Microstructural evolution on oxide-rich gabbros from the Atlantis Bank oceanic core complex, Southwest Indian Ridge (ODP Hole 735B)

The Atlantis bank is an oceanic core complex that was exhumed by a large-scale detachment fault on the Southwest Indian Ridge. Deformation in these rocks is localized along hypersolidous and HT shear zones that evolve to low-temperature shear zones, being finallyoverprinted by brittle faults of different scales. Here we present the results of a detailed microstructural and crystallographic preferred orientation study in five samples from the oxide-rich gabbros that occur along the core. These gabbros occur with higher frequency toward the top of the section and are interpreted to have intruded the more primitive gabbros at a later stage. Composition of the studied samples varies considerably from sample to sample, between clinopyroxene, amphibole and plagioclase-rich gabbros, but the content of oxides (mostly magnetite and ilmenite) is >5%. All the studied samples have well-developed foliation, but lineation is not well marked. In general, clinopyroxene (and later amphibole) occur as porphyroclasts in the plagioclase bearing gabbros, but as discontinuous layers in the cpx/amphibolte-rich terms. Plagioclase occur preferentially as equigranular aggregates, whereas ilmenite, and at last extend magnetite, occur along bands parallel or oblique to the foliation. Ilmenite normally occur as interstitial grains. CPOs determined via EBSD shows a weak but always present CPO for clinopyroxene, plagioclase and ilmenite. In most of the samples the clinopyroxene CPO is characterized by the alignment of poles to (100) subparallel or oblique to the pole of foliation (Z), whereas the poles of (010) or (001) are parallel to X, depending on the sample. Plagioclase CPO is characterized by the alignment of poles to (010) parallel to Z and poles to (001) close to X. Ilmenite (0001) poles are systematically aligned parallel to Z and <11-20> parallel to X. Magnetite CPO is rather weak and may either show <100> or <111> parallel to the pole of foliation. All phases show systematic peaks at low misorientation angles, in agreement with the observation of undulose extinction and subgrain boundaries. Microstructures and CPOs suggest the oxide-rich gabbros were deformed under the presence of melt (possibly with rock-melt reactions) progressively evolving to continuous HT-LT solid state deformation.