The role of silica in the hydrous metamorphism of chromite
Por:
Colas, Vanessa, Alberto Padron-Navarta, Jose, Maria Gonzalez-Jimenez, Jose, Fanlo, Isabel, Lopez Sanchez-Vizcaino, Vicente, Gervilla, Fernado, Castroviejo, Ricardo
Publicada:
1 nov 2017
Resumen:
Retrograde hydrous metamorphism ``has produced three types of
microstructures in chromite grains from chromitites and enclosing rocks
of the Tapo Ultramafic Massif (Central Peruvian Andes). In semi-massive
chromitites (60-80 vol% chromite), (i) partly altered chromite with
homogeneous cores surrounded by lower Al2O3 and MgO but higher Cr2O3 and
FeO porous chromite with chlorite filling the pores. In serpentinites
(ii) zoned chromite with homogeneous cores surrounded by extremely
higher Fe2O3 non-porous chromite and magnetite rims, and (iii)
non-porous chromite grains. The different patterns of zoning in chromite
grains are the consequences of the infiltration of reducing and
SiO2-rich fluids and the subsequent heterogeneous interaction with more
oxidizing and Fe-bearing fluids. During the first stage of alteration
under reduced conditions magmatic chromite is dissolved meanwhile new
metamorphogenic porous chromite crystallizes in equilibrium with
chlorite. This reaction that involves dissolution and precipitation of
minerals is here modeled thermodynamically for the first time.
ptSiP(2)-MgO pseudosection calculated for unaltered semi-massive
chromitites at 2 kbar and 300 degrees C, the lowest P-T conditions
inferred from the Tapo Ultramafic Massif and Marafion Complex, predicts
that chromite + chlorite (i.e., partly altered chromite) is stable
instead of chromite + chlorite + brucite at progressive higher piSiO(2)
but lower ilMgO. Our observation is twofold as it reveals that the
important role of SiO2 and MgO and the open-nature of this process.
P-T-X diagrams computed using the different P-T pathways estimated for
the enclosing Tapo Ultramafic Massif reproduce well the partial
equilibrium sequence of mineral assemblages preserved in the
chromitites. Nevertheless, it is restricted only to the P-T conditions
of the metamorphic peak and that of the latest overprint. Our
estimations reveal that a high fluid/rock ratio (1:40 ratio) is required
to produce the microstructures and compositional changes observed in the
chromitites from the Tapo Ultramafic Massif. The circulation of
SiO2-rich fluids and the mobilization of MgO from the chromitite bodies
are linked with the formation of garnet amphibolites and carbonate
silica hydrothermalites (i.e., listwaenites and birbirites) in the
ultramafic massif. The origin of these fluids is interpreted as a result
of the dissolution of orthopyroxene and/or olivine from the
metaharzburgites and metagabbros enclosed in the Tapo Ultramafic Massif.
(C) 2017 Elsevier B.V. All rights reserved.
Filiaciones:
Colas, Vanessa:
Univ Nacl Autonoma Mexico, Inst Geol, Ciudad De Mexico 04510, Mexico
Alberto Padron-Navarta, Jose:
CNRS, Geosci Montpellier, F-34095 Montpellier, France
Univ Montpellier, UMR5243, F-34095 Montpellier, France
Maria Gonzalez-Jimenez, Jose:
Univ Granada, Fac Ciencias, Dept Mineral & Petr, Avda Fuentenueva S-N, Granada 18002, Spain
Fanlo, Isabel:
Univ Zaragoza, Dept Ciencias Tierra, Pedro Cerbuna 12, Zaragoza 50009, Spain
Lopez Sanchez-Vizcaino, Vicente:
Univ Jaen, Escuela Politecn Super, Dept Geol, Avda Univ S-N, Linares 23700, Spain
Gervilla, Fernado:
Univ Granada, Fac Ciencias, Dept Mineral & Petr, Avda Fuentenueva S-N, Granada 18002, Spain
Castroviejo, Ricardo:
Univ Politecn Madrid, ETS Ing Minas Energia, Rios Rosas 21, Madrid 28003, Spain
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