Val-Nalps-Gneiskomplex

Représentation et statut

Index

G-Au

Couleur CMYK

(0%,20%,30%,10%)

Couleur RGB

R: 230 G: 185 B: 160

Rang

Formation lithostratigraphique

Usage

Ce terme est en usage.

Status

terme informel

Nomenclature

Deutsch

Val-Nalps-Gneiskomplex

Français

Complexe gneissique du Val Nalps

Italiano

Complesso gneissico della Val Nalps

English

Val Nalps Gneiss Complex

Origine du nom

Val Nalps (GR), Tujetsch

Variantes historiques

Sericitschiefer oberhalb Hospenthal (Waindziok 1906), Nördliche Paragneise, Series I + II (Niggli 1944), Metasedimentary meta-ophiolitic unit (Mercolli et al. 1994), Val Nalps Gneiss Complex (Berger et al. 2017)

Hiérarchie et succession

Unités hiérarchiquement subordonnées

• Gneiss di Prato
• Distelgrat-Zone
• Guspis-Gneis
• Gurschen-Gneis
• Paragneiszug Crap Stagias – Fuorcla da Vallesa – Piz Rentiert
• Paragneiszug Val Lavaz – Val Sumvitg
• Paragneiszug Piz Cristallina – Piz Valdraus – Piz Cugn
• Chämleten-Serpentinit
• Fuorcla-Paradis-Serpentinit

Âge

Âge au sommet

• Cambrien

Âge à la base

• Précambrien

Méthode de datation

Protolit: Precambrien - Cambrien. Ca. 950 Ma (Sm/Nd, Biino et al. 1994).

Paléogéographie et tectonique

• Socle polycyclique anté-varisque de l'Helvétique

Paléogéographie

plaque continentale européenne

Termes génériques

• Gotthard

Type de protolithe

• sédimentaire
• ophiolitique

Conditions de formation

Accretionary prism (Abrecht et al. 1991a).

Métamorphisme

polycyclique

Faciès métamorphique

• faciès à amphibolites
• faciès à granulites
• faciès à éclogites

Remarque sur le facies

Eklogit- und Granulit-Fazies (prae-Ordoviz), sowie Amphibolit-Fazies (variszisch).

Références

Définition

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.64: The Val Nalps Gneiss Complex constitutes the lithostratigraphic core of the Gotthard Nappe. Biotite-hornblende-garnet-bearing andesine-labradorite gneiss with inclusions of calcsilicate rocks and fine-banded or hornfelsitic garnet-sillimanite-bearing albite-oligoclase gneiss, both of metasedimentary origin, dominate this gneiss complex (Tab. 3). The present-day complex variation of mineralogical composition of these gneisses is due to the overlap of three metamorphic events expressed by different metamorphic degrees and subsequent retrograde alterations. Consequently, a number of local names have been used to describe these gneisses (Tab. 3). AMBÜHL (1929), WINTERHALTER (1930), HUBER (1943), NIGGLI (1944), OBERHOLZER (1955), FEHR (1956), HAFNER (1958) and ARNOLD (1970) described in great detail the petrography and the structures of the Val Nalps Gneiss Complex of the Ausserbinn-Piz Cavel Zone.
Frequent intercalations of amphibolite and rare ultramafic rocks are associated with the metasedimentary gneisses of the Val Nalps Gneiss Complex. ABRECHT et al.(1991) stressed the ophiolitic character of the amphibolite /ultramafic rock association, and BIINO (1994) described the metamorphic evolution of these rocks. A metamorphic loop starting in eclogite facies crossing the granulite facies field and ending in upper amphibolite facies is recorded in these rocks, suggesting a classical subduction–collision geodynamic context for the Ordovician cycle (NIGGLI 1944, BIINO 1994). Beside the meta-ophiolitic sequences, ABRECHT & BIINO (1994) found the same metamor-phic evolution in some gabbro bodies of island-arc affinity intruding the metasedi-mentary gneisses (e.g., the Chastelhorn Metagabbro, Fig.13). OBERLI et al. (1994) dated the intrusion of these gabbros in a time interval between 475 and 467 Ma. This Early Ordovician age is in agreement with an age of ~468 Ma obtained for the MORB-type eclogite by OBERLI et al.(1994). These ages indicate a probable Neoproterozoic age for the oceanic protoliths of the meta-ophiolites.
The metamorphic evolution of the metasedimentary Val Nalps Gneiss Complex fits in with this Ordovician orogenic scenario. NIGGLI (1944) and ARNOLD (1970) described upper amphibolite- to granulite-facies parageneses in these rocks and in particular, in the calcsilicate inclusions. From this it follows that the Neoproterozoic ophiolitic sequence was subducted to eclogite-facies conditions, and was subsequently wedged into the clastic metasedimentary rocks (most probably in the accretionary prism) during collision and the following high-temperature metamorphism (MERCOLLI et al.1994).