A 19 to 17 Ma amagmatic extension event at the Mid-Atlantic Ridge: ultramafic mylonites from the Vema Lithospheric Section

A >300 km long lithospheric section (Vema Lithospheric Section or VLS) is exposed south of the Vematransform at 11N in the Atlantic. It is oriented along a seafloor spreading flow line and represents 26 Maof accretion at a single 80 km long segment (EMAR) of the Mid-Atlantic Ridge. The basal part of the VLSexposes a mantle unit made mostly of relatively undeformed coarse-grained/porphyroclastic peridotitesthat were sampled at close intervals. Strongly deformed mylonitic peridotites were found at 14 contiguoussites within a 80 km stretch (4.7 Ma interval); they are dominant in a time interval of 1.4 Ma, fromcrustal ages of 16.8 to 18.2 Ma (mylonitic stretch). Some of the mylonites are ‘‘dry,’’ showing anhydroushigh-T deformation, but most contain amphibole. The mylonitic peridotites tend to be less depleted thanthe porphyroclastic peridotites on the basis of mineral major and trace elements composition, suggestingthat the mylonites parent was a subridge mantle that underwent a relatively low degree of melting. The Sr,Nd, and O isotopic composition of the amphiboles is MORB-like and suggests either that seawater did notcontribute to their isotopic signature or that their isotopic ratios re-equilibrated during fluid circulation inthe upper mantle. Four 40Ar/39Ar ages, on three amphiboles separated from the peridotites, are close tocrustal ages predicted from magnetic anomalies, confirming that the amphiboles formed close to ridge axis.We propose that crustal accretion at the EMAR segment has been mostly symmetrical for the 26 Ma of itsrecorded history, except for the 1.4 Ma interval of prevalent ultramafic mylonites (mylonitic stretch) thatmay record a period of quasi-amagmatic asymmetric accretion of oceanic lithosphere close to the ridge–Vema transform intersection, possibly with development of detachment faults. This interval maycorrespond to a thermal minimum of the subridge upwelling mantle, marking the transition from a periodof decreasing to one of increasing mantle melting below the EMAR segment.