Paragneiss

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Paragneiss of the Kutná Hora Crystalline (Bohemian Mass), overlaid with discordant angles by Cretaceous sandstones, Kutná Hora , Czech Republic.

A paragneiss is a metamorphic rock with a gneiss structure , the parent rock ( protolith ) of which is a siliciclastic sedimentary rock . Paragneiss are quartz , feldspar and mica- rich rocks that can contain kyanite (= thistle), staurolite , garnet and other minerals as sub-mixtures. In contrast to paragneiss, orthogneiss originated from magmatites .

etymology

The term paragneiss is a compound from the Greek prefix παρα- ( para -) with the meaning with , minor, extra or over- and the rock name gneiss .

Parent rocks

Paragneiss usually originate from terrigenous, detritic, often poorly developed sediments or sedimentary rocks. These can be arkoses or greywacken and pelite , the last two often mentioned as components of flysch or flyschoid sequences that were deposited in the marine environment , on the shelf and on the continental slope . Pure quartz sandstone or claystone, d. H. Sediments with a high degree of ripeness do not change into gneiss under the appropriate conditions, but into quartzite or mica schist .

distribution

Contact between a paragneiss (top right) and a dolerite dike (bottom left), Svekonorwegids of the Baltic Shield (Koster Islands, Southwest Sweden).

Paragneiss occurs worldwide in medium to highly metamorphic basement complexes . They are very common in certain areas of the Variscan basement, within which they can be found, for example, in the Bohemian Massif , the Black Forest , the Vosges or in the French Massif Central . They are also found in metamorphic complexes in the Alps , for example in the Dent-Blanche-Sesia Nappe, which, among other things, forms the summit area of ​​the Matterhorn and Dent Blanche .

education

Tightly folded paragneiss from the French Massif Central , Arverni area near Nontron . The rock shows a clear parasitic small fold.

Like orthogneiss, paragneiss are massive and hard crystalline rocks that are characterized by a characteristic granoblastic or granolepidoblastic layer texture ( foliation ). This layer texture is normally a rhythmic alternation of bright quartz-feldspar-rich layers and dark, mica-rich layers. The special thing about paragneiss is that the original stratification of the parent rock can be preserved in the layer texture. In “forgotten” Grauwacken claystone sequences, the Grauwacken banks are noticeable as extremely massive intermediate layers with a granite-like appearance.

composition

Mineral inventory

Essential minerals in paragneiss are quartz and feldspar ( alkali feldspar and plagioclase , usually oligoclase or andesine ), which usually occur together and build up the quartz feldspar layers mentioned above. There are also biotite , muscovite and amphiboles in the dark, mica-rich layers. Since paragneiss was recorded by a regional metamorphosis, they usually also contain typical metamorphic minerals such as sillimanite (fibroolite), andalusite , thistle , cordierite , staurolite or garnet ( almandine and spessartine ). As accessories , they usually contain apatite , titanite , allanite , epidote , tourmaline and zircon as well as opaque ore minerals such as B. Ilmenite .

Chemism

Due to the wide range of protoliths, the chemical composition is very variable and subject to great fluctuations. The SiO 2 content can vary between 57 and 80%. The Al 2 O 3 content varies between 11 and 18%.

The following table lists the chemical composition of paragneiss using the example of two gneiss units from the southern Black Forest (each Metagrauwacken; from Wimmenauer, 1985) and the Massif Central ( Bas Limousin with 11 and lower gneiss unit with 7 samples):

Oxide
wt.%		 Ibach
southern Black Forest		 Murgtal
Southern Black Forest		 Bas Limousin
Massif Central		 Lower gneiss unit
Massif Central
SiO 2 66.04 77.18 68.10 75.16
TiO 2 0.75 0.56 0.65 0.13
Al 2 O 3 15.35 11.68 15.15 13.65
Fe 2 O 3 1.53 0.24 1.61 0.51
FeO 4.34 2.26 3.45 1.02
MnO 0.09 0.05 0.08 0.12
MgO 1.94 1.01 2.08 0.43
CaO 1.73 1.61 1.43 0.51
Na 2 O 3.51 3.42 2.89 2.56
K 2 O 2.22 1.18 2.93 4.40
P 2 O 5 0.12 0.18 0.17
H 2 O + 2.37 0.71 1.52 1.40
Na + K 5.73 4.60 5.82 6.96

Metamorphic facies and anatexis

Anatectic, quartz-feldspar-rich lens in the plagioclase-rich paragneiss of Nontron. Their spatial arrangement suggests dextral shear sense

Be sediments or emerged from sedimentary rocks sunk deep into the earth's crust, such as by subduction or during the collision of two continental blocks, they are due to the high pressure and high temperatures at these depths converted ( Regional metamorphosis ). In the case of paragneiss (as with gneiss in general), the transformation takes place under the medium to high-grade (“mesozonal” to “catazonal”) pressure-temperature conditions of the amphibolite facies .

Sometimes pressure-temperature conditions are run through during the metamorphosis, under which the rock partially melts ( anatexis ). The gneisses then show migmatitic secretions of granitoid composition (quartz and feldspathic bands and lenses concordant with the layer texture, but also discordant passages and larger granitoid enclaves). In the case of a very close association with granitoids, which have a similar geochemical signature ( trace elements , isotope distribution), a direct genetic relationship between these granitoids and paragneiss can be concluded. It can be assumed that in these cases the paragneiss separated the granitoids with partial melting. Such an association can be found, for example, in the upper gneiss unit of the Livradois (Massif Central).

Natural stone types

polished pattern of Serizzo Antigorio, approx. 24 × 15 cm

The following varieties are considered to be paragneiss:

literature

  • W. Wimmenauer: Petrography of igneous and metamorphic rocks . Ferdinand Enke Verlag, 1985, ISBN 3-432-94671-6 .

References and comments

  1. ↑ Note: “Little developed” in this context means that the sediment has a low grain sorting in terms of grain size and mineral composition as well as a slight grain rounding , so it applies above all to the Arkosen and Grauwacken mentioned below. One speaks here of “immature” sediments.
  2. ↑ Fabien Solgadi, Jean-François Moyen, Olivier Vanderhaeghe, Edward W. Sawyer, Laurie Reisberg: The Role of Crustal Anatexis and Mantle-Derived Magmas in the Genesis of Synorogenic Hercynian Granites of the Livradois Area, French Massif Central. In: The Canadian Mineralogist. Vol. 45, No. 3, 2007, pp. 581–606, doi : 10.2113 / gscanmin.45.3.581 (alternative full text access : free.fr )
  3. ^ Friedrich Müller: INSK compact. The international natural stone index for the current market. Volume 2. Index sheet 48.2. Ebner Verlag Ulm 1997.
  4. ^ Friedrich Müller: INSK compact. The international natural stone index for the current market. Volume 2. Index sheet 48.1. Ebner Verlag Ulm 1997.
  5. ^ Friedrich Müller: INSK compact. The international natural stone index for the current market. Volume 2. Index sheet 47.1. Ebner Verlag Ulm 1997.
  6. ^ Friedrich Müller: INSK compact. The international natural stone index for the current market. Volume 2. Index sheet 47.2. Ebner Verlag Ulm 1997.