In a region affected by regional metamorphism, the initial planar surface: the stratification (called S0), is rarely preserved. It is most often substituted by a surface of tectonic origin: the foliation. This marks the plane of flattening of the ellipsoid of the finite deformation.

On the crest of the Taillante, in Queyras, the prevailing surface dipping to the west (i.e. to the left of the photo) is a foliation surface..

This surface is sometimes marked with a lineation that indicates the direction of the stretching. It is thus possible to map the orientation of the ellipsoid of the finite deformation. 

Sometimes several episodes of deformation have succeeded each other in time and several surfaces of foliations can be observed on the thin slides. They are referred to as S1, S2, etc. The geometric relationships between these different surfaces enable a relative chronology to be made.

Similarly, several generations of metamorphic minerals may crystallize before, during or after these surfaces. These minerals make it possible to evaluate the P and T conditions under which the deformation episodes responsible for these textures took place. Tectonics cannot assess these parameters.

By studying a thin section of rock, the petrologist tries to define the relationships between the different deformation episodes and the metamorphic recrystallisation.

On a thin section of a metapelite under GreenSchists conditions, an S1 surface is folded by a second deformation phase responsible for an S2. (see its position on the metamorphic gradient - sample 1 )

Note that the biotite crystal on the right is partially retromorphosed to chlorite, probably during S2; PPL.

The Quartz is white in the photo above and grey in the one below.The muscovite (=white mica) and the chlorite (grey in the photo above) are green on the picture below. The biotite is brown.

Muscovite and chlorite are parallel to both foliation surfaces: these two minerals are therefore contemporary with the two deformation episodes, which means that these deformation episodes took place under conditions of thermal stability of these minerals.

The biotite in large crystals is oriented obliquely on both surfaces. The large size of these crystals shows that they formed in a static regime, without any deformation episodes. The crystal in the middle of the picture shows that the biotite forms on S1 and is therefore posterior to it, but is "cut" by S2, therefore prior to S2.The biotite crystallized between the two deformation phases.

Of the three phyllosilicates, biotite is the highest temperature mineral. Consequently, the crystallisation of this mineral between the 2 deformation phases indicates that the peak T reached by this rock was between these 2 phases. This T peak corresponds to the emplacement of a granite after S1 and deformed by S2.

The following photos are of a garnet metapelite (see its position on the metamorphic gradient - sample 2) in the conditions of the end of the greenschist facies - beginning of the Amphibolite facies.

The three phyllosilicates (chlorite, muscovite and biotite) define the foliation. In the centre of the image, a rounded aggregate of chlorite contains corroded relics of a garnet crystal.

Note the anomalous purple polarised tints of the chlorite aggregate. The relics of garnet, an isotropic mineral, are black in this photo under CPL.

The garnet, contemporaneous with the mica-marked foliation, shows early amphibolite facies conditions, while the chlorite that forms at its expense shows a retromorphosis under greenschist facies conditions.

The long side of the microphotographs is about 3mm. The dark spots in the chlorite and biotite are pleochroic (black) halos around the zircon crystals.

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