Manioc : Portail | resultats 1 - 20 pour la recherche: Bergen Arcs

Éditeur(s) : HAL CCSD Elsevier
Résumé : Understanding chemical element mobility during high-pressure metamorphism is paramount to the knowledge of the transformations occurring during the course of eclogite-facies metamorphism. In particular, the role of deformation and fluid circulation appear essential. In order to better decipher this role, we investigated two eclogite-facies samples from the Lindas Nappe in the Bergen arcs (Norwegian Caledonides), which are related to a fluid/deformation metamorphic event. This study, based on detailed microtextural investigations, coupled with major and trace element analyses, either in situ EMPA and LA-ICP-MS or by conventional ICP-MS nebulisation on mineral fractions, illustrates the complexity of chemical reactions accompanying the eclogite- and amphibolite-facies metamorphism. A single eclogite-facies mineral species two eclogite-facies (i.e. phengite and epidote) depicts various chemical features depending on its textural location at thin-section scale. The correlation with the mineralogical reaction occurring in each textural site demonstrates that the compositional variations are inherited from the precursor minerals. Thus, the composition of eclogite-facies minerals is locally controlled by the composition of few hundred-micron domains. On the contrary, alteration phases of the amphibolite facies (symplectite and calcic amphibole) display clear enrichment in LILE, Pb, Sr and LREE compared to their precursor minerals (omphacite and garnet). This supports that the transport of elements at hand-sample scale was enhanced during the retrogression and elements were efficiently delocalised from one textural site to another. The contrasting behaviour of element mobility between peak eclogite-facies metamorphism and retrogression can be explained in terms of rate processes and fluid/deformation activity. During eclogitisation,deformation allows the formation of textural and mineralogical microdomains. Contemporaneously the fluid circulation favours the development of heterogeneities by transport of elements in excess towards areas protected from deformation where phengite-bearing quartz lenses crystallise. This, together with the very fast recrystallisation of eclogite-facies minerals, results in a heterogeneous redistribution of elements at sample scale and in a chemical disequilibrium between different microdomains of hundred-micron size. During retrogression in the amphibolite facies, coeval with long-term ductile deformation, the fluids act as an efficient vector to redistribute the elements from a given textural site to another and, thus, allow to reach, at hand-sample scale, a new chemical equilibrium between the different crystallising phases.
ISSN: 0024-4937

hal-00404602
https://hal.archives-ouvertes.fr/hal-00404602
DOI : 10.1016/j.lithos.2006.10.001

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Narrative of a five years' expedition against the revolted negroes of Surinam in Guiana, on the wild coast of South America from the year 1772 to 1777, elucidating the history of that country and describing its productions... with an account of the Indians of Guiana and negroes of Guinea... Tome 2
Auteur(s) : Stedman, John Gabriel (1744-1797)
Année de publication :

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Petrogenesis of crustal wehrlites in the Oman ophiolite: Experiments and natural rocks
Auteur(s) : Koepke, J. Schoenborn, S. Oelze, M. Wittmann, H. Feig, S. T. Hellebrand, E. Boudier, Françoise Schoenberg, R.
Auteurs secondaires : Institut für Mineralogie [Hannover] ; Leibniz Universität Hannover [Hannover] (LUH) ARC Centre of Excellence in Ore Deposits (CODES) ; University of Tasmania Department of Geology and Geophysics [Manoa] ; University of Hawaii at Manoa (UHM) Géosciences Montpellier ; Université des Antilles et de la Guyane (UAG) - Institut national des sciences de l'Univers (INSU - CNRS) - Université de Montpellier (UM) - Centre National de la Recherche Scientifique (CNRS) Centre for Geobiology ; University of Bergen (UIB) Department of Earth Science [Bergen] ; University of Bergen (UIB)

Éditeur(s) : HAL CCSD AGU and the Geochemical Society
Résumé : International audience
In the Wadi Haymiliyah of the Oman ophiolite (Haylayn block), discordant wehrlite bodies ranging in size from tens to hundreds of meters intrude the lower crust at different levels. We combined investigations on natural wehrlites from the Wadi Haymiliyah section with an experimental study on the phase relations in a wehrlitic system in order to constrain the petrogenesis of the crustal wehrlites of the Oman ophiolite. Secondary ion mass spectrometry analyses of clinopyroxenes from different wehrlite bodies imply that the clinopyroxenes were crystallized from tholeiitic, mid-ocean ridge (MORB)-type melts. The presence of primary magmatic amphiboles in some wehrlites suggests a formation under hydrous conditions. Significantly enhanced Sr-87/Sr-86 isotope ratios of separates from these amphiboles imply that the source of the corresponding magmatic fluids was either seawater or subduction zone-related. The experiments revealed that under wet conditions at relatively low temperatures, a MORB magma has the potential to produce wehrlite in the ocean crust by accumulation of early olivine and clinopyroxene. These show typically high Mg# which is a consequence of the oxidizing effect of the prevailing high aH(2)O. First plagioclases crystallizing after clinopyroxene under wet conditions are high in An content, in contrast to the corresponding dry system. Trace element compositions of clinopyroxenes of those wehrlites from the Moho transition zone are too depleted in HREE to be in equilibrium with present-day MORB, implying a genetic relation to the V2 lavas of the Oman ophiolite, which are interpreted to be the result of fluid-enhanced melting of previously depleted mantle. We present a model on the petrogenesis of the crustal wehrlites in an upper mantle wedge above an initial, shallow subduction zone at the beginning of the intraoceanic thrusting.
ISSN: 1525-2027

hal-00429373
https://hal.archives-ouvertes.fr/hal-00429373
DOI : 10.1029/2009GC002488

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Flore phanérogamique des Antilles françaises : Martinique et Guadeloupe
Auteur(s) : Duss, Antoine (1840-1924)
Auteurs secondaires : Heckel, Edouard Marie (1843-1916)
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La France et ses colonies. Tome 1: En France
Auteur(s) : Reclus, Onésime (1837-1916)
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Flora brasiliensis : enumeratio plantarum in Brasilia hactenus detectarum quas suis aliorumque botanicorum studiis descriptas et methodo naturali digestas partim icone illustratas. Volumen XI, Pars II, Euphorbiaceae
Auteur(s) : Müller, Joannes
Auteurs secondaires : Martius, Karl Friedrich Philipp von (1794-1868) Eichler, August Wilhelm (1837-1887)
Année de publication :

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