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| Titel |
Alteration Mechanism of Chromite in Podiform Chromitites from two Metamorphosed Ophiolitic Complexes: Golyamo Kamenyane (Bulgaria) and Tapo (Peru) |
| VerfasserIn |
Fernando Gervilla, Isabel Fanlo, Thomas N. Kerestedjian, Ricardo Castroviejo, Jose Maria Gonzalez-Jimenez, Jose Alberto Padron, José Feliciano Rodrigues |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250053862
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| Zusammenfassung |
| Chemical zoning related with alteration of chromite from podiform chromitites in metamorphosed ophiolites
from Golyamo Kamenyane (Rhodope Mountains, SE Bulgaria) and Tapo (Eastern Cordillera, Peru) reveals
similar, distinct, two stages of alteration: 1) mass loss of chromite under reducing conditions and 2) addition
of magnetite component. During the first stage, chromite looses Al2O3 by reaction with associated serpentine
to form chlorite and Fe2O3 is reduced to FeO. Because the volume of chromite grains does not change, Al2O3
loss led to development of porous texture in chromite. Most of these pores are filled by chlorite. The second
stage must take place under relatively oxidizing conditions and involves the addition of magnetite component to
the altered, porous chromite giving rise to what is known as ferritchromite (or ferrian chromite). This process
is associated with lowering the pores volume of chromite and the replacement of chlorite by antigorite. Since
chromite grains preserve their original shape and volume the amount of magnetite component that can be added
to the porous chromite generated in the first stage is limited by its pores volume. Further addition of magnetite
would result in the formation of a magnetite-rich, ferritchromite shell overgrowing chromite-ferritchromite grains.
This ferritchromite shell either shows subhedral shape (in chromite-rich disseminated chromitites) or evolves into
a mosaic-like texture with straight grain boundaries and triple points (in nearly massive chromitites).
The unusual chemical trends of altered chromite in podiform chromitites from the two studied, metamorphosed
ophiolitic complexes suggest that this type of alteration should be related with the metamorphic evolution suffered
by such rocks. In fact, the Golyamo Kamenyane massif is hosted by high-pressure, metamorphic rocks (at least
650-750ºC and 8-12kb; Mogessie et al., 2008) and recent geothermometric data indicate that the Tapo massif also suffered similar pressure and temperature conditions (around 535ºC and 12.5kb; Willner et al., 2010).
References:
Mogessie, A., Scheipl, G., Bauer, C. Krenn, K. and Georgieva, M. (2008). Petrology and Geochemistry of the Avren Complex, Rhodope Massif, Bulgaria. International Geológical Congress, Oslo.
Willner, A.P., Castroviejo, R., Rodrigues, J.F., Acosta, J. and Rivera, M. (2010). High pressure metamorphic conditions in garnet amphibolite from a collisional shear zone related to the Tapo ultramafic body, Eastern Cordillera of Central Peru. Simposio S3_Bloques litosféricos de la Cadena Andina. XV Congr. Peruano de Geología. Cusco. |
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