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| Titel |
Temperature micro-mapping and redox conditions of a chlorite zoning pattern in green-schist facies fault zone |
| VerfasserIn |
Vincent Trincal, Pierre Lanari, Brice Lacroix, Martine D. Buatier, Delphine Charpentier, Pierre Labaume, Manuel Muñoz |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250095028
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| Publikation (Nr.) |
 EGU/EGU2014-10468.pdf |
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| Zusammenfassung |
| Faults are major discontinuities driving fluid flows and playing a major role in precipitation of
ore deposits. Mineral paragenesis and crystal chemistry depend on Temperature (T)
condition, fluid composition but also on the redox environment of precipitation.
The studied samples come from the Pic de Port Vieux thrust sheet, a minor
thrust sheet associated to Gavarnie thrust fault zone (Central Pyrenees).The Pic de
Port Vieux Thrust sheet comprises a 1-20 meter thick layer of Triassic red beds
and mylonitized Cretaceous limestone. The thrust sheet is affected by faults and
cleavage; the other important deformation product is a set of veins filled by quartz and
chlorite. Microstructural and mineralogical investigations were performed based on
the previous work of Grant (1992). The crystallization of chlorite is syn-tectonic
and strongly controlled by the fluid circulation during the Gavarnie thrust sheet
emplacement.
Chlorite precipitated in extension veins, crack-seal shear veins or in open cavities.
The chlorite filling the open cavities occurs as pseudo-uniaxial plates arranged in
rosette-shaped aggregates. These aggregates appear to have developed as a result
of radial growth of the chlorite platelets. According to point and microprobe
X-ray images, these chlorites display oscillatory chemical zoning patterns with
alternating iron rich and magnesium rich bands. The chlorite composition ranges
from Fe rich pole (Si2.62Al1.38O10(Al1.47Fe1.87Mg2.61)6(OH)8) to Mg rich pole
(Si2.68Al1.31O10(Al1.45Fe1.41Mg3.06)6(OH)8). In metamorphic rocks, zoning pattern or
rimmed minerals results for varying P or T conditions and can be used to unravel the P-T
history of the sample.
In the present study, temperature maps are derived from standardized microprobe X-ray
images using the program XMapTools (Lanari et al 2014). The (Fe3+/Fetot) value in
chlorite was directly measured using μXANES spot analyses collected at the Fe-K
edge.
The results indicate a homogeneous temperature of 300-350°C throughout the
crystallization. This result excludes the T as the main parameter to explain the Fe and Mg
zoning patterns. Several other origins can be proposed and discussed in order to understand
zoning patterns such as fluid chemistry, pressure, pH or redox variations of the
fluid.
Grant, N.T., 1992. Post-emplacement extension within a thrust sheet from the central
Pyrenees. Journal of the Geological Society 149, 775–792.
Lanari, P., Vidal, O., De Andrade, V., Dubacq, B., Lewin, E., Grosch, E.G., Schwartz, S.,
2014. XMapTools: A MATLAB©-based program for electron microprobe X-ray
image processing and geothermobarometry. Computers & Geosciences 62, 227–240. |
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