Fossil oceanic core complexes in the Alps. New field, geochemical and isotopic constraints from the Tethyan Aiguilles Rouges Ophiolite (Val d\u2019H\xe9rens, Western Alps, Switzerland)

Thierry Decrausaz,Paola Manzotti,Othmar Müntener,Carl Spandler,Romain Lafay

doi:10.1186/s00015-020-00380-4

Abstract

Exhumation of basement rocks on the seafloor is a worldwide feature along passive continental margins and (ultra-) slow-spreading environments, documented by dredging, drilling or direct observations by diving expeditions. Complementary observations from exhumed ophiolites in the Alps allow for a better understanding of the underlying processes. The Aiguilles Rouges ophiolitic units (Val d’Hérens, Switzerland) are composed of kilometre-scale remnants of laterally segmented oceanic lithosphere only weakly affected by Alpine metamorphism (greenschist facies, Raman thermometry on graphite: 370–380 °C) and deformation. Geometries and basement-cover sequences comparable to the ones recognized in actual (ultra-) slow-spreading environments were observed, involving exhumed serpentinized and carbonatized peridotites, gabbros, pillow basalts and tectono-sedimentary cover rocks. One remarkable feature is the presence of a kilometric gabbroic complex displaying preserved magmatic minerals, textures and crosscutting relationships between the host gabbro and intruding diabase, hornblende-bearing dikelets or plagiogranite. The bulk major and trace element chemistry of mafic rocks is typical of N-MORB magmatism (CeN/YbN: 0.42–1.15). This is supported by in-situ isotopic signatures of magmatic zircons (εHf = + 13 ± 0.6) and apatites (εNd = + 8.5 ± 0.8), determined for gabbros and plagiogranites. In-situ U–Pb dating was performed on zircons by laser ablation-ICP-MS, providing ages of 154.9 ± 2.6 Ma and 155.5 ± 2.8 Ma, which are among the youngest for oceanic gabbros in the Alps. Our study suggests that the former Aiguilles Rouges domain was characterized by tectonism and magmatism resembling present-day (ultra-) slow-spreading seafloor. It also suggests that the Tethyan lithosphere is laterally segmented, with punctuated magmatism such as the Aiguilles Rouges gabbros and carbonated ultramafic seafloor covered by basalts and Jurassic tectono-sedimentary deposits.

Highlights

Over the last decades, various types of spreading systems (Dick et al 2003) have been identified worldwide, highlighting diverse modes for the generation of oceanicIt has long been recognized that Alpine ophiolites are different from what has later become the Penrose type ophiolites (e.g. Decandia and Elter 1969; Bonatti 1971)
An important extension of our understanding of ophiolites formed in slow-spreading systems within orogens is that some ophiolitic units were identified as representing ancient ocean-continent transition zones (OCT) preserved in the Eastern Central Alps and Northern Apennines (Florineth and Froitzheim 1994; Molli 1996; Müntener and Hermann 1996; Manatschal and Nievergelt 1997; Epin et al 2019)
Primary magmatic texture and mineralogy are preserved in the kilometre-scale unstrained gabbro exposed in the Aiguilles Rouges Ophiolite

Summary

Introduction

Various types of spreading systems (Dick et al 2003) have been identified worldwide, highlighting diverse modes for the generation of oceanicIt has long been recognized that Alpine ophiolites are different from what has later become the Penrose type ophiolites (e.g. Decandia and Elter 1969; Bonatti 1971). An important extension of our understanding of ophiolites formed in (ultra-) slow-spreading systems within orogens is that some ophiolitic units were identified as representing ancient ocean-continent transition zones (OCT) preserved in the Eastern Central Alps and Northern Apennines (Florineth and Froitzheim 1994; Molli 1996; Müntener and Hermann 1996; Manatschal and Nievergelt 1997; Epin et al 2019). Lombardo et al 2002; Tribuzio et al 2016), regardless of later Alpine metamorphic overprint. In most places, they intrude as dikes into mantle rocks and, locally, they form 10 to 100’s of metre-sized bodies with internal intrusive relationships Kilometre-sized gabbroic bodies in Alpine ophiolites are not common and are mostly overprinted by high-pressure metamorphism (e.g. Lagabrielle et al 2015; Festa et al 2015); their magmatic construction is poorly constrained

Methods

Discussion

Conclusion