Hydration of an active shear zone: interactions between deformation, metasomatism and magmatism—the spinel-lherzolites from the Montferrier (southern France) Oligocene basalts

Abstract

Geochemical and textural investigations have been simultaneously performed on spinel-lherzolite xenoliths from the Oligo-Miocene alkali basalts of Montferrier (southern France). All the investigated samples have undergone a deformation very particular by intense shearing under high stresses (up to 1.75 kbar), low temperatures (⩽ 900°C) and strain rates of about 10 −18 to 10 −15 s −1. Mineral chemistry reveals that the Montferrier lherzolites are fragments of an undepleted relatively shallow upper mantle level located at a depth of 50 km (15 kbar). Moreover, Na and Ti enrichment in diopside would reflect a metasomatic event, also emphasized by the common occurrence of pargasite in 50–70% of the investigated samples. Crystallization of this amphibole is attributed to a hydrous infiltration which is related in time and space to the deformation. Indeed, amphibole is preferentially concentrated in strongly deformed zones and in kink-band boundaries of orthopyroxene porphyroclasts. Moreover, the grain boundaries were used by the pervasive agent to percolate into the lherzolite: significant chemical variations (increase in MgO: 15% and decrease in Al 2O 3: 55%) are observed within the range of 7–5 μm adjacent to the grain boundary. Finally, Sr isotopic data ( 87Sr/ 86Sr) demonstrate that the amphibole, i.e. the metasomatic agent, is genetically related to the host lava of the xenoliths. Thus, the hydrous silicate liquid from which the amphibole has crystallized may be an early percolation of the ascending alkali magma. This silicate liquid hydrated the shear zone, located at a depth of 50 km, induced the hydraulic fracturation of the lherzolite and the magmatic conduit opening. Subsequently, the alkali magma sampled some fragments of this strongly deformed and metasomatized undepleted upper mantle level and brought them to the surface.