Ocean‐floor hydrothermal metamorphism in the Limousin ophiolites (western French Massif Central): evidence of a rare preserved Variscan oceanic marker

Abstract

The Limousin ophiolite is located at the suture zone between two major thrust sheets in the western French Massif Central. This ophiolitic section comprises mantle-harzburgite, mantle-dunite, wehrlites, troctolites and layered gabbros. It has recorded a static metamorphic event transforming the gabbros into undeformed amphibolites and the magmatic ultramafites into serpentinites and/or pargasite-bearing chloritites. With various thermobarometric methods, it is possible to show that the different varieties of amphibole have registered low-P (c. 0.2 GPa) conditions with temperature ranging from high-T, latemagmatic conditions to greenschist-zeolite metamorphic facies. The abundance of undeformed metamorphic rocks (which is typical of the lower oceanic crust), the occurrence of Ca-Al (-Mg) metasomatism illustrated by the growth of Ca-Al silicates in veins or replacing the primary magmatic minerals, the P-T conditions of the metamorphism and the numerous similarities with oceanic crustal rocks from Ocean Drilling Program and worldwide ophiolites are the main arguments for an ocean-floor hydrothermal metamorphism in the vicinity of a palaeo-ridge. Among the West-European Variscan ophiolites, the Limousin ophiolites constitute an extremely rare occurrence that has not been involved in any HP (subduction-related) or MP (orogenic) metamorphism as observed in other ophiolite occurrences (i.e. France, Spain and Germany).

The birth of the Rheic Ocean — Early Palaeozoic subsidence patterns and subsequent tectonic plate scenarios

“…http://doc.rero.ch the Massif Central domain, (Limousin area, Pin and Marini, 1993;Briand et al,1995;Berger et al, 2005). The Upper Cambrian oceanisation of some marginal basins in a context of back-arc rifting, scattered through the Iberian Allochthonous (Arenas et al, 2007a,b) and the formerly nearby areas of rifting like the Ossa Morena Zone (Sánchez García et al, 2003;Quesada, 2006) could equally represent the westward continuation of the Chamrousse Zone, if a more cylindrical model is applied for the Gondwana margin.…”

Amphibolitization within the lower crust in the termination area of the Godzilla Megamullion, an oceanic core complex in the Parece Vela Basin

“…2005). One conclusion of these investigations is that an ocean‐floor hydrothermal metamorphism is ubiquitous in the vicinity of a paleo‐ridge (Berger et al . 2005).…”

“…Mével et al 1978;Girardeau & Mével, 1982;Berger et al 2005). One conclusion of these investigations is that an ocean-floor hydrothermal metamorphism is ubiquitous in the vicinity of a paleo-ridge (Berger et al 2005). In the Godzilla Megamullion, Harigane et al (2008) showed that hydrothermal alteration resulted in retrograde metamorphism associated with deformation on the Fe-Ti oxide gabbroic body in the West Leg Ridge (Fig.…”

“…Stakes & Vanko 1986;Mével 1987;Manning & MacLeod 1996;Harigane et al 2008) and in ophiolites (e.g. Mével et al 1978;Girardeau & Mével, 1982;Berger et al 2005). One conclusion of these investigations is that an ocean-floor hydrothermal metamorphism is ubiquitous in the vicinity of a paleo-ridge (Berger et al 2005).…”

Discovery of metamorphic microdiamonds from the parautochthonous units of the Variscan French Massif Central

“…Indeed, although controversial (Berger et al, 2005), the nappe stacking model is still the prevailing one (Faure et al, 2009). …”