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The Geology of the Montagne Noire and the Caroux Massif

Map of the main geological  units of the area
From: BRGM and Demanger, M (quoted from Tormo N).
For more information on the spectacular "augen gneiss" in the photo below, and the setting in which it was created, scroll down.

Large close up photograph of a group of three augen
Close up photograph of a group of large augen in a gneiss which is a "metamorphic rock". Note that this photograph is the right way up. This may indicate that this rock was subjected to compressive forces acting horizontally relative to the present orientation of the rock The stripes are due to metamorphic change in the rock and are variations in the balance of minerals. These changes occur under the influence of such compressive forces at elevated temperatures.

geol map
Structural sketch map of the eastern part of the Montagne Noire (Drawn by Paul Proudman after Van den Driessche and Brun (1991-2))
(1) schists X; (2) Mont St-Gervais unit; (3) southern fold and thrust belt; (4) orthogneiss or "augen gneiss";
(5) mylonites; (6) migmatitic gneiss; (7) migmatite; (8) Anatectic granite; (9) Stephanian sediments (10) Permian sediments; (11) post Palaeozoic sediments; (13) high angle normal faults; (14) Pyrenean thrust.

The Gorges d'Héric form a natural section through the Caroux Massif which offers the best opportunity to see its spectacular "augen gneisses". The massif is the south eastern part of a much larger gneiss (see location 3 and 4) dome structure with an axis with a direction of (trending) NE-SW to ENE-WSW which forms part of the Montagne Noire. The local Caroux dome is also elongated in the ENE- WSW direction and the bulk of the interior consists of gneiss, much of which is "augenitic"(with yeux, or eyes). The whole is surrounded by nappes (large overturned thrusted folds with almost parallel sided limbs) of Palaeozoic low grade metamorphosed sedimentary rock (metasediments) (see location 1 for more explanation). These nappes have been associated with southwards thrusting related to the Variscan continental collision when France and Spain collided (Van den Driessche and Brun 1991-2).

Map of location and main structural features .Location and main structural features of the surroundings.

Drawn by Paul Proudman after Van den Driessche and Brun (1991-2)
The origin of the augen and the rock texture
The texture of this rock is likely to have been created in conditions of "regional metamorphism". The large ovoid to lozenge shaped megacrysts are probably relict porphyroblasts. Quartz is easily recrystallized and feldspar is brittle and more enduring (Spry,1969). Quartz forms relatively fine grained, recrystallized aggregates in the matrix where it may be accompanied by feldspar, mica and sometimes amphiboles as foliated layers swirling around the large porphyroblasts. The banding is compositional, i.e. due to modal variations in phases present and created by partial melting and recrystallization, a process called anatexis. The bands alternate between quartz rich layers which are light in colour and thinner biotite rich layers (dark and flecked in colour). This type of rock is associated with high grade metamorphism of aluminous sediments.
The gneissic banding shapes
The characteristic shape of the augen is remarkably constant which is evidence of a process independent of small scale compositional variations. It's likely that the relict megacrysts of feldspar were present at the time of deformation and metamorphism and that they resisted the deformation to a greater extent than the other phases. The partial melting which accompanied the deformation occurred at crystal boundaries so the large single crystals at the centre of the eyes form a pocket of constant size and shape within an environment of change. As the rock was squashed, the banding was formed. As the space in the matrix diminished, the bands became more deformed round the large crystals (Barker 1990) This all happened very slowly although some of the shapes are reminiscent of liquid movements which seem to be swirling around them.

section showig the evolution of a dome in an extensional environment

Drawn by Paul Proudman after Van den Driessche and Brun (1991-2)

Four sections (above) which show the evolution of an uplifted dome in an extensional environment. The series shows the development of a similar fault pattern to Basin and Range
Radiometric Dating
The climax of the Variscan shortening and thickening event has been dated at about 320 Ma by Rb-Sr data and by 40Ar-39Ar (Maluski et al 1991). Three structural zones have been identified using 40Ar-39Ar
  1. the migmatites and gneisses of the core which give mica dates of 315 Ma,
  2. mylonites of the northern and southern shear zones which give an age of 310 Ma,
  3. the nappe structures which give an age of 279 Ma
Evidence for late extension
On the small scale the many dykes, some up to 5m, which are seen on the route is strong evidence for extension in some form. This extension can occur on the outside curve of folds. If these dykes extend far enough they can can be distinguished from fold cracks and here they do appear to be planar on a quite large scale. On a larger scale there are substantial coal bearing basins to both the north and the south. The presence of basins does constitute strong evidence for extension. The date of these basins is early enough to be contemporary with the main dome building event at the Upper Carboniferous to the Permian.
group of three augens
A group of augen showing complex but regular curving of the gneissic schistosity in accordance with "orderly" deformation
The augen
The gneiss of the Montagne Noire dome contains augen of alkali feldspar which is a crystalline mineral composed mostly of silicon aluminium potassium and oxygen. These large crystals (porphyroblasts) which are set in a finer "matrix" appear to be lozenge shaped (although this may in part be a textural artefact). They can be up to 10 cm long but are more typically 5 or 6 cm If you get the sunlight in the right direction it is clear that the auge is one large crystal as the whole reflects at the same angle. Although the crystal is of alkali feldspar there are inclusions of other minerals such as biotite (a dark form of mica, see location 1.


Three stages in the development of foliation around relict feldspar porphyroblasts. Deformation is in the direction of the arrows.
(1) Near to the beginning of the deformation there is very little banding and very little space has been lost.
(2) The top to bottom dimension is reduced by approximately 40% and banding is now pronounced. The curved forms around the feldspar crystal are developing
(3) Top to bottom dimension is more than halved and banding is sharply defined. Curved shapes around the crystal are fully formed.
Composition of the Montagne dome
As can be seen from the first large map the Montagne Noire dome contains mostly anatectic granites (that is, granites arising out of the partial melting of high grade metamorphic rock), migmatites (mixtures of anatectic granite and gneiss) and high grade metamorphic gneisses, overlain by lower grade Cambrian metamorphosed sedimentary rock (metasedimentary) cover, which is not widely exposed. This means that the Caroux dome of the Gorges is not made of quite the same material as the rest of the Montagne dome.
Composition of the Caroux dome
The Caroux dome of the Gorges is of a different composition, the anatectic granite and migmatites being largely replaced by the augen gneiss. Although there are some pockets of migmatites, in general the highest metamorphic grade is slightly lower. (See the first large map).The Espinouse Dome to the north contains mostly anatectic granites (that is, granites arising out of the partial melting).
Geological history
All models propose a late doming in Stephanian-Permian times. Earlier models agree with a doming event associated with late regional compression after earlier Variscan thrusting. For more details on these models go to:
Nav. button to previous models

These explanations based on compressional doming are now being largely replaced by:

  • a crustal thickening coinciding with the Variscan collision and contemporary with the emplacement of the surrounding nappe structures, followed by
  • an extensional tectonic phase in Stephanian-Permian times which caused the doming to develop as a response to gravity collapse due to crustal thickening.

The extensional phase includes strongly asymmetrical strain and metamorphic patterns. The extension is caused by gravity spreading of the thickened crust, with accompanying basin and range formation. Ductile fabrics acquired in the Variscan compressive phase are retained. The Montagne Noire is now considered to be an important example of the spreading of a thickened crust and the strain and metamorphic patterns which can go with this process.

A synthetic cross section through the Stephanian-Permian Extensional System. The Caroux dome is Ca and the Espinouse dome is Es. From Van den Driessche and Brun (1991-2)


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This page was written by Paul Proudman, Geology, School of Biology and Molecular Sciences, Oxford Brookes University, Gipsy Lane Campus, Headington, Oxford, OX3 OBP UK

Last Modified: 25 June 03. Minor editing by Roger Suthren, March 2013