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Metamorphism on Chromite Ores from the Dobromirtsi ultramafic massif, Rhodope Mountains (SE Bulgaria)
González-Jiménez, J. M.; Kerestedjian, T.; Proenza Fernández, Joaquín Antonio; Gervilla Linares, Fernando
Universitat de Barcelona
Podiform chromitite bodies occur in highly serpentinized peridotites at Dobromirtsi Ultramafic Massif (Rhodope Mountains, southeastern Bulgaria). The ultramafic body is believed to represent a fragment of Palaeozoic ophiolite mantle. The ophiolite sequence is associated with greenschist - lower-temperature amphibolite facies metamorphosed rocks (biotitic gneisses hosting amphibolite). This association suggests that peridotites, chromitites and metamorphic rocks underwent a common metamorphic evolution. Chromitites at Dobromirtsi have been strongly altered. Their degree of alteration depends on the chromite/silicate ratio and to a lesser extent, on the size of chromitite bodies. Alteration is recorded in individual chromite grains in the form of optical and chemical zoning. Core to rim chemical trends are expressed by MgO- and Al2O3- impoverishment, mainly compensated by FeO and/or Fe2O3 increases. Such chemical variations correspond with three main alteration events. The first one was associated with ocean-floor metamorphism and was characterized by a lizardite replacement of olivine and the absence of chromite alteration. The second event took place during greenchist facies metamorphism. During this event, MgO- and SiO2-rich fluids (derived from low temperature serpentinization of olivine and pyroxenes) reacted with chromite to form chlorite; as a consequence, chromite became altered to a FeO- and Cr2O3-rich, Al2O3-poor chromite. The third event, mainly developed during lower temperature amphibolite facies metamorphism, caused the replacement of the primary and previously altered chromite by Fe2O3-rich chromite (ferritchromite).
2011-03-09
Ofiolites
Cromita
Metamorfisme
Roques ultrabàsiques
Bulgària
Ophiolite
Chromite
Metamorphism
Ultrabasic rocks
Bulgaria
(c) Gonzalez-Jimenez et al., 2009
Article
Universitat de Barcelona (UB). Institut de Ciències de la Terra Jaume Almera (ICTJA). Institut de Diagnosi Ambiental i Estudis de l'Aigua (IDEA). Universitat Autonònoma de Barcelona (UAB). Consejo Superior de Investigaciones Científicas (CSIC)
         

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