Val-Nalps-Gneiskomplex

Representation and status

Index

G-Au

Color CMYK

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

Color RGB

R: 230 G: 185 B: 160

Rank

lithostratigraphic Formation

Validity

Unit is in Use

Status

informal term

Nomenclature

Deutsch

Val-Nalps-Gneiskomplex

Français

Complexe gneissique du Val Nalps

Italiano

Complesso gneissico della Val Nalps

English

Val Nalps Gneiss Complex

Origin of the Name

Val Nalps (GR), Tujetsch

Historical Variants

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)

Hierarchy and sequence

Subordinate units

• 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

Age

Age at top

• Cambrian

Age at base

• Precambrian

Dating Method

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

Palaenography and tectonic

• Pre-Variscan polycyclic basement of the Helvetic

Paleogeography

European continental plate

Tectonic unit (resp. main category)

• Gotthard

Kind of protolith

• sedimentary
• ophiolitic

Conditions of formation

Accretionary prism (Abrecht et al. 1991a).

Metamorphism

polycyclic

Metamorphic facies

• amphibolite facies
• granulite facies
• eclogite facies

Note on facies

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

References

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.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).