Transform tectonics, metamorphic plagioclase and amphibolitization in ultramafic rocks of the Vema transform fault (Atlantic Ocean)

Based on microstructures and mineral chemistry, we show that ultramafic rocks sampled in the south wall of the Vema transform have recorded a suite of deformation events that had approximately the same geometry but occurred successively in spinel, plagioclase and amphibolite facies conditions. This tectonic and metamorphic evolution is inferred to have taken place at depth near the eastern intersection of the Vema transform with the Mid-Atlantic Ridge. We discuss the origin of plagioclase-bearing ultramafic mylonites and conclude that they resulted from dynamic recrystallization of moderately depleted spinel peridotites in the lower pressure plagioclase stability field. Low degrees of partial melting in these samples were likely due to the cold thermal regime of the transform region. Strain softening that accompanied recrystallization of plagioclase may be a common effect in the upper mantle below large-offset transforms, favouring strain localization and the initiation of ductile shear zones. Finally, amphibolite facies recrystallization followed the introduction into the ultramafic rocks of a metasomatic hydrous fluid enriched in titanium, sodium and calcium. We propose that this fluid was hydrothermal in origin, and had been chemically modified by interactions with crustal rocks, prior to its circulation in the ultramafics.