Streifengneis-Komplex

Representation and status

Index

GO-Au (GO-S)

Color CMYK

(0%,31%,29%,4%)

Color RGB

R: 245 G: 170 B: 175

Rank

lithostratigraphic Formation

Validity

Unit is in Use

Status

incorrect name (though informally used)

Status discussion

Nomenclature

Deutsch

Streifengneis-Komplex

Français

Complexe du Streifengneis

Italiano

Complesso dello Streifengneis

English

Streifengneis Complex

Origin of the Name

Charakteristische Bänderung / Streifung des Gesteins, die durch eine ausgeprägte Lineartextur entseht.

Historical Variants

Bänder- oder Streifengneis (Heim 1891), Zone der streifigen Gneise (Winterhalter 1930), Streifengneise (Huber 1943), Zweiglimmer-Na-K-Feldspatgneise (Oberholzer 1955), Streifen-Orthogneis (Grünenfelder et al. 1964), Zentraler Streifengneis, Streifengneiss (Beffa 1993), Late Ordovician granitoids [Early Silurian metagranitoids] (Mercolli et al. 1994), Streifen gneiss (Sergeev & Steiger 1996), Streifengneis = frühsilurische Metagranite (Labhart & Renner 2012), Prävariszische Orthogneise = «Streifengneis» s.l. (Vögeli et al. 2013), "Streifengneis" Complex (Berger et al. 2017), Complesso dello Gneiss listato

Description

Description

polymetamorph, engadinitgranitischen Chemismus

Mercolli et al. 1994: The term Streifengneis is well established in the Alpine literature. Nevertheless, it contains ambiguities since it is constrained by a peculiar texture more than by genetic or chronological criteria. The Streigengneises are locally less deformed and, therefore, are not banded othogneisses but augengneisses, or even at outcrop scale they have preserved igneous tectures. Another quite common feature is the presence of a finer grained marginal facies that is transitional to the augengneisses.

Hierarchy and sequence

Subordinate units

• Sassina-Augengneis
• Vitgira-Gneis
• Alp-Ramosa-Granitgneis
• Miniera-Schiefer

Age

Age at top

• Llandovery

Age at base

• Late Ordovician

Dating Method

445.2 +3.6/-4.6 Ma (U/Pb zircon: Sergeev & Steiger 1996), 439 +/-5 Ma (Sergeev & Steiger 1993), 436 +/-17 Ma (Rb/Sr Rtot: Arnold 1970), 421 +/-17 Ma (Rb/Sr Rtot) + metamorphism at 275 +/-11 Ma (Rb/Sr muscovite) (Arnold 1970).

Palaenography and tectonic

• Pre-Variscan polycyclic basement of the Helvetic

Tectonic unit (resp. main category)

• Gotthard

Kind of protolith

• plutonic

Conditions of formation

volcanic-arc S-type granitoid (Sergeev & Steiger 1993)

Metamorphism

polycyclic

Metamorphic facies

• amphibolite facies

Note on facies

Nur variszisch (und alpin) metamorph.

References

Definition

Heim Albert (1891) : Geologie der Hochalpen zwischen Reuss und Rhein. Mit einem Anhang von Petrogr. Beiträgen von C. Schmidt. Beitr. Geol. Karte Schweiz 25, 605 Seiten

Definition

Berger Alfons, Mercolli Ivan, Herwegh Marco, Gnos Edwin (2017) : Geological Map of the Aar Massif, Tavetsch and Gotthard Nappes. Geological Special Map 1:100'000, Explanatory Notes 129

p.66: The Streifengneis Complex is composed of granitoid gneisses. The most well-known of these granitoid gneisses is the “Streifengneis”. In spite of differing textures reflecting varying deformation conditions, these granitoid gneisses outcrop over the entire length of the Ausserbinn-Piz Cavel Zone. They represent Early Silurian granitic intrusions. Radiometric ages reported in the literature vary between 439± 5 Ma (SERGEEV & STEIGER 1993) and 436± 17 Ma (ARNOLD 1970). The importance of the “Streifengneis” Complex consists in its key structural position. These Early Silurian granites intruded discordantly into the high-temperature metamorphic sequences of the Val Nalps and Paradis Gneiss Complexes. As a consequence of this crosscutting relationship, the highest-grade metamorphic event was attributed to the Ordovician. On the other hand, the rocks of the “Streifengneis” Complex were deformed together with the Val Nalps and Paradis Gneiss Complexes. This deformation led to a very characteristic structure. Today, this is exposed as kilometre-wide, steep and nearly isoclinal folds or “Schlingen”. These “Schlingen” and related structures are discordantly intruded by the late to post-Variscan (Asselian–Sakmarian) plutonic rocks of the Val Lavaz and Pesciora Groups. This type of “Schlingen structure” must therefore be assigned to the Variscan overprint of the polycyclic metamorphic basement of the Gotthard Nappe. Such “Schlingen” are widespread in the polycyclic basement of the Central Alps (ZURBRIGGEN 2015, 2017). Similar structures, but at smaller scale, are frequent in all polycyclic metamorphic basement units of the Gotthard Nappe and of the Aar Massif.

Important Publications

Huber Heinrich M. (1943) : Physiographie und Genesis der Gesteine im südöstlichen Gotthardmassiv Schweiz. Min. Petr. Mitt. 23, 72-260