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Digitale Medien

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

Quelle: Blackwell Publishing Journal Backfiles 1879-2005

Thema: Geologie und Paläontologie

Notizen: 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.

Materialart: Digitale Medien

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

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2

Digitale Medien

Leucitites from Gaussberg, Antarctica (1980)

Sheraton, J. W. ; Cundari, A.

Springer

Contributions to mineralogy and petrology 71 (1980), S. 417-427

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ISSN: 1432-0967

Quelle: Springer Online Journal Archives 1860-2000

Thema: Geologie und Paläontologie

Notizen: Abstract Leucitities of probable late Pleistocene age from the extinct volcano of Gaussberg on the coast of East Antarctica consist of microphenocrysts of olivine, diopside, and leucite in a largely glassy groundmass. Their extreme K contents and K/Na ratios, as well as high Rb, Ba, and possibly Ti, are compatible with an origin by small degrees of partial melting of phlogopite-rich mantle below the level of amphibole stability, whereas their enrichment in P, Sr, Zr, Nb, La, Ce, Pb, and Th indicates the presence of minor phases such as apatite in the source material. Although the Mg/(Mg+Fe) values (∼0.70) and relatively high Ni and Cr contents suggest that the Gaussberg lavas represent near-primary melts, some degree of fractionation involving an aluminous phase (probably garnet) may be necessary to produce liquids with atomic K〉Al. Alternatively, an as yet poorly understood process such as wall-rock reaction, liquid immiscibility, or mantle metasomatism may have been a critical factor in the genesis of these unusual rocks. Gaussberg is situated on a passive continental margin and does not appear to be related to any other known area of Cainozoic volcanic activity.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1007/BF00374713

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3

Digitale Medien

Osumilite-sapphirine-quartz granulites from Enderby Land Antarctica — Mineral assemblages and reactions (1980)

Ellis, D. J. ; Sheraton, J. W. ; England, R. N. ; [weitere]

Springer

Contributions to mineralogy and petrology 72 (1980), S. 123-143

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ISSN: 1432-0967

Quelle: Springer Online Journal Archives 1860-2000

Thema: Geologie und Paläontologie

Notizen: Abstract The pre-Cambrian granulites of Enderby Land Antarctica, contain coexisting spinel-quartz, sapphirine-quartz, hypersthene-sillimanite-quartz and osumilite on a regional extent. Osumilite is present in a variety of mineral assemblages, most of which are documented in granulites for the first time. The mineral assemblages, reactions and compositional zoning in minerals are discussed in terms of continuous and discontinuous reactions in response to changing conditions of metamorphism. The development of many of the mineral coronas can be explained by continuous rather than discontinuous reactions, due to the effects of Mg-Fe and (Mg,Fe)-2Al exchange equilibria with decreasing temperature. The highest P-T conditions of metamorphism (8–10 kb, 900 °–980 ° C, Ellis, in preparation) were beyond the stability limit of coexisting garnet-cordierite. Secondary cordierite has developed through a large number of mineral reactions in response to cooling of these granulites. A theoretical analysis of the phase relations involving osumilite in the chemical systems K2O-MgO-Al2O3-SiO2 and K2O-MgO-FeO-Al2O3-SiO2 is presented. In the pure Mg-system the lower temperature stability limit of Mg-osumilite is inferred to be defined with increasing pressure by the reactions Os→Cd+En+Kfeld+Qtz, Os→Sa+En+Kfeld+Qtz, Os→Sill+En+Kfeld+Qtz. In iron-bearing systems an important reaction involving osumilite is Os+Gt→Cd+Hy+Kfeld+Qtz. At moderate temperatures and pressures, osumilite is limited to rocks which lie on the Mg-rich side of the Cd-Hy stable tie line on an AFM diagram. At higher pressures and temperatures osumilite occurs in a widerrange of rock compositions because of the stability of coexisting garnet and osumilite. Petrographic data, as well as chemographic relations indicate that for many common rock compositions, garnet, cordierite, hypersthene, sapphirine and sillimanite cannot coexist with both osumilite and K-feldspar.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1007/BF00399473

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Digitale Medien

Geochemistry and geochronology of proterozoic tholeiite dykes of east antarctica: evidence for mantle metasomatism (1982)

Sheraton, J. W. ; Black, L. P.

Springer

Contributions to mineralogy and petrology 78 (1982), S. 305-317

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ISSN: 1432-0967

Quelle: Springer Online Journal Archives 1860-2000

Thema: Geologie und Paläontologie

Notizen: Abstract Two episodes of tholeiite dyke emplacement have been identified in Archaean high-grade metamorphics of the Napier Complex in Enderby Land. Middle Proterozoic Amundsen dykes are typical continental tholeiites and most of the chemical variation in individual suites can be explained in terms of different degrees of partial melting and low-pressure crystal fractionation. Group I Amundsen tholeiites were derived from a relatively homogeneous source region 1,190±200 m.y. ago, whereas that of the group II Amundsen tholeiites was chemically and isotopically heterogeneous. Group II dykes have various degrees of enrichment in incompatible elements, and commonly show normalised trace element abundance patterns with negative Nb anomalies. These features imply variable metasomatism of the source region by a volatile-rich fluid phase (rather than a melt of any observed igneous composition) enriched in K, Rb, Ba, Th, and possibly La and Ce. Early Proterozoic (2,350±48 m.y.) tholeiites were emplaced at considerable depths in the crust during the waning stages of granulite-facies metamorphism and include a high-Mg suite of possible komatiitic affinity, ranging in composition from hypersthene-rich tholeiite (norite) to quartz-rich tholeiite. They tend to have higher ratios of highly to moderately incompatible elements (e.g., K/Zr, K/Ce), and larger Nb anomalies (i.e., higher K/Nb) compared with middle Proterozoic tholeiites, suggesting derivation from more enriched source regions. Isotopic data are not compatible with significant crustal contamination, but constrain source metasomatism to a time immediately before emplacement. Metasomatism of the source region of the much younger group I tholeiites may have been contemporaneous with that of the high-Mg suite.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1007/BF00398925

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5

Digitale Medien

Geochemical evolution of Archaean granulite-facies gneisses in the Vestfold Block and comparisons with other Archaean gneiss complexes in the East Antarctic Shield (1984)

Sheraton, J. W. ; Collerson, K. D.

Springer

Contributions to mineralogy and petrology 87 (1984), S. 51-64

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ISSN: 1432-0967

Quelle: Springer Online Journal Archives 1860-2000

Thema: Geologie und Paläontologie

Notizen: Abstract The Vestfold Block, like other Archaean cratons in East Antarctica and elsewhere, consists predominantly of felsic orthogneiss (Mossel and Crooked Lake gneisses), with subordinate mafic granulite (Tryne metavolcanics) and paragneiss (Chelnok supracrustals). Two major periods of continental crust formation are represented. The Mossel gneiss (metamorphosed about 3,000 Ma ago) is mainly of tonalitic composition, and is similar to much of the roughly coeval Napier Complex in Enderby Land. The Crooked Lake gneiss was emplaced under high-grade conditions about 2,450 Ma ago and comprises a high proportion of more potassic rocks (monzodioritic and monzonitic suites), as well as tonalite and minor gabbro and diorite. Both Mossel and Crooked Lake gneisses are depleted in Y and have moderate to high Sr, Ce/Y, and Ti/Y, consistent with melting of a mafic source (?subducted hydrated oceanic crust) leaving major residual hornblende (± garnet). Most Crooked Lake gneisses are more enriched in incompatible elements (P, Sr, La, Ce, and particularly Rb, Ba, and K) than Mossel gneisses, suggesting derivation from a more enriched mafic source. The Vestfold Block contains few orthogneisses derived by melting of older felsic crustal rocks, in marked contrast to the Archaean Napier Complex and, in particular, southern Prince Charles Mountains. Both Mossel and Crooked Lake tonalites are strongly depleted in Rb, K, Th, and U, and have very low Rb/Sr and high K/Rb; more potassic orthogneisses are depleted in Th, U, and, to lesser extents, Rb. Tryne metavolcanics are depleted in Th and Rb, but appear to have been enriched in K (and probably Na), possibly during early low-grade alteration.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1007/BF00371402

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