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

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