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

Abstract 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