Mazerolles unit

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The Mazerolles unit is a tectonic nappe unit in the Haute-Charente on the north-western edge of the French Massif Central .

etymology

The Mazerolles unit is named after the French commune of Mazerolles in the Charente department .

geography

Heavily tectonically stressed mica schist of the Mazerolles unit

The Mazerolles unit is open in a 10 kilometer long and 6 kilometer wide, narrow triangle that tapers to the north. It lies on the western edge of the Massif Central and dips to the west under the Jurassic deck sediments of the northern Aquitaine Basin . In the south, it is cut off in a north - easterly direction by the Le Lindois fault opposite the Saint-Mathieu leukogranite and by the Orgedeuil fault opposite the recrystallized Jura. The eastern boundary to paragneiss of the lower gneiss cover follows the north-south direction from Cherves-Châtelars to Le Lindois . Two smaller booms a little over a kilometer long and around 500 meters wide can be found north of Le Lindois in the middle of the paragneiss.

geology

Geological overview map of the Saint-Mathieu-Doms with the surrounding ceilings

The Mazerolles unit, also known as the Mazerolles mica schist or Mazerolles formation, is composed primarily of mica schists and quartz-rich mica schists. Because of its tectonic position and relatively low degree of metamorphosis , it is considered a lateral equivalent of the Thiviers-Payzac Unit ( TPU ). In particular, it shows similarities with the Semblat schist in the syncline on the Puy des Ages near Payzac . In the limousin's pile of ceilings , it comes to rest in the hanging walls of the two gneiss ceilings . On the opposite geological map of the Saint-Mathieu-Dom , its location is shown in green as MU .

Petrology

In addition to mica slate and quartz-rich mica slate, there are also dark to black, graphite-rich inclusions in the centimeter to decimeter range. The latter are often associated with sulphide ores and represent former organic layers in the originally clayey and silty sediments.

There are two facies of mica schists . It dominates a gray-greenish, the foliation surfaces silky facies. West of Mazerolles it shows a very fine alternation of mica and quartz-rich levels. These represent the original stratification (S 0) and can form an oblique angle with the foliation (S 1). A gray-black to dark, fine-grained, quite massive-looking facies appear subordinate to the gray-green facies. Their mica content is much lower. The facies transitions are progressive, but still occur quite quickly.

Enclosed within the mica schist are lenticular features of the anatectic Roussines granite west of Le Lindois and south of Montembœuf . They reach up to 1.5 kilometers in length and strike predominantly northwest to north-northwest. There are also graphite veins up to one kilometer long, with a predominant strike direction northeast, north and northwest. Two quartz veins in the kilometer range move north-northwest.

mineralogy

The mineralogical composition of the gray-green mica schist with a lepidoblastic structure is based on the minerals quartz, muscovite and biotite as well as accessory tourmaline , zircon and opaque minerals. Metamorphic formations are garnet , staurolite and chlorite . Chlorite is an alteration mineral that emerged from biotite and garnet. All these minerals also occur in the dark facies, which however also contain plagioclase (usually sericized), apatite , pyrite and graphite.

geochemistry

The following analyzes are intended to clarify the chemical composition of the Mazerolles unit. For comparison, two analyzes of the parautochthonous mica slate unit PMU are added:

Oxide
wt.%		 Mazerolles
mica slate 1		 Mazerolles
mica slate 2		 Mazerolles
mica slate 3		 Mazerolles
quartz mica schist		 Parautochthonous
mica slate		 Parautochthonous
quartz mica schist		 Roussines
granite
SiO 2 62.50 67.90 71.00 64.80 58.30 65.05 71.94
TiO 2 0.80 0.69 0.75 0.60 0.95 0.82 0.16
Al 2 O 3 17.86 15.85 15.49 14.74 20.90 17.00 15.32
Fe 2 O 3 1.89 1.44 1.03 1.03 2.46 2.32 0.98
FeO 4.66 3.50 4.11 3.00 5.40 4.24 0.14
MnO 0.13 0.08 0.10 0.07 0.39 0.32 0.06
MgO 2.19 1.72 2.20 1.39 2.20 2.15 0.38
CaO 1.05 1.35 1.78 1.37 1.06 1.27 0.84
Na 2 O 2.48 2.87 3.42 3.72 0.91 1.87 4.00
K 2 O 3.08 2.85 2.00 1.99 4.15 3.43 5.40
P 2 O 5 0.14 0.10 0.00 0.08 0.13 0.22 0.08
H 2 O - 0.10 0.11 0.02 0.06 0.03 0.32
H 2 O + 2.94 1.70 1.15 1.18 2.60 1.63 1.11

The SiO 2 content of the Mazerolles unit fluctuates between 62.5 and 71 percent by weight, so it has an acidic or rockic character. With around 15 to 18 percent by weight Al 2 O 3 , the rocks are quite rich in aluminum . The total iron content varies between 4 and 6.5 percent by weight and is lower than in the parautochthonous mica schists. The alkalis Na + K are around 5.5 percent by weight. The parautochthonous mica schists differ in their higher TiO 2 , MnO and P 2 O 5 .

metamorphosis

The western two thirds of the Mazerolles unit are located in the biotite-almandine zone , while the edge area is to be counted as part of the staurolite-almandine zone . Both zones are separated by the staurolite isograde, the exceeding of which indicates the entry into high-grade metamorphic conditions of the amphibolite facies. The edge area to the lower gneiss cover, which already belongs to the sillimanite zone, is consequently higher metamorphic than the middle-grade western section.

tectonics

The cleavage of the mica schist is relatively flat and dips to the northwest at an average of 20 ° (range 0 to 50 °). Wrinkles - sometimes even isoclinal - and small folds can be observed. The fold axes are horizontal and oriented to east-southeast and north-northwest. In the case of the small folds, the axes are also more or less horizontal, but point to the northeast and northwest - and are thus perpendicular to one another. The mica-rich areas are primarily affected by the small folds - so muscovite is found in the hinges. The Le Lindois Fault is split into several branches and is cataclastic-brecciated to mylonitic .

See also

literature

  • J. - P. Floc'h and others: La Rochefoucauld . In: Carte géologique de la France at 1/50 000 . BRGM, 1983.
  • G. Le Pochat et al .: Montbron . In: Carte géologique de la France at 1/50 000 . BRGM, 1986.

Individual evidence

  1. ↑ J. - P. Floc'h et al: La Rochefoucauld . In: Carte géologique de la France at 1/50 000 . BRGM, 1983.