⁺⁺+Na⁺.
3. Plagioclase+(Mg, Fe)⁺⁺+Na⁺=Spinel + more sodic plagioclase+Ca⁺⁺.

These occur in the inner and outer corona shell and the surrounding plagioclase, respectively, and involve the products of reactions (a)-(d). Alumina and silica are again relatively immobile. Balanced, and generalized to account for aluminous pyroxenes and variable An content of plagioclase, they are equivalent to:

1. Orthopyroxene+Anorthite+Spinel=Garnet (Green and Ringwood, 1967).

Amphibole coronas about opaque oxides in rocks of both areas are the result of oxide/plagioclase reactions with addition of magnesium from coexisting olivine. Based on published experimental data, pressure and temperature at the time of corona formation were on the order of 8 kb and 800° C for the garnet bearing coronas, with somewhat lower pressures indicated for the clinopyroxene/spinel coronas.     

Development of corona textures around olivine in anorthosites of the Korosten’ pluton, Ukrainian Shield: Mineralogy, geochemistry, and fluid inclusions

New data on the composition of minerals in corona textures around olivine and crystal-fluid inclusions in olivine from anorthosites of the Korosten’ pluton (sampled in the Golovino quarry), Ukrainian Shield were obtained using electron and ion microprobe analyses, Raman spectroscopy, scanning electron microscopy, and cryo- and thermometry. The corona textures developed around olivine grains in contact with plagioclase and consist of inner orthopyroxene rims around olivine and outer rims of orthopyroxene-clinopyroxene-orthoclase-plagioclase symplectites. The symplectites and orthopyroxene rims most probably developed nearly simultaneously and grew in the opposite directions from the original contact of the magmatic olivine and plagioclase and replaced both olivine and plagioclase. The Al₂O₃ and CaO concentrations in the symplectitic orthopyroxene increase toward the contact with magmatic plagioclase, whereas the Al₂O₃ and CaO concentrations in the symplectitic plagioclase simultaneously decrease and its Na₂O and K₂O increase. Optically discernible crystalline and fluid phases of crystal-fluid inclusions in olivine were identified as pyroxenes (orthopyroxene and clinopyroxene), actinolite, Ca-and Fe, Mg-carbonates, and magnetite, along with practically pure highdensity CO₂. The mineral assemblages of corona texture in the Korsten’ anorthosites were produced by autometasomatic processes at a high CO₂ activity, and the local variations in the chemistry of corona minerals were likely controlled by the content and chemistry of the interstitial fluid and primary minerals. The coronas developed under subsolidus conditions, via the reaction interaction of olivine and plagioclase under the effect of an integranular fluid, with the dissolution of olivine and plagioclase at T = 980–860°C and P > 5 kbar. Inasmuch as corona textures do not occur ubiquitously in the rocks, the origin of the former was most probably controlled by the amount of the intergranular fluid.     

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.     

The origin of brown hornblende in the Artfjället gabbro and dolerites

Brown hornblende occurs in minor amounts in the Artfjället gabbro and dolerites, except in quartz-dolerites where a pale green hornblende occurs. In the gabbro, brown hornblende is mostly Ti-bearing pargasite or kaersutite. It occurs along veins of orthopyroxene, as rims around and blebs in pyroxenes, with orthopyroxene in coronas between olivine and plagioclase and in coronas between ilmenite and plagioclase. In the olivine-dolerites and orthopyroxene-dolerites brown hornblende is ferroan titanian pargasite or ferroan kaersutite. The pale green hornblende in the quartz-dolerites is a magnesio-hornblende. The hornblendes in the dolerites are interstitial or granular, in some dolerites occurring as coarse oikocrysts. It is proposed that under certain conditions the Ti content of hornblende can be used as a thermometer, derived from experimental data of Helz (1973). Microstructures, compositions and formation temperatures (< 1,040° C) show that the brown hornblende in the gabbro is not magmatic, but of subsolidus origin. Probably it formed as a result of the introduction of water into the gabbro during a deformation event that occurred early in the cooling history of the gabbro. Least-squares modelling of hornblende formation indicates that all magmatic minerals must have participated in the reaction and that the reaction probably was not isochemical. Microstructures, compositions and formation temperatures (1,030-965° C) of brown hornblende in the dolerites are consistent with late-stage crystallization from the magma. For the pale green hornblende in the quartz-dolerites a magmatic origin is likely, but cannot be proven.     

Metasomatism and origin of glass in the lithospheric mantle xenoliths beneath Ain Temouchent area (North-West Algeria)

A spinel ± amphibole ± feldspar bearing Iherzolites, a spinel ± amphibole ± feldspar bearing harzburgites, and a spinel ± amphibole ± phlogopite bearing wehrlites are metasomatized peridotitic mantle xenoliths from Ain Temouchent volcanic complex (North-West Algeria). These xenoliths are metamorphic/deformed rocks with a strong planar fabric typical of mantle tectonites. The wehrlites are not the result of a simple model of partial melting. The spinel ± amphibole ± feldspar bearing harzburgites and lherzolites exhibit asymmetric concave-shaped REE patterns. These indicate that an earlier partial melting event was followed by metasomatic processes. The wehrlites have higher REE concentrations and LREE/HREE fractionations, indicating a sequential evolution of wehrlites from previous refractory material with melting as an addition process. This process reflects the interaction of the lithospheric mantle beneath the Ain Temouchent area with basaltic melt. Metasomatism is expressed by the formation of amphibole, phlogopite, and increased abundances of clinopyroxene at the expense of orthopyroxene, in lherzolite and harzburgite. In the Ain Temouchent area, metasomatizing agents are Na-alkali silicates. The similarities observed between the glasses studied in this paper, and the basaltic host rocks of the Ain Temouchent area, may suggest a common mantle source, or with chemical similarities but with relatively different evolutions pathways. The formation of glass in wehrlites from the Ain Temouchent area has an origin formed by the breakdown of amphibole or phlogopite as a result of decompressional melting and production of silica-undersaturated glasses. The glass reacts with essentially orthopyroxene to produce silica-rich glasses. This study has contributed to highlighting a relationship between glass, and the processes that caused the formation of metasomatic phases.     

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.     

Transformation of garnet epidote amphibolite to eclogite, western Dabie Mountains, China

Omphacite and garnet coronas around amphibole occur in amphibolites in the Hong'an area, western Dabie Mountains, China. These amphibolites consist of an epidote–amphibolite facies assemblage of amphibole, garnet, albite, clinozoisite, paragonite, ilmenite and quartz, which is incompletely overprinted by an eclogite facies assemblage of garnet, omphacite and rutile. Coronas around amphibole can be divided into three types: an omphacite corona; a garnet–omphacite–rutile corona; and, a garnet–omphacite corona with less rutile. Chemographic analysis for local reaction domains in combination with petrographical observations show that reactions Amp + Ab + Pg = Omp +Czo + Qtz + H₂O, and Amp + Ab = Omp ± Czo + Qtz + H₂O may lead to the development of omphacite coronas. The garnet–omphacite–rutile corona was formed from the reaction Amp + Ab + Czo + Ilm ± Qtz = Omp + Grt + Rt + H₂O. In garnet–omphacite coronas, the garnet corona grew during an early stage of epidote amphibolite facies metamorphism, whereas omphacite probably formed by the reactions forming the omphacite corona during the eclogite facies stage. It is estimated that these reactions occurred at 0.8–1.4 GPa and 480–610 °C using the garnet–clinopyroxene thermometer and omphacite barometer in the presence of albite.     

Coronas in olivine gabbros and iron ores from Susimäki and Riuttamaa,Finland

Coronas have been studied by petrographie and microprobe techniques in metamorphosed olivine gabbros and associated iron ores from Susimäki and Riuttamaa in Southwest Finland. Three types of coronas are distinguished occurring between the following primary minerals: (1) olivine-plagioclase, (2) opaque oxides-plagioclase, (3) opaque oxides-clinopy-roxene. Secondary corona minerals are, in order of decreasing abundance, hornblende, orthopyroxene, spinel, olivine, ilmenite, and magnetite. This is the first reported occurrence of coexisting primary and secondary olivines in coronas. Quantitative approximations of the corona-producing reactions are given by chemical equations of the analyzed reactant and product mineral phases. Individual coronas of all three types developed essentially as allochemical systems open to mass transfer by an intergranular fluid phase. The overall corona formation within the volume of a handspecimen likely involved only a net gain of water and a loss of Na.     

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.     

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.     

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.     

Petrology of the Udayagiri anorthosite complex, Eastern Ghats Belt, India

The Eastern Ghats Belt (EGB), characterised by pervasive Grenvillian granulite facies metamorphism, is the host to several 950–1000 Ma old massif-type anorthosite complexes. The present work describes one such complex near Udayagiri from the northern margin of the EGB, reported for the first time as “Udayagiri anorthosite complex” (UAC). The ‘massif type’ UAC comprises mainly of anorthosite, leuconorite-olivine leuconorite and norite in the decreasing order of areal extent. Mineralogically, these rocks dominantly consist of cumulates of moderately calcic plagioclase (～An_50–60), moderately magnesian intercumulus olivine (X_Mg: ～0.6) and orthopyroxene (X_Mg: 0.47 to 0.70). Metamorphic garnet (Alm: ～50 mol%) is also common in these rocks. Anorthosite and leuconorite of the UAC exhibit a moderate ‘+ve’ Eu anomaly. Norite occurs locally as schlierens and is relatively rich in Fe, P, Rb, Sr, Th, Nb, Ta, Y and REE which could be a residual melt product. These rocks exhibit both relict magmatic mineralogy and textures with a metamorphic impress manifested by the development of multilayered corona involving olivine, orthopyroxene, garnet, phlogopite, ilmenite and plagioclase during cooling of the pluton. The corona development is a result of combination of significant magmatic and metamorphic reactions which have the potential to provide important clues for deciphering the magmatic and metamorphic evolution of such plutons in ambient granulite facies conditions.     

The role of trace element partitioning between garnet,zircon and orthopyroxene on the interpretation of zircon U–Pb ages: an example from high-grade basement in Calabria (Southern Italy)

The recognition of the coeval growth of zircon, orthopyroxene and garnet domains formed during the same metamorphic cycle has been attempted with detailed microanalyses coupled with textural analyses. A coronitic garnet-bearing granulite from the lower crust of Calabria has been considered. U–Pb zircon data and zircon, garnet and orthopyroxene chemistries, at different textural sites, on a thin section of the considered granulite have been used to test possible equilibrium and better constrain the geological significance of the U–Pb ages related to zircon separates from other rocks of the same structural level. The garnet is very rich in REE and is characterised by a decrease in HREE from core to outer core and an increase in the margin. Zircons show core–overgrowth structures showing different chemistries, likely reflecting episodic metamorphic new growth. Zircon grains in matrix, corona around garnet and within the inner rim of garnet, are decidedly poorer in HREE up to Ho than garnet interior. Orthopyroxene in matrix and corona is homogeneously poor in REE. Thus, the outer core of garnet and the analysed zircon grains grew or equilibrated in a REE depleted system due to the former growth of garnet core. Zircon ages ranging from 357 to 333 Ma have been determined in the matrix, whereas ages 327–320 Ma and around 300 Ma have been determined, respectively, on cores and overgrowths of zircons from matrix, corona and inner rim of garnet. The calculated DREE_zrn/grt and DREE_opx/grt are largely different from the equilibrium values of literature due to strong depletion up to Ho in zircon and orthopyroxene with respect to garnet. On the other hand, the literature data show large variability. In the case study, (1) the D _zrn/grt values define positive and linear trends from Gd to Lu as many examples from literature do and the values from Er to Lu approach the experimental results at about 900 °C in the combination zircon dated from 339 to 305 Ma with garnet outer core, and (2) D _opx/grt values define positive trends reaching values considered as suggestive of equilibrium from Er to Lu only with respect to the outer core of garnet. The presence of a zircon core dated 320 Ma in the inner rim of garnet suggests that it, as well as those dated at 325–320 Ma in the other textural sites and, probably, those dated at 339–336 Ma showing depletion of HREE, grew after the garnet core, which sequestered a lot of HREE and earlier than the HREE rich margin of garnet. The quite uniform REE contents in orthopyroxene from matrix and corona and the low and uniform contents of HREE in the zircon overgrowths dated at about 300 Ma allow to think that homogenisation occurred during or after the corona formation around this age. The domains dated around 325–320 Ma would approximate the stages of decompression, whereas the metamorphic peak probably occurred earlier than 339 Ma.     

Genesis of the Katugin rare-metal ore deposit: Magmatism versus metasomatism

Arguments in favor of magmatic or metasomatic genesis of the Katugin rare-metal ore deposit are discussed. The geological and mineralogical features of the deposit confirm its magmatic origin: (1) the shape of the ore-bearing massif and location of various types of granites (biotite, biotite–amphibole, amphibole, and amphibole–aegirine); (2) the geochemical properties of the massif rocks corresponding to A type granite (high alkali content (up to 12.3% Na₂O + K₂O), extremely high FeO/MgO ratio (f = 0.96–1.00), very high content of the most incoherent elements (Rb, Li, Y, Zr, Hf, Ta, Nb, Th, U, Zn, Ga, and REE) and F, and low concentrations of Ca, Mg, Al, P, Ba, and Sr); (3) Fe–F-rich rock-forming minerals; (4) no previously proposed metasomatic zoning and regular replacement of rock-forming minerals corresponding to infiltration fronts of metasomatism. The similar ages of the barren (2066 ± 6 Ma) and ore-bearing (2055 ± 7 Ma) granites along with the features of the ore mineralization speak in favor of the origin of the ore at the magmatic stage of the massif’s evolution. The nature of the ore occurrence and the relationships between the ore minerals support their crystallization from F-rich aluminosilicate melt and also under melt liquation into aluminosilicate and fluoride (and/or aluminofluoride) fractions.