Ductile deformation of gabbroic rocks drilled on the west wall of the Mid-Atlantic Ridge south of the Kane Fracture Zone (Sites 921-923) localizes preferentially in gabbronoritic intervals. The least evolved gabbroic lithologies (troctolites) are rarely deformed, and the most evolved ones (leucocratic segregations) are not deformed. A close look at ductile shear zones and at their mineral chemistry suggests that, in many cases, they developed in gabbronoritic dikelets intruded into less evolved lithologies. Recrystallized phase assemblages are similar to igneous ones in most deformed samples. This indicates that ductile deformation occurred in high-temperature nearly anhydrous conditions similar to magmatic conditions. Compositional variations in igneous and recrystallized minerals in many shear zones also suggest that ductile deformation took place with some residual melt present. The presence of interstitial melt in shear zones during deformation would produce a strong rheological contrast between material in the shear zone and the surrounding rocks. The range of gabbroic lithologies drilled at Sites 921-923, their relative proportions in the core, the composition of igneous minerals, the preferred localization of ductile deformation in gabbronoritic lithologies, and the probable role of trapped fractionated melts in favoring this strain localization recall observations made in gabbroic rocks from Site 735 in the Southwest Indian Ocean.

Date of initial receipt: 7 August 1995
Date of acceptance: 26 January 1996

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