Val-Rondadura-Gruppe (prä- und frühvariszische Sedimente und Vulkanite der Gotthard-Decke)

Rappresentazione e statuto

Colore CMYK

(12%,0%,15%,33%)

Colore RGB

R: 150 G: 170 B: 145

Rango

Gruppo litostratigrafico

Uso

Unità in uso.

Status

termine informale

Nomenclatura

Deutsch

Val-Rondadura-Gruppe (prä- und frühvariszische Sedimente und Vulkanite der Gotthard-Decke)

Français

Groupe du Val Rondadura

Italiano

Gruppo della Val Rondadura

English

Val Rondadura Group

Origine del nome

Val Rondadura (GR)

Varianti storiche

Palaeozoic metasedimentary unit / Middle Paleozoic metasediments (Mercolli et al. 1994), Mittelpaläozoische Metasedimente (Labhart 2005), Metasedimente (Spillmann et al. 2011), Variszische Paragneise und Schiefer (Vögeli et al. 2013), Pre- and early Variscan metasedimentary and volcaniclastic rocks = Val Rondadura Group (Berger et al. 2017)

Descrizione

Descrizione

Gemäss Niggli (1944) können folgende Kriterien für die Zuordnung zu den jüngeren, variszischen Paragesteinen herangezogen werden: die lediglich niederiggradige, grünschieferfazielle präalpine Metamorphose; das Fehlen von Zonen mit gehäuftem Auftreten von Pegmatiten («Injektionen») sowie das Auftreten von Metaquarzit. Eine eindeutige Alterseinstufung basierend auf den genannten Kriterien ist jedoch nicht unbestritten (Fehr 1956, Arnold 1970).

Gerarchia e successione

Unità di rango inferiore

• Alp-Cavradi-Gneiskomplex
• Borel-Gneiskomplex
• Tenelin-Gneiskomplex
• Laiets-Gneiskomplex
• Giubine-Gneiskomplex
• Complesso gneissico della Tremola
• Prüsfa-Gneiskomplex

Età

Geomorfologia

• Paleozoico medio

Osservazioni sul tetto

Dévonien?

Età alla base

• Paleozoico medio

Osservazioni sulla base

Silurien?

Metodo di datazione

Mittelpaläozoisch (Mercolli et al. 1994)

Paleogeografia e tettonica

Paleogeografia

Continente Europeo

Termini generici

• Gotthard

Tipo di origine

• sedimentaria
• vulcanica

Metamorfismo

policiclico (alpino e varisico)

Facies metamorfica

• facies a scisti verdi
• facies anfibolitica

Referenze

Definizione

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.68: Various gneisses of sedimentary origin form elongated structures of varying thickness (a few hundred m to two km) concordantly interlayered with rocks of the polycyclic metamorphic basement. The common characteristic of these rocks is their lack of high-grade metamorphic relics. Three of these steeply lying intercalations, the Giubine, Tenelin and Laiets Gneiss Complexes, are crosscut by intrusions of late to post-Variscan Medel Granite and Cristallina Granodiorite, indicating the pre-Mesozoic age of the protoliths and their tectonic attitude. STEIGER (1962) suggested a double overprint at similar lower amphibolite-facies metamorphic conditions as for the Tremola Gneiss Complex. Since the younger metamorphic event is assumed to be of Alpine age, the second and older is considered to be Variscan. In this case, the sedimentary protoliths of these sequences must have been deposited after the Ordovician high-grade metamorphism and before the onset of the Variscan metamorphic overprint (MERCOLLI et al.1994 and references therein).
These sedimentary gneisses were grouped together into the Val Rondadura Group (Tab. 3). The protoliths of the gneisses of the Val Rondadura Group have a similar relative age as those of the Cavardiras Group in the Aar Massif. Several zones (tectonic elements) belonging to this group are exposed in the central part of the Gotthard Nappe. They are distributed relatively regularly from the northern boundary of the nappe to its southern limit. Where possible, the original names of these units were kept, exchanging the old denominations like “serie” or “zone” with the term “gneiss complex”. Only two units have been renamed (see below).
HUBER (1943, Fig. 6, p. 93) was the first to recognise the tectonic importance of these units and called them the “ younger trough zones” to distinguish them from the structures in the polycyclic metamorphic basement. NIGGLI (1944, Tab.V, p. 128) classified all the pre-Mesozoic metasedimentary gneisses of the central Gotthard Nappe in a noteworthy way (incl. listing of localities, main rock type, guide rock, pre-Alpine metamorphic grade, relative age referred to the intrusion of the “Streifengneis” and the Variscan granitoids and finally the likely primary sedimentary facies of the protoliths). Only a selection of these data are summarised in Table 3.
In general, the sedimentary protoliths of the greenschist- to lower amphibolite-facies schists and gneisses of the Val Rondadura Group are evident already in the field. In fact, compositional banding at different scales (cm to mm) is the major characteristic of these rocks. Garnet-mica schist, hornblende-garnet-mica schist (frequent with the characteristic “hornblende-garben” structure), calcite-bearing mica schist, amphibolite, quartzite, sericite-biotite gneiss, phyllite and graphitic schist are the rock types alternating in the different gneiss complexes. NIGGLI (1944, p.128) indicated shale, sandy shale, dolomitic marl and quartz sandstone as the protoliths of the metasedimentary gneisses. HUBER (1943) and NIGGLI (1944) used tectonic and petrographic criteria to distinguish these metasedimentary gneisses from those of the polycyclic metamorphic basement. The older metasedimentary gneisses are intruded by Early Silurian granitoids (“Streifengneis” Complex), grading into migmatites and often bearing high-grade relics (granulite and eclogite), testifying to peak metamorphic grades much higher than those reached during Variscan and Alpine events. It is important also to note that the rocks of the Val Rondadura Group have suffered two metamorphic overprints. STEIGER (1962) suggested a pre-Alpine metamorphism of probable Variscan age, very similar in intensity as the second, Alpine overprint (lower amphibolite facies). These two metamorphic phases, and the intrusive contact of the late to post-Variscan Medel Granite and Cristallina Granodiorite with the Giubine, Tenelin and Laiets Gneiss Complexes, set a pre-Late Carboniferous age for the protoliths and the first metamorphic overprint of the rocks of the Val Rondadura Group (MERCOLLI et al.1994 and references therein). Furthermore, in spite of the lithological diversity, these two stages of metamorphic overprint indicate a link of the Val Rondadura Group to the Cavardiras Group of the Aar Massif. It must be kept in mind that all these arguments are based on singular local observations extrapolated to an entire group of rocks. It remains to be demonstrated whether all gneiss complexes forming the Val Rondadura Group have an identical evolution, i.e., pre- to Early Carboniferous sedimentation followed by Variscan deformation and metamorphism, and then the final Alpine overprint.