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Pressure–temperature evolution of metapelitic granulites in a polymetamorphic terrane: the Rauer Group, East Antarctica (1991)

HARLEY, S. L. ; FITZSIMONS, I. C. W.

Oxford, UK : Blackwell Publishing Ltd

Journal of metamorphic geology 9 (1991), S. 0

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ISSN: 1525-1314

Source: Blackwell Publishing Journal Backfiles 1879-2005

Topics: Geosciences

Notes: Distinctive lithological associations and geological relationships, and initial geochronological results indicate the presence of an areally extensive region of reworked Archaean basement containing polymetamorphic granulites in the Rauer Group, East Antarctica.Structurally early metapelites from within this reworked region preserve complex and varied metamorphic histories which largely pre-date and bear no relation to a Late Proterozoic metamorphism generally recognized in this part of East Antarctica. In particular, magnesian metapelite rafts from Long Point record extreme peak P–T conditions of 10–12 kbar and 100–1050°C, and an initial decompression to 8 kbar at temperatures of greater than 900°C. Initial garnet–orthopyroxene–sillimanite assemblages contain the most magnesian (and pyrope-rich) garnets (XMg= 0.71) yet found in granulite facies rocks. A high-temperature decompressional P–T history is consistent with reaction textures in which the phase assemblages produced through garnet breakdown vary systematically with the initial garnet XMg composition, reflecting the intersection of different divariant reactions in rocks of varied composition as pressures decreased. This history is thought to relate to Archaean events, whereas a lower-temperature (c. 750–800°C) decompression to 5 kbar reflects Late Proterozoic reworking of these relict assemblages.The major Late Proterozoic (c. 1000 Ma) granulite facies metamorphism is recorded in a suite of younger Fe-rich metapelites and associated paragneisses in which syn- to post-deformational decompression, through 2–4 kbar from maximum recorded P–T conditions of 7–9 kbar and 800–850°C, is constrained by geothermobarometry and reaction textures. This P–T evolution is thought to reflect rapid tectonic collapse of crust previously thickened through collision.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1111/j.1525-1314.1991.tb00519.x

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Stable isotopic signatures of superposed fluid events in granulite facies marbles of the Rauer Group, East Antarctica (1994)

BUICK, I. S. ; HARLEY, S. L. ; CARTWRIGHT, I. ; [et al.]

Oxford, UK : Blackwell Publishing Ltd

Journal of metamorphic geology 12 (1994), S. 0

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ISSN: 1525-1314

Source: Blackwell Publishing Journal Backfiles 1879-2005

Topics: Geosciences

Notes: The role of volatiles in the stabilization of the lower (granulite facies) crust is contentious. Opposing models invoke infiltration of CO2-rich fluids or generally vapour-absent conditions during granulite facies metamorphism. Stable isotope and petrological studies of granulite facies metacarbonates can provide constraints on these models. In this study data are presented from metre-scale forsteritic marble boudins within Archaean intermediate to felsic orthogneisses from the Rauer Group, East Antarctica.Forsteritic marble layers and associated calcsilicates preserve a range of 13C- and 18O-depleted calcite isotope values (δ13C= -9.9 to -3.0% PDB, δ18O = 4.0 to 12.1% SMOW). A coupled trend of 13C and 18O depletion (∼2%, ∼5%, respectively) from core to rim across one marble layer is inconsistent with pervasive CO2 infiltration during granulite facies metamorphism, but does indicate localized fluid-rock interaction. At another locality, more pervasive fluid infiltration has resulted in calcite having uniformly low, carbonatite-like δ18O and δ13C values. A favoured mechanism for the low δ18O and δ13C values of the marbles is infiltration by fluids that were derived from, or equilibrated with, a magmatic source. It is likely that this fluid-rock interaction occurred prior to high-grade metamorphism; other fluid-rock histories are not, however, ruled out by the available data. Coupled trends of 13C and 18O depletion are modified to even lower values by the superposed development of small-scale metasomatic reaction zones between marbles and internally folded mafic (?) interlayers. The timing of development of these layers is uncertain, but may be related to Archaean high-temperature (〉1000d̀C) granulite facies metamorphism.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1111/j.1525-1314.1994.tb00023.x

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Two-stage decompression in orthopyroxene–sillimanite granulites from Forefinger Point, Enderby Land, Antarctica: implications for the evolution of the Archaean Napier Complex (1990)

HARLEY, S. L. ; HENSEN, B. J. ; SHERATON, J. W.

Oxford, UK : Blackwell Publishing Ltd

Journal of metamorphic geology 8 (1990), S. 0

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ISSN: 1525-1314

Source: Blackwell Publishing Journal Backfiles 1879-2005

Topics: Geosciences

Notes: Magnesian metapelites of probable Archaean age from Forefinger Point, SW Enderby Land, East Antarctica, contain very-high-temperature granulite facies mineral assemblages, which include orthopyroxene (8–9.5 wt% Al2O3)–sillimanite ± garnet ± quartz ± K-feldspar, that formed at 10 ± 1.5 kbar and 950 ± 50°C. These assemblages are overprinted by symplectite and corona reaction textures involving sapphirine, orthopyroxene (6–7 wt% Al2O3), cordierite and sometimes spinel at the expense of porphyroblastic garnet or earlier orthopyroxene–sillimanite. These textures mainly pre-date the development of coarse biotite at the expense of initial mesoperthite, and the subsequent formation of orthopyroxene (4–6 wt% Al2O3)–cordierite–plagioclase rinds on late biotite.The early reaction textures indicate a period of near-isothermal decompression at temperatures above 900°C. Decompression from 10 ± 1.5 kbar to 7–8 kbar was succeeded by biotite formation at significantly lower temperatures (800–850°C) and further decompression to 4.5 ± 1 kbar at 700–800°C.The later parts of this P–T evolution can be ascribed to the overprinting and reworking of the Forefinger Point granulites by the Late-Proterozoic (c. 1000 Ma) Rayner Complex metamorphism, but the age and timing of the early high-temperature decompression is not known. It is speculated that this initial decompression is of Archaean age and therefore records thinning of the crust of the Napier Complex following crustal thickening by tectonic or magmatic mechanisms and preceding the generally wellpreserved post-deformational near-isobaric cooling history of this terrain.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1111/j.1525-1314.1990.tb00490.x

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Garnet coronas in scapolite-wollastonite calc-silicates from East Antarctica: the application and limitations of activity-corrected grids (1994)

FITZSIMONS, I. C. W. ; HARLEY, S. L.

Oxford, UK : Blackwell Publishing Ltd

Journal of metamorphic geology 12 (1994), S. 0

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ISSN: 1525-1314

Source: Blackwell Publishing Journal Backfiles 1879-2005

Topics: Geosciences

Notes: Calc-silicate boudins within Proterozoic granulite facies gneisses of the northern Prince Charles Mountains, East Antarctica, preserve a number of reaction textures including garnet coronas between calcite and scapolite; garnet-quartz coronas between scapolite and wollastonite and between plagioclase and wollastonite; calcite-quartz intergrowths in wollastonite; and calcite-plagioclase symplectites in scapolite. These textures have been modelled using petrogenetic grids for reactions in the CaO-Al2,O3-SiO2-CO2 system, but with reduced mineral activities to account for additional components in real mineral compositions. Such fixed-composition reduced-activity grids are strictly valid only at the point in P-T-aCO2 space where an assemblage last equilibrated, and do not show the true positions of reactions away from this point because mineral compositions change with reaction progress. In this case, however, mineral compositions close to end-member values and low extents of reaction progress mean that compositional change was limited and the grids are good approximations to true pseudosections over the entire P-T-aco2 range of interest.The grids show that the textures are consistent with near-isobaric cooling from about 850 to 700d̀ C at 7 kbar, a P-T path compatible with thermobarometric studies of other lithologies from the area. Phase relationships indicate that CO2 activities were buffered by the local mineral assemblage during peak and retrograde metamorphism, either under fluid-absent conditions or within a non-pervasive fluid phase. Previous studies of garnet coronas in scapolite-wollastonite calc-silicates have used qualitative grids based on limited experimental data to invoke garnet growth during water infiltration at high temperature, but the grids used here show that garnet coronas can form on cooling, without any need for water influx.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1111/j.1525-1314.1994.tb00058.x

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5

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Stable isotopes as tracers of metamorphic processes: introduction (1994)

HARLEY, S. L. ; GRAHAM, C. M.

Oxford, UK : Blackwell Publishing Ltd

Journal of metamorphic geology 12 (1994), S. 0

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ISSN: 1525-1314

Source: Blackwell Publishing Journal Backfiles 1879-2005

Topics: Geosciences

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1111/j.1525-1314.1994.tb00016.x

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6

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Accessory phase controls on the geochemistry of crustal melts and restites produced during water-undersaturated partial melting (1993)

Watt, G. R. ; Harley, S. L.

Springer

Contributions to mineralogy and petrology 114 (1993), S. 550-566

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ISSN: 1432-0967

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences

Notes: Abstract The profound geochemical conseqences of accessory phase behaviour during partial melting of highgrade metapelites are demonstrated with reference to two geochemically distinct crustal melts produced by biotite dehydration melting reactions under granulite facies (kbar, 860°CC) conditions. These two leucogneiss suites, from the Brattstrand Bluffs coastline, eastern Antarctica, have similar field relations, transport distances (10–100 s of metres) and major element chemistry. Type 1 leucogneisses have low Zr, Th and LREE, positive Eu anomalies and Zr/Zr* and LREEt/LREEt * values less than 1.0 (i.e. less than required to saturate the melt). Mass balance constraints suggest that these melts have been extracted before equilibration with host melanosomes. The dry, peraluminous nature of vapour-undersaturated melts inhibits monazite and zircon solubility and results in concentration of these phases in the residue. Melts are consequently depleted in LREE and HREE. Melanosomes show complementary enrichment in LREE, while HREE patterns are dominated by residual garnet. Type 2 leucogneisses, in contrast, have strongly enriched Zr, Th and LREE abundances, negative Eu anomalies and Zr/Zr* and LREEt/LREEt *〉1 resulting from accessory phase entrainment. Vapour-absent partial melting under moderate (6–8 kbar) pressure granulite-facies conditions of a pelitic source containing monazite is likely to give disequilibrium melts depleted in LREE and HREE as monazite and garnet are concentrated in the residue. If temperatures are high enough (850–870° C) to permit relatively large degrees of partial melting then the feldspar component of the source will be removed almost completely, giving melts with large positive Eu anomalies. Melts formed under vapour-present conditions are unlikely to show such extreme LREE and HREE depletion or positive Eu anomalies, even at high degrees of partial melting. Disequilibrium melting coupled with source entrainment could fortuitously produce REE and trace element signatures similar to those typical of S-type granites and usually ascribed to equilibrium melting conditions.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF00321759

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