Martin, S., Godard, G. and Rebay, G. 2004.   Walking on a Palaeo Ocean Floor. The Subducted Tethys in the Western Alps - An Excursion Guide. In: (eds.) Beltrando, M., Lister, G., Ganne, J., and Boullier, A., Evolution of the western Alps: insights from metamorphism, structural geology, tectonics and geochronology, Journal of the Virtual Explorer, Electronic Edition, ISSN 1441-8142, Volume 16, Paper 2.

Walking on a Palaeo Ocean Floor. The Subducted Tethys in the Western Alps - An Excursion Guide

Silvana Martin

Dipartimento di Scienze chimiche ambientali, Università dell’Insubria, Via Lucini, 3, 22100 Como, Italy.

Gaston Godard

Laboratoire des fluides géologiques, case 89, Institut de Physique du Globe de Paris, 4, Place Jussieu, 75252 Paris, France.

Gisella Rebay

Dipartimento di Scienze della Terra, Università di Pavia, via Ferrata, 1, 27100 Pavia, Italy.


This excursion in the Italian Western Alps presents an overview of the eclogitized ophiolite of the Zermatt-Saas unit, focusing on the relationships between various alpine units and on metamorphic features of different lithologies (serpentinite, metagabbro and metabasalt, Mn-rich quartzite, hydrothermal sulphide deposits and metasediments). These rocks underwent high- to ultrahigh-pressure metamorphism during subduction of the Tethys ocean, and developed peculiar high-pressure peak assemblages according to their composition.

First, in order to acquire a general view of the main Alpines and their lithologies, we cross the Tethys ocean suture (the Zermatt-Saas and Combin units of the ophiolitic Piedmont Zone) from the European passive continental margin (now represented by the Monte Rosa unit) to the Adria (Africa) continental margin (Sesia-Lanzo and Dent Blanche nappes). Then, we visit some of the most typical outcrops of Alpine geology in the NW Alps: (i) the Saint-Marcel Fe-Cu hydrothermal sulphide deposits; (ii) the famous Praborna Mn mine with its unique high-pressure minerals; (iii) the Cignana coesite site and the Crepin metagabbro (Valtournanche). Attention is especially focussed on the eclogitized ophiolite and hydrothermal oceanic deposits. In addition, historical aspects, Alpine views and regional geology are taken in consideration. The field trip is concluded with the breathtaking view from Plateau Rosa, south of Cervino (Matterhorn), where it is possible to summarise the regional relationships of the collisional nappe stack and the ophiolitic suture (remnants of the “lost Tethyan ocean”), and to have the opportunity to enjoy some last eclogite outcrops.

Keywords: eclogite facies, ophiolite, western Alps, hydrothermal ore deposits

Table of Contents

First itinerary (Gressoney valley):
Second itinerary (Saint-Marcel valley):
Third itinerary (Valtournanche):
Regional geology
The European continental crust (Monte Rosa unit)
The Zermatt-Saas meta-ophiolite unit
The intermediate continental slices
The Combin meta-ophiolite unit
The Adria continental crust (Sesia-Lanzo and Dent Blanche nappes)
Field Itineraries
Itinerary 1
STOP 1.1 – Eclogitized migmatites, at Rifugio Città di Mantova (alt. 2498 m)
STOP 1.2 – Monte Rosa basement, at Punta Indren (N 45° 53.01’; E 7° 51.89’; alt. 3109 m)
STOP 1.3 – Panoramic view, near Stolemberg (N 45° 52.934; E 7° 51.874)
STOP 1.4 – Tectonic contact between Monte Rosa gneiss and Zermatt-Saas ophiolite, at the klippe of Punta Stolemberg (N 45° 52.762’; E 7° 51.936’; alt. 3068 m)
STOP 1.5 – Contact between Monte Rosa gneiss and Zermatt-Saas ophiolite, at Corno Camoscio (N 45° 52.481’; E 7° 52.136’; alt. 2952 m)
STOP 1.6 – Monte Rosa folded garnet micaschists and gneiss of Istituto Mosso (N 45° 52.469’; E 7° 52.308’; alt. 2902 m)
STOP 1.7 – Metabasic dyke in serpentinite, north of Rifugio Città di Vigevano (N 45° 52.321’; E 7° 52.371’; alt. 2861 m)
STOP 1.8 – Contact between Zermatt-Saas and Combin meta-ophiolites, at Col d’Olen (N 45° 52.229’; E 7° 52.129’; alt. 2885 m)
STOP 1.9 – Zermatt-Saas and Combin meta-ophiolites, on the pathway towards Gabiet (e.g., N 45° 52.25’; E 7° 51.76’; alt. 2800 m)
STOP 1.10 – Panoramic view, on the upper contact between the meta-ophiolite and the Sesia-Lanzo zone, at Lago di Gabiet (alt. 2367 m)
STOP 1.11 – Sesia-Lanzo unit (i.e., Adria microplate), at Lillianes (N 45° 37.985’; E 7° 50.634’; alt. 646 m).
Itinerary 2 - Eclogitized hydrothermal deposits in ophiolites: the Servette and Praborna mines
STOP 2.1 – Panoramic view, on the road between Druges Basses and Druges Hautes (N 45° 42.469’; E 7° 28.608’; 1585 m).
STOP 2.2 – Old foundry of Treves (N 45° 42.444’; E 7° 27.432’; alt. 1672 m)
STOP 2.3 – Slag cumulus (N 45° 42.449’, E 7° 27.304’, alt. 1659 m)
STOP 2.4 – High-pressure rocks, at Fontillon (N 45° 42.111’; E 7° 27.327’, alt. 1819 m)
STOPS 2.5 – Eclogitized oceanic hydrothermal ore of the Servette mine
STOP 2.6 – Praborna Mn mine (N 45° 40.774’; E 7° 26.968’, alt. 1894 m).
STOP 2.7 – abandoned mine village of Chuc (N 45° 42.044’; E 7° 26.863’; 1422 m)
STOP 2.8 – Green fountain” of Acqua verde (N 45° 42.094’; E 7° 26.940’; 1371 m)
Itinerary 3 - Regional structures and (ultra)high-pressure metamorphism in the Combin and Zermatt-Saas ophiolites of Valtournanche
STOP 3.1 – Panoramic views from Gillaray (alt. 2186 m.)
STOP 3.2 – Panorama of the Cignana valley, from Rifugio Barmasse (N 45° 52,590’; N 7° 35,385’; alt. 2182 m).
STOPS 3.3 – Ultrahigh-pressure meta-ophiolite of the Cignana lake
STOP 3.4 – Zermatt-Saas meta-ophiolite, below Cignana dam (e.g., N 45° 52.509’; E 7° 36.530’; alt. 1891 m)
STOP 3.5 – Rodingitized gabbro dykes in serpentinite (N 45° 52.878’; E 7° 36,954’; alt. 1804 m.)
STOP 3.6 – Serpentinite lens within metagabbro, along the path to Liortere (N 45° 53.144’; E 7° 36,893’; 1826 m.)
STOP 3.7 – Crepin eclogite-facies gabbro and troctolite, (e.g., N 45° 52.619’; E 7° 37.095’; alt. 1630 m)
STOP 3.8 – Panoramic views from Plateau Rosà (3480 m, Testa Grigia-Plateau Rosà)