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| Titel |
ALKALI-METASOMATISM AND PHYLLONITE DEVELOPMENT ALONG A MAJOR ALPINE SHEAR ZONE: THE EAST TENDA SHEAR ZONE (ALPINE CORSICA, FRANCE) |
| VerfasserIn |
Matteo Maggi, Federico Rossetti, Thomas Theye |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250038433
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| Zusammenfassung |
| Here we present results of an integrated study which combines structural geology, petrology
and geochemistry addressed to assess modes and regimes of fluid-rock interaction during
shear fabric development along the East Tenda Shear Zone (ETSZ), Haute Corse,
France. This shear zone marks the overthrusting of the oceanic-derived Schistes
Lustrés nappe onto the Hercynian granitic basement of the western Corsica. The
granitic protolith (Casta Granodiorite) consists of K-feldspar, plagioclase, quartz and
minor hornblende, and accessory apatite and zircon. A progressive, ductile-to-brittle
top-to-the-W/SW shearing affects the Casta granodiorite, evolving from blueschist-facies
(S-L tectonites) to upper crustal (brittle thrust faults) conditions. Apart the variably
retrogressed mafic blueschist boudins, within the ETSZ two main rock types were
recognised: weakly-foliated gneisses (hereafter referred as massive bodies) and
phengite-dominated mylonites (hereafter referred as phyllonites). Both rock types consist of a
high-variance mineral assemblage made up of feldspar (albite and K-feldspar) +
quartz + phengite. Accessory minerals include relict zircons, and syn-kinematic
andradite, epidote, monazite, leucoxene and titanite. At a regional scale, phyllonites
envelop the massive bodies that, commonly, form sigma-shaped shear lenses wrapped
within the mylonitic foliation. Phyllonites consist of alternating, micro-crystalline
quartz-feldspar-phengite-bearing layers and by different generation of variably deformed
quartz and composite quartz-feldspar (albite and microcline) vein segregations (up to 30 cm
in thickness). Late-stage, laminated sub-horizontal quartz-feldspar vein arrays also occur,
suggesting their origin as thrust-related shear veins. Microtextures are indicative of
pseudomorphic growth of phengite after relict igneous K-feldspar. Nevertheless, new
growth of microcline is ubiquitous along the rims of porphyroclastic K-feldspar
grains. EMPA of phengite systematically revealed a high celadonitic substitution,
ranging from 3.52 to 3.65 a.p.f.u. without any systematic core-rim zonation. P --T
pseudosections in the NKFMASHO (Na2O-K2O-FeO-MgO-Al2O3-SiO2 -H2O-O2)
for the P --T range 250–600 Ë C and 1.5–9.5 kbar was computed using the
Perple_X software (Connolly 2005; http://www.perplex.ethz.ch/) for phyllonite
bulk rock compositions. These calculations indicate that the phyllonite assemblage
(quartz-microcline-albite-phengite-epidote) is stable in the 3-7 kbar pressure interval for
temperatures between 250 and 450 Ë C, indicating P - T conditions compatible
with the low-grade greenschist facies field. Activity diagrams in the NKFMASH
(Na2O-K2O-FeO-MgO-Al2O3-SiO2-H2O) compositional system indicate that the phyllonite
assemblage represents an invariant point, which argues for a rock-buffered fluid circulation
system. Mass balance calculations comparing whole rock chemical composition of a
progressively deformed rock sequence and assuming Al as immobile, documents a
progressive gain in Si (up to 29%), Na and K (both up to 43%), a depletion in all other major
elements, and a volume increase up to about 20%. This implies a strong alkaline
metasomatism operated by the circulating fluids during phyllonite development. In this
light, the matrix, highly substituted phengite compositions of phyllonites (i.e. not
compatible with the overall greenschist-facies P - T conditions) appear strongly
influenced by the intense fluid-rock interaction during progress of shear deformation. |
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