Cap Creus in Spain is located at the eastern end of the Pyrenees, near the town of Cadaques. A very pleasant place, with, among other things, the omnipresence of Dali! And geologically fascinating!

Map of Cap Creus after J. Carreiras

Cap Creus belongs to the Hercynian Axial Zone of the Pyrenean chain. It consists almost exclusively of metapelites and pegmatites.

The lithological contrast is sharp in the Cape between the dark metapelites and the white pegmatites.

    The metapelites are metamorphosed under HT-LP gradient conditions with the succession of typical minerals: biotite, andalousite, cordierite (which forms small rugby balls), sillimanite. Locally, anatexis conditions are reached, producing garnet migmatites (in black on the map above). With the sole exception of these anatexites, muscovite is stable in these rocks. These show 3 tectonic surfaces: 2 foliations S and a shear plane C (oriented NW-SE). It can be seen, on the map above, that the sillimanite isograde (black line) is shifted by these dextral shears.

Near Cala Culip, metapelites and pegmatites are cut by dextral shears

Metamorphism is prograde during the first two phases of deformation, while shearing is contemporaneous with retromorphosis.

The shear zones delimit lens in which (relatively) early metamorphic conditions are preserved

It is these deformation-recrystallisation relationships that we will discuss here.

Cordierite and andalusite patches contain an internal foliation parallel to the S. This suggests that these minerals are contemporary with this foliation.

The andalusite patches contain an internal foliation sub-parallel to S1 and are surrounded by a crenulation foliation S2.

However, these minerals are wrapped by surface 2 which carries fibrolite.

Andalusite crystals mark the "horizontal" S1 and are enveloped by the S2 which bears fibrous sillimanite (fibrolite) - PPL, microphotograph about 3mm wide.

This suggests that the peak of the metamorphism coincides with the second phase of deformation. (see the diagram below)

The polymorphic andalusite-sillimanite transition occurs in the S2 foliation plane in the next picture. This shows the role of deformation in the metamorphic recrystallisation.

Poecilitic Andalusite (in grey) changes to sillimanite
PPL Microphotograph about 3mm wide.

Muscovite grows statically on the S2 foliation at the expense of fibrolite.

Bright spots of muscovite develop secant to the foliation.

These new post-cinematic crystals are larger than those developed during deformation and the orientation of their cleavage are sometimes perpendicular to the foliation.

The post-cinematic Muscovite after S2 develops on the sillimanite (CPL microphotograph)

However, this muscovite is deformed in shear zones: in this case, it is ante kinematic to the deformation that produced these shear zones.

(Microphotograph in CPL)

The following figure summarises the relationships between the minerals and the different deformation phases:

This can be represented in the following diagram:

Thus, deformation induces the recrystallisation of new phases and the disappearance of old ones. Early paragenesis is best preserved in the least deformed areas (1 and 2) at a late stage. However, in this least deformed domain, the polymorphic transformation And=Sill takes place in the foliation plane, confirming the role of deformation on the reaction kinetics. Muscovite forms as a result of fluid circulation (see below). In contrast, in the late shear zones (3 et 4), the early minerals (andalusite, sillimanite and muscovite) disappear.

We will now look at the transformation of sillimanite into muscovite ... We mentioned above that muscovite is always stable in metapelites. Thus, the destabilization of sillimanite is not achieved by the reaction Sill + Fk + V = Musc + Q. In addition, we notice that the late static muscovite is abundant near the pegmatites.

The pegmatites are linked to a network of fractures that sometimes guide their emplacement::

These fractures are rimmed, over a few centimetres, with tourmaline from the metasomatic fluids released by the pegmatites during their crystallisation.

It is these fluids that are most likely the cause of the fractures by hydraulic fracturing.

These metasomatic fluids, released during the crystallisation of the pegmatite, favoured the destabilisation of the sillimatite during a "leaching reaction" of the form :

3Sill+3Q+2K+3H2O=2Musc+2H.

Jordie Carreiras, from the University of Barcelona, shows that pegmatites are emplaced throughout deformation (and metamorphism).Indeed, in spite of a pegmatitic grained texture, some clues demonstrate the synkinematic character of these magmatic rocks. It can be seen that the tourmaline rods have a preferential orientation that is parallel to the mineral lineation in the metapelites:

Tourmalines are oriented and sometimes truncated in the direction of the mineral lineation of metapelites. These two photos are taken from two perpendicular planes: the ZY plane (left photo) and the ZX plane (right photo) of the ellipsoid of the deformation.

See also the late shear zones, the sheath folds, the pseudotachylites at the Cap...

And among these shears, see also the ductile normal faults.

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