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Now showing items 1 - 9 of 9

  • MONICA BLACKMUN VISONà, Constructing African Art Histories for the Lagoons of C?te d\"Ivoire. Farnham and Burlington VT: Ashgate Publishing (hb £65 – 978 1 40940 440-8). 2010, 216 pp.

    F?RSTER   TILL  

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  • SITUATING ART AND ART HISTORY Constructing African Art Histories for the Lagoons of C?te d\"Ivoire. By Monica Blackmun Visonà. Surrey, UK: Ashgate, 2010. Pp. xiii+201. $109.95, hardback (ISBN 978-1-4094-0440-8).

    BRETT-SMITH   SARAH C.  

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  • Constructing African Art Histories for the Lagoons of C?te d\"Ivoire\r . by Monica Blackmun Visonà . Burlington, VT: Ashgate Publishing Company, 2010. 216 pages, 41 b/w illustrations, bibliography, index. $109.95 hardcover

    Soppelsa   Robert T.  

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  • Thermal and baric evolution of garnet granulites from the Kharta region of S Tibet, E Himalaya

    Borghi, A   Castelli, D   Lombardo, B   Visona, D  

    Granulite-facies garnet-bearing metapelites, metabasics and calc-silicate rocks from the lower metamorphic complex (Kharta Gneiss) of the Greater Himalayan Crystallines in the Kharta region of S Tibet, E Himalaya, preserve textural and chemical evidence for prograde equilibration at temperatures of at least 700-720degreesC and pressures around 8 kbar during the main event of the Himalayan metamorphism. Post-deformational reaction textures include clinopyroxene (+/- orthopyroxene) - plagioclase symplectites after garnet in calc-silicate rocks, and cordierite +/- spinel coronas on sillimanite and garnet in metapelite granulites. These assemblages indicate a decompressional pressure-temperature path that is confirmed by the geothermobarometry of zoned and symplectite minerals as well as by calculated phase equilibria. Isothermal decompression through ca. 3 kbar occurred at temperatures of about 700degreesC, and was followed by further decompression to P similar to 3 kbar, and T similar to 710degreesC. At this point, decompression was replaced by quasi-isobaric cooling ending in the andalusite stability field at P ca. 2.5 kbar. The P-T path of the Kharta Gneiss appears to be similar to those inferred for the lower Greater Himalayan Crystallines exposed in the nearby Dudh Kosi and middle Arun valleys of eastern Nepal. This type of clockwise P-T path, with most of the exhumation occurring at relatively constant metamorphic temperatures, requires a high exhumation rate and suggests that extrusion tectonics of crustal-scale wedges may have been operative during post-collisional exhumation of the Greater Himalayan Crystallines.
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  • Is there any detachment in the Lower Dolpo (western Nepal)?

    Carosi, R   Montomoli, C   Visona, D  

    A structural transect in the Lower Dolpo highlights that the deformation and metamorphism of the Tibetan Zone (TZ) increase toward the bottom of the sequence. The contact with the underlying HHC is marked by a metamorphic jump from amphibolite facies in the carbonatic rocks of the upper part of the HHC to greenschist facies marbles in the TZ. Moreover, the HHC and the TZ show different metamorphic histories. The contact zone shows a strain increase accompanied by asymmetric folds with a top-to-the-northeast vergence, connected to a down-to-the-northeast tectonic transport. The contact is interpreted as an extensional shear zone, connected to the South Tibetan Detachment System. (C) 2002 Academie des sciences / Editions scientifiques et medicales Elsevier SAS.
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  • CUMULATE-LIKE TEXTURES AND CHEMICAL RELATIONSHIPS IN THE BRESSANONE (BRIXEN) GRANODIORITE (EASTERN ALPS, ITALY) - A NEW GENETIC APPROACH

    VISONA, D  

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  • New geochemical and petrographic data on the Gabbro-Syenite Suite between Hargeysa and Berbera-Shiikh (northern Somalia)

    Abdalla, JA   Said, AA   Visona, D  

    The Somali crystalline basement in northern Somalia contains a Neoproterozoic igneous suite essentially composed of gabbros with minor syenitic and granitic bodies. This Gabbro-Syenite Suite (GSS) was emplaced at a relatively high crustal level and is related to Late Precambrian crustal extension. The basic plutons are composed mostly of gabbro with petrographic and geochemical characteristics ranging from N-type to T- and P-type MORB. This compositional variability points to heterogeneous sub-continental mantle sources. The plutons do not show significant in situ differentiation, suggesting that the cumulitic rocks may be related to cooling processes developed in deeper magma chambers and/or during magma ascent. Copyright (C) 1997 Elsevier Science Ltd.
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  • Some constraints on geochemical features in the Triassic mantle of the easternmost Austroalpine-Southalpine domain: evidence from the Karawanken pluton (Carinthia, Austria)

    Visona, D   Zanferrari, A  

    The Karawanken pluton, near Eisenkappel (Carinthia, Austria), is composed of closely alternating, E/W-running, essentially granitic and dioritic bands, with minor gabbro, monzonite, and hybrid rocks, and is cut by diabase dykes. This pluton, of Triassic age, is a shallow-emplaced intrusion: the rocks of which belong to a series of alkaline affinity and, despite local evidence of mixing and mingling of magmas, the bulk of the pluton in the examined area evolved mainly by fractional crystallization. The shift in the initial Sr isotope ratio from gabbro (0.70313) to monzonite (0.70525) and steady values from monzonite to granite (0.70473) suggest a predominant assimilation and fractional crystallization (AFC) process in the mafic stage, followed by a dominant fractional crystallization (FC) process when residual liquids became felsic. The geochemical characteristics of the Karawanken pluton point to a mantle source enriched both in LILE and HFSE, whereas the coeval magmas of the nearby Dolomites (NE Italy), emplaced in the same extensional-transtensional geodynamic framework, derive from a mantle modified by preceding (Variscan) subductive processes. These geochemical differences in the Triassic mantle of this part of the Eastern Alps may involve the different Paleozoic geodynamic evolution of these two sectors, e.g., the fact that they belong to two different microplates linked in the Middle Carboniferous. In any case, both the magmas of the Karawanken pluton and of the Dolomites clearly fit the mid-Triassic transtensional-extensional tecronism which acted in the Southalpine-Austroalpine and Dinaric domains during the initial stages of Mesozoic rifting.
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  • Occurrence and origin of andalusite in peraluminous felsic igneous rocks

    Clarke, DB   Dorais, M   Barbarin, B   Barker, D   Cesare, B   Clarke, G   El Baghdadi, M   Erdmann, S   Forster, HJ   Gaeta, M   Gottesmann, B   Jamieson, RA   Kontak, DJ   Koller, F   Gomes, CL   London, D   Morgan, GB   Neves, LJPF   Pattison, DRM   Pereira, AJSC   Pichavant, M   Rapela, CW   Renno, AD   Richards, S   Roberts, M   Rottura, A   Saavedra, J   Sial, AN   Toselli, AJ   Ugidos, JM   Uher, P   Villaseca, C   Visona, D   Whitney, DL   Williamson, B   Woodard, HH  

    Andalusite occurs as an accessory mineral in many types of peraluminous felsic igneous rocks, including rhyolites, aplites, granites, pegmatites, and anatectic migmatites. Some published stability curves for And = Sil and the water-saturated granite solidus permit a small stability field for andalusite in equilibrium with felsic melts. We examine 108 samples of andalusite-bearing felsic rocks from more than 40 localities world-wide. Our purpose is to determine the origin of andalusite, including the T-P-X controls on andalusite formation, using eight textural and chemical criteria: size-compatibility with grain sizes of igneous minerals in the same rock; shape-ranging from euhedral to anhedral, with no simple correlation with origin; state of aggregation-single grains or clusters of grains; association with muscovite-with or without rims of monocrystalline or polycrystalline muscovite; inclusions-rare mineral inclusions and melt inclusions; chemical composition-andalusite with little significant chemical variation, except in iron content (0.08-1.71 wt % FeO); compositional zoning-concentric, sector, patchy, oscillatory zoning cryptically reflect growth conditions; compositions of coexisting phases-biotites with high siderophyllite-eastonite contents (Al-iv approximate to 2.68 +/- 0.07 atoms per formula unit), muscovites with 0.57-4.01 wt % FeO and 0.02-2.85 wt % TiO2, and apatites with 3.53 +/- 0.18 wt % F. Coexisting muscovite-biotite pairs have a wide range of F contents, and F-Bt = 1.612F(Ms) + 0.015. Most coexisting minerals have compositions consistent with equilibration at magmatic conditions. The three principal genetic types of andalusite in felsic igneous rocks are: Type 1 Metamorphic-(a) prograde metamorphic (in thermally metamorphosed peraluminous granites), (b) retrograde metamorphic (inversion from sillimanite of unspecified origin), (c) xenocrystic (derivation from local country rocks), and (d) restitic (derivation from source regions); Type 2 Magmatic-(a) peritectic (water-undersaturated, Tup arrow) associated with leucosomes in migmatites, (b) peritectic (water-undersaturated, Tdown arrow), as reaction rims on garnet or cordierite, (c) cotectic (water-undersaturated, Tdown arrow) direct crystallization from a silicate melt, and (d) pegmatitic (water-saturated, Tdown arrow), associated with aplite-pegmatite contacts or pegmatitic portion alone; Type 3 Metasomatic-(water-saturated, magma-absent), spatially related to structural discontinuities in host, replacement of feldspar and/or biotite, intergrowths with quartz. The great majority of our andalusite samples show one or more textural or chemical criteria suggesting a magmatic origin. Of the many possible controls on the formation of andalusite (excess Al2O3, water concentration and fluid evolution, high Be-B-Li-P, high F, high Fe-Mn-Ti, and kinetic considerations), the two most important factors appear to be excess Al2O3 and the effect of releasing water (either to strip alkalis from the melt or to reduce alumina solubility in the melt). Of particular importance is the evidence for magmatic andalusite in granites showing no significant depression of the solidus, suggesting that the And = Sil equilibrium must cross the granite solidus rather than lie below it. Magmatic andalusite, however formed, is susceptible to supra- or sub-solidus reaction to produce muscovite. In many cases, textural evidence of this reaction remains, but in other cases muscovite may completely replace andalusite leaving little or no evidence of its former existence.
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