Wagnerite in a cordierite-gedrite gneiss: Witness of long-term fluid-rock interaction in the continental crust (Ile d'Yeu, Armorican Massif, France)

Research areas:
Year:
2008
Authors:
  • Pavel Pitra
  • Philippe Boulvais
  • Vladimir Antonoff
  • Hervé Diot
Journal:
AMERICAN MINERALOGIST
Volume:
93
Number:
2-3
Pages:
315-326
Month:
FEB-MAR
ISSN:
0003-004X
BibTex:
Abstract:
We describe the first occurrence in the Variscan Belt of Western Europe
of the relatively rare phosphate wagnerite, ideally Mg2PO4F. It occurs
in albite-rich, cordierite-gedrite-bearing gneisses on the island of Ile
d'Yeu, southern Armorican Massif, France. These gneisses are associated
with a network of shear zones that crosscut granitoid orthogneisses of
calc-alkaline affinity. Wagnerite is zoned and displays a rimward
decrease of Fe/(Fe + Mg) from 0.16 to 0.08 and a concomitant increase in
F. The F content ranges 0.46-1.05 apfu, but critically depends on the
choice of the analytical standard. Based on phase diagrams calculated
with THERMOCALC, we infer that the wagnerite-bearing orthoamphibole +
cordierite + biotite + chlorite paragenesis equilibrated at ca. 550
degrees C, and pressures lower than 4 kbar. The presence of staurolite
relics requires similar temperatures, but pressures higher than 4 kbar,
implying an evolution dominated by decompression. On the basis of
whole-rock chemistry and stable isotopes, we suggest that superimposed
periods of metasomatic alteration throughout the metamorphic history led
to the prograde stabilization of the cordierite-gedrite gneiss at the
expense of the orthogneiss. This alteration involved aqueous fluids in
isotopic equilibrium with local rocks and caused significant loss of Ca,
K, and Si, and gain of Mg and Na. We argue that the Na-enrichment is the
most significant difference between wagnerite-bearing and wagnerite-free
Mg-rich, Ca-poor rocks on Ile d'Yeu. This emphasizes the possible
importance of Na metasomatism for the formation of wagnerite. In light
of comparisons with other wagnerite occurrences, we conclude that a
long-term fluid-rock interaction, typically associated with shear-zones,
may be the rule rather than the exception for the formation of wagnerite
in metamorphic rocks unaffected by anatexis.