M. Cannat et M. Seyler, TRANSFORM TECTONICS, METAMORPHIC PLAGIOCLASE AND AMPHIBOLITIZATION INULTRAMAFIC ROCKS OF THE VEMA TRANSFORM-FAULT (ATLANTIC-OCEAN), Earth and planetary science letters, 133(3-4), 1995, pp. 283-298
Based on microstructures and mineral chemistry, we show that ultramafi
c 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 faci
es conditions. This tectonic and metamorphic evolution is inferred to
have taken place at depth near the eastern intersection of the Vema tr
ansform with the Mid-Atlantic Ridge. We discuss the origin of plagiocl
ase-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 partia
l melting in these samples were likely due to the cold thermal regime
of the transform region. Strain softening that accompanied recrystalli
zation of plagioclase may be a common effect in the upper mantle below
large-offset transforms, favouring strain localization and the initia
tion of ductile shear zones. Finally, amphibolite facies recrystalliza
tion followed the introduction into the ultramafic rocks of a metasoma
tic hydrous fluid enriched in titanium, sodium and calcium. We propose
that this fluid was hydrothermal in origin, and had been chemically m
odified by interactions with crustal rocks, prior to its circulation i
n the ultramafics.