We combined structural analysis, thermobarometry and oxygen isotope
geochemistry to constrain the evolution of kyanite and/or
andalusite-bearing quartz veins from the amphibolite facies metapelites
of the Simano nappe, in the Central Alps of Switzerland. The Simano
nappe records a complex polyphase tectonic evolution associated with
nappe stacking during Tertiary Alpine collision (D1). The second
regional deformation phase (132) is responsible for the main penetrative
schistosity and mineral lineation, and formed during top-to-the-north
thrusting. During the next stage of deformation (D3) the
aluminosilicate-bearing veins formed by crystallization in tension
gashes, in tectonic shadows of boudins, as well as along shear bands
associated with top-to-the-north shearing. D2 and D3 are coeval with the
Early Miocene metamorphic peak, characterised by kyanite + staurolite +
garnet + biotite assemblages in metapelites. The peak pressure (P) and
temperature (T) conditions recorded are constrained by
multiple-equilibrium thermobarometry at 630 +/- 20 degrees C and 8.5 +/-
1 kbar (similar to 27 km depth), which is in agreement with oxygen
isotope thermometry indicating isotopic equilibration of quartz-kyanite
pairs at 670 +/- 50 degrees C. Quartz-kyanite pairs from the
aluminosilicate-bearing quartz veins yield equilibration temperatures of
645 +/- 20 degrees C, confirming that the veins formed under conditions
near metamorphic peak. Quartz and kyanite from veins and the surrounding
metapelites have comparable isotopic compositions. Local intergranular
diffusion in the border of the veins controls the mass-transfer and the
growth of the product assemblage, inducing local mobilization of SiO2
and Al2O3. Andalusite is absent from the host rocks, but it is common in
quartz veins, where it often pseudomorphs kyanite. For andalusite to be
stable at T-max, the pressure in the veins must have been substantially
lower than lithostatic. An alternative explanation consistent with
structural observations would be inheritance by andalusite of the
kyanite isotopic signature during polymorphic transformation after the
metamorphic peak.