Prograde and retrograde history of eclogites from the Eastern Blue
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The prograde metamorphism of eclogites is typically obscured by chemical equilibration at peak conditions and by partial requilibration during retrograde metamorphism. Eclogites from the Eastern Blue Ridge of North Carolina retain evidence of their prograde path in the form of inclusions preserved in garnet. These eclogites, from the vicinity of Bakersville, North Carolina, USA are primarily comprised of garnet-clinopyroxene-rutile-hornblende-plagioclase-quartz. Quartz, clinopyroxene, hornblende, rutile, epidote, titanite and biotite are found as inclusions in garnet cores. Included hornblende and clinopyroxene are chemically distinct from their matrix counterparts. Thermobarometry of inclusion sets from different garnets record different conditions. Inclusions of clinozoisite, titanite, rutile and quartz (clinozoisite + titanite ¼ grossular + rutile + quartz + H2O) yield pressures (6-10 kbar, 400-600 � C and 8-12 kbar 450-680 � C) at or below the minimum peak conditions from matrix phases (10-13 kbar at 600-800 � C). Inclusions of hornblende, biotite and quartz give higher pressures (13-16 kbar and 630-660 � C). Early matrix pyroxene is partially or fully broken down to a diopside-plagioclase symplectite, and both garnet and pyroxene are rimmed with plagioclase and hornblende. Hypersthene is found as a minor phase in some diopside + plagioclase symplectites, which suggests retrogression through the granulite facies. Two-pyroxene thermometry of this assemblage gives a temperature of c. 750 � C. Pairing the most Mg-rich garnet composition with the assemblage plagioclase-diopside-hypersthene-quartz gives pressures of 14-16 kbar at this temperature. The hornblende-plagioclase-garnet rim-quartz assemblage yields 9-12 kbar and 500-550 � C. The combined P-T data show a clockwise loop from the amphibolite to eclogite to granulite facies, all of which are overprinted by a texturally late amphibolite facies assemblage. This loop provides an unusually complete P-T history of an eclogite, recording events during and following subduction and continental collision in the early Palaeozoic.Keywords:
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Abstract A mid‐ocean ridge basalt (MORB)‐type eclogite from the Moldanubian domain in the Bohemian Massif retains evidence of its prograde path in the form of inclusions of hornblende, plagioclase, clinopyroxene, titanite, ilmenite and rutile preserved in zoned garnet. Prograde zoning involves a flat grossular core followed by a grossular spike and decrease at the rim, whereas Fe/(Fe + Mg) is also flat in the core and then decreases at the rim. In a pseudosection for H 2 O‐saturated conditions, garnet with such a zoning grows along an isothermal burial path at c. 750 °C from 10 kbar in the assemblage plagioclase‐hornblende‐diopsidic clinopyroxene‐quartz, then in hornblende‐diopsidic clinopyroxene‐quartz, and ends its growth at 17–18 kbar. From this point, there is no pseudosection‐based information on further increase in pressure or temperature. Then, with garnet‐clinopyroxene thermometry, the focus is on the dependence on, and the uncertainties stemming from the unknown Fe 3+ content in clinopyroxene. Assuming no Fe 3+ in the clinopyroxene gives a serious and unwarranted upward bias to calculated temperatures. A Fe 3+ ‐contributed uncertainty of ±40 °C combined with a calibration and other uncertainties gives a peak temperature of 760 ± 90 °C at 18 kbar, consistent with no further heating following burial to eclogite facies conditions. Further pseudosection modelling suggests that decompression to c. 12 kbar occurred essentially isothermally from the metamorphic peak under H 2 O‐undersaturated conditions ( c. 1.3 mol.% H 2 O) that allowed the preservation of the majority of garnet with symplectitic as well as relict clinopyroxene. The modelling also shows that a MORB‐type eclogite decompressed to c. 8 kbar ends as an amphibolite if it is H 2 O saturated, but if it is H 2 O‐undersaturated it contains assemblages with orthopyroxene. Increasing H 2 O undersaturation causes an earlier transition to SiO 2 undersaturation on decompression, leading to the appearance of spinel‐bearing assemblages. Granulite facies‐looking overprints of eclogites may develop at amphibolite facies conditions.
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Abstract Eclogite boudins occur within an orthogneiss sheet enclosed in a Barrovian metapelite‐dominated volcano‐sedimentary sequence within the Velké Vrbno unit, NE Bohemian Massif. A metamorphic and lithological break defines the base of the eclogite‐bearing orthogneiss nappe, with a structurally lower sequence without eclogite exposed in a tectonic window. The typical assemblage of the structurally upper metapelites is garnet–staurolite–kyanite–biotite–plagioclase–muscovite–quartz–ilmenite ± rutile ± silli‐manite and prograde‐zoned garnet includes chloritoid–chlorite–paragonite–margarite, staurolite–chlorite–paragonite–margarite and kyanite–chlorite–rutile. In pseudosection modelling in the system Na 2 O–CaO–K 2 O–FeO–MgO–Al 2 O 3 –SiO 2 –H 2 O (NCKFMASH) using THERMOCALC, the prograde path crosses the discontinuous reaction chloritoid + margarite = chlorite + garnet + staurolite + paragonite (with muscovite + quartz + H 2 O) at 9.5 kbar and 570 °C and the metamorphic peak is reached at 11 kbar and 640 °C. Decompression through about 7 kbar is indicated by sillimanite and biotite growing at the expense of garnet. In the tectonic window, the structurally lower metapelites (garnet–staurolite–biotite–muscovite–quartz ± plagioclase ± sillimanite ± kyanite) and amphibolites (garnet–amphibole–plagioclase ± epidote) indicate a metamorphic peak of 10 kbar at 620 °C and 11 kbar and 610–660 °C, respectively, that is consistent with the other metapelites. The eclogites are composed of garnet, omphacite relicts (jadeite = 33%) within plagioclase–clinopyroxene symplectites, epidote and late amphibole–plagioclase domains. Garnet commonly includes rutile–quartz–epidote ± clinopyroxene (jadeite = 43%) ± magnetite ± amphibole and its growth zoning is compatible in the pseudosection with burial under H 2 O‐undersaturated conditions to 18 kbar and 680 °C. Plagioclase + amphibole replaces garnet within foliated boudin margins and results in the assemblage epidote–amphibole–plagioclase indicating that decompression occurred under decreasing temperature into garnet‐free epidote–amphibolite facies conditions. The prograde path of eclogites and metapelites up to the metamorphic peak cannot be shared, being along different geothermal gradients, of about 11 and 17 °C km −1 , respectively, to metamorphic pressure peaks that are 6–7 kbar apart. The eclogite–orthogneiss sheet docked with metapelites at about 11 kbar and 650 °C, and from this depth the exhumation of the pile is shared.
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Abstract Orthopyroxene‐free garnet + clinopyroxene + plagioclase ± quartz‐bearing mineral assemblages represent the paragenetic link between plagioclase‐free eclogite facies metabasites and orthopyroxene‐bearing granulite facies metabasites. Although these assemblages are most commonly developed under P–T conditions consistent with high pressure granulite facies, they sometimes occur at lower grade in the amphibolite facies. Thus, these assemblages are characteristic but not definitive of high pressure granulite facies. Compositional factors favouring their development at amphibolite grade include Fe‐rich mineral compositions, Ca‐rich garnet and plagioclase, and Ti‐poor hornblende. The generalized reaction that accounts for the prograde development of garnet + clinopyroxene + plagioclase ± quartz from a hornblende + plagioclase + quartz‐bearing (amphibolite) precursor is Hbl + Pl + Qtz=Grt + Cpx + liquid or vapour, depending on whether the reaction occurs above or below the solidus. There are significant discrepancies between experimental and natural constraints on the P–T conditions of orthopyroxene‐free garnet + clinopyroxene + plagioclase ± quartz‐bearing mineral assemblages and therefore on the P–T position of this reaction. Semi‐quantitative thermodynamic modelling of this reaction is hampered by the lack of a melt model and gives results that are only moderately successful in rationalizing the natural and experimental data.
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Abstract The prograde metamorphism of eclogites is typically obscured by chemical equilibration at peak conditions and by partial requilibration during retrograde metamorphism. Eclogites from the Eastern Blue Ridge of North Carolina retain evidence of their prograde path in the form of inclusions preserved in garnet. These eclogites, from the vicinity of Bakersville, North Carolina, USA are primarily comprised of garnet–clinopyroxene–rutile–hornblende–plagioclase–quartz. Quartz, clinopyroxene, hornblende, rutile, epidote, titanite and biotite are found as inclusions in garnet cores. Included hornblende and clinopyroxene are chemically distinct from their matrix counterparts. Thermobarometry of inclusion sets from different garnets record different conditions. Inclusions of clinozoisite, titanite, rutile and quartz (clinozoisite + titanite = grossular + rutile + quartz + H 2 O) yield pressures (6–10 kbar, 400–600 °C and 8–12 kbar 450–680 °C) at or below the minimum peak conditions from matrix phases (10–13 kbar at 600–800 °C). Inclusions of hornblende, biotite and quartz give higher pressures (13–16 kbar and 630–660 °C). Early matrix pyroxene is partially or fully broken down to a diopside–plagioclase symplectite, and both garnet and pyroxene are rimmed with plagioclase and hornblende. Hypersthene is found as a minor phase in some diopside + plagioclase symplectites, which suggests retrogression through the granulite facies. Two‐pyroxene thermometry of this assemblage gives a temperature of c. 750 °C. Pairing the most Mg‐rich garnet composition with the assemblage plagioclase–diopside–hypersthene–quartz gives pressures of 14–16 kbar at this temperature. The hornblende–plagioclase–garnet rim–quartz assemblage yields 9–12 kbar and 500–550 °C. The combined P–T data show a clockwise loop from the amphibolite to eclogite to granulite facies, all of which are overprinted by a texturally late amphibolite facies assemblage. This loop provides an unusually complete P–T history of an eclogite, recording events during and following subduction and continental collision in the early Palaeozoic.
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The thermodynamic properties of 154 mineral end‐members, 13 silicate liquid end‐members and 22 aqueous fluid species are presented in a revised and updated data set. The use of a temperature‐dependent thermal expansion and bulk modulus, and the use of high‐pressure equations of state for solids and fluids, allows calculation of mineral–fluid equilibria to 100 kbar pressure or higher. A pressure‐dependent Landau model for order–disorder permits extension of disordering transitions to high pressures, and, in particular, allows the alpha–beta quartz transition to be handled more satisfactorily. Several melt end‐members have been included to enable calculation of simple phase equilibria and as a first stage in developing melt mixing models in NCKFMASH. The simple aqueous species density model has been extended to enable speciation calculations and mineral solubility determination involving minerals and aqueous species at high temperatures and pressures. The data set has also been improved by incorporation of many new phase equilibrium constraints, calorimetric studies and new measurements of molar volume, thermal expansion and compressibility. This has led to a significant improvement in the level of agreement with the available experimental phase equilibria, and to greater flexibility in calculation of complex mineral equilibria. It is also shown that there is very good agreement between the data set and the most recent available calorimetric data.
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Abstract Garnet–clinopyroxene intermediate granulites occur as thin layers within garnet–kyanite–K–feldspar felsic granulites of the St. Leonhard granulite body in the Bohemian Massif. They consist of several domains. One domain consists of coarser‐grained coexisting ternary feldspar, clinopyroxene, garnet, quartz and accessory rutile and zircon. The garnet has 16–20% grossular, and the clinopyroxene has 9% jadeite and contains orthopyroxene exsolution lamellae. Reintegrated ternary feldspar and the Zr‐in‐rutile thermometer give temperatures higher than 950 °C. Mineral equilibria modelling suggests crystallization at 14 kbar. The occurrence and preservation of this mineral assemblage is consistent with crystallization from hot dry melt. Between these domains is a finer‐grained deformed matrix made up of diopsidic clinopyroxene, orthopyroxene, plagioclase and K‐feldspar, apparently produced by reworking of the coarser‐grained domains. Embedded in this matrix, and pre‐dating the reworking deformation, are garnet porphyroblasts that contain clinopyroxene, feldspar, quartz, rutile and zircon inclusions. In contrast with the garnet in the coarser‐grained domains, the garnet generally has >30% grossular, the included clinopyroxene has 7–27% jadeite and the Zr content of rutile indicates much lower temperatures. Some of these high‐grossular garnet show zoning in Fe/(Fe + Mg), decreasing from 0.7 in the core to 0.6 and then increasing to 0.7 at the rim. These garnet are enigmatic, but with reference to appropriate pseudosections are consistent with localized new mineral growth from 650 to 850 °C and 10 to 17 kbar, or with equilibration at 20 kbar and 770 °C, modified by two‐stage diffusional re‐equilibration of rims, at 10–15 and 8 kbar. The strong pervasive deformation has obscured relationships that might have aided the interpretation of the origin of these porphyroblasts. The evolution of these rocks is consistent with formation by igneous crystallization and subsequent metamorphism to high‐ T and high‐ P , rather than an origin by ultrahigh‐ T metamorphism. Regarding the petrographic complexity, combination of the high grossular garnet with the ternary feldspar to infer ultrahigh‐ T metamorphism at high pressure is not justified.
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Garnet-bearing mafic granulite and amphibolite exposed as lenses, boudins, or interlayers within metasediments in the Qingshigou area, Dunhuang block, southernmost Central Asian orogenic belt, record important information for understanding the tectono-metamorphic evolution of subduction and collision zones in the southern Central Asian orogenic belt during the mid-Paleozoic. Three stages of metamorphic assemblages (M1, M2, M3) are recognized in the high- and medium-pressure mafic granulite and amphibolite. In the high-pressure mafic granulite, the prograde assemblage (M1) is represented by inclusion minerals (hornblende + plagioclase + quartz) preserved in garnet porphyroblasts; the metamorphic peak assemblage (M2) is characterized by garnet porphyroblasts and matrix minerals (garnet + clinopyroxene + plagioclase + quartz ± zircon ± titanite); and the retrograde assemblage (M3) is marked by coronitic symplectite (hornblende + plagioclase + quartz ± magnetite) rimming the garnet porphyroblasts. In the medium-pressure mafic granulite, the prograde assemblage (M1) of hornblende + plagioclase + quartz is included in the garnet porphyroblasts; the peak assemblage (M2) consists of garnet + orthopyroxene + clinopyroxene + plagioclase + quartz ± zircon ± titanite (M2) in the matrix; and the retrograde assemblage (M3) of hornblende + orthopyroxene + plagioclase + quartz (M3) surrounds the garnet porphyroblasts. In the amphibolite, the prograde assemblage (hornblende + plagioclase + quartz + ilmenite) is preserved as inclusions in garnet (M1); the peak assemblage (M2) is composed of garnet + hornblende + plagioclase + quartz ± zircon ± titanite; and the retrograde assemblage (M3), consisting of hornblende + biotite + plagioclase + quartz + epidote + magnetite, rings the garnet porphyroblasts. Geothermobarometric calculations suggest that the metamorphic pressure-temperature paths pass from 568 °C and 8.8 kbar through 607 °C and 10.6 kbar and 861 °C and 16.9 kbar and finally to 598 °C and 4.4 kbar for the high-pressure mafic granulite; from 756 °C and 9.0 kbar through 750–874 °C and 9.3–11.6 kbar to 675 °C and 4.7 kbar for the medium-pressure mafic granulite; and from 686 °C and 7.6 kbar through 715–766 °C and 10.6–11.2 kbar to 671 °C and 5.6 kbar for the amphibolite, and the paths show clockwise pressure-temperature loops typical of an orogenic process. The metamorphic peak of the high-pressure mafic granulite lies in the eclogite facies, which is indicative of a subduction zone environment. High-resolution secondary ion mass spectrometry (SIMS) U-Pb dating of metamorphic zircon indicates that the metamorphism occurred in the Early Silurian (ca. 430 Ma) and lasted for at least 65 m.y. This study reveals a possible southward subduction history of a branch of the Paleo–Asian Ocean, the Liuyuan Ocean, from the Silurian to Late Devonian, which may be an important event in the accretionary history of the Central Asian orogenic belt.
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and often show inclusions of small grains of bluish clinopyroxene (up to ca 20 % of Jd component), rutile, ilmenite and apatite. Other, not so common accesoric minerals are zircon, sphene, minerals of epidote group and quartz. Anhedral chlorite and biotite are rare retrograde phases in garnet-free amphibolites, while symplectites of green amphibole and plagioclase forming diablastic textures around garnet are typical for garnet amphibolites. Comparing basicity of plagioclases, inside symplectites is higher than outside (less than An50) and is increasing towards the garnet, as a result of reactions producing An-rich plagioclase e.g.: 3 Prp + 11 Di + 7 Qtz + 4 H2O = 4 Tr + 3 An. The replacement of clinopyroxene by green to green-brownish amphibole on the rims is also documented in several samples of garnet amphibolites. No orthopyroxene or its relics was not found. Binary diagrams based on the major oxides vs. SiO2 chemistry suggest the existence of at least two suites of studied rocks. Amphibolites have characteristics corresponding to tholeiite series, only one sample is displayed in the calc-alkaline field of the AFM diagram. The REE values normalized by chondrite (Boynton, 1984) are characterised by Eu/Eu* (0.87–1.35), CeN/YbN (0.97–12.81), CeN/SmN (0.52–3.38) and EuN/YbN (1.14–2.40). Hornblende-plagioclase geothermobarometry applied to pairs of horblende and plagioclase from symplectites surrounding garnets (Holland and Blundy, 1994), combined with amphibole geobarometry (Johnson and Rutherford, 1989; Schmidt, 1992) gave the interval 780.5 °C, 6.7 kbar and 797.7 °C, 8.5 kbar for the sample from the locality near the Rokytno village and 833.6 °C, 8 kbar and 861 °C, 9.9 kbar for the sample from the locality Pohledec. Although temperature values seem to be slightly overestimated, due to hornblende high persistence (up to 1000 °C, 10 kbar) the plagioclase-garnet-clinopyroxene ± hornblende assemblage is typical for both upper amphibolite and high-P granulite facies. Thus, it is no reason to be in doubt that some samples of garnet-clinopyroxene amphibolites represent rocks corresponding to amphibolite/granulite facies transition or that their peak metamorphism could be under granulite facies conditions.
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