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Título

MICROSTRUCTURAL CHARACTERIZATION OF MYLONITIC GABBROS FROM THE UPPER SECTION OF HOLE U1473A, SOUTHWEST INDIAN RIDGE (IODP EXPEDITION 360)

Texto do resumo

The Atlantis Bank is a 300 km² Oceanic Core Complex (OCC) located in the Southwest Indian Ridge,
formed between 10 and 13 Ma. This OCC was the target of the International Ocean Discovery Program
(IODP) Expedition 360, in which approximately 789 meters of in situ oceanic crust was recovered through
hole U1473A, exposing a thick sequence of intensely deformed gabbroic rocks. This work focuses on the
microstructural characterization and mineral chemistry of mylonitic gabbros samples recovered from the
upper portion (the first 300 meters below ocean floor) of the core, in order to investigate the relationships
between deformation mechanisms and fluid flow. The upper portion of hole U1473A consists essentially of
protomylonitic to mylonitic gabbros, with major plagioclase, clinopyroxene, orthopyroxene and olivine;
amphibole occurs as a secondary phase. The protomylonitic domain is characterized by coarse grained
gabbro with clinopyroxene, orthopyroxene, plagioclase and, to a lesser extent, fractured olivine
porphyroclasts that are locally mantled by neoblasts of the same mineral phase, in addition to a
monomineralic matrix of plagioclase. The mylonitic domain is characterized by fine to medium grained
mylonitic gabbro with clinopyroxene and plagioclase porphyroclasts within a fine grained polyphasic matrix
composed of plagioclase, amphibole, chlorite, sericite, epidote, and Fe-Ti oxides, intercalated with a fine
monomineralic layer of plagioclase. Kink bands in plagioclase porphyroclasts, rotated grains within the
mylonitic foliation, secondary amphibole growth, fractures that crosscut clinopyroxene porphyroclasts and
fractures filled by low-temperature minerals suggest that shear zones acted as channels for fluid migration.
Transformation of primary mafic minerals into deformed weaker hydrated phases that are mixed into fine-
grained aggregates suggest that fluid-rock interaction played a crucial role in the deformation history
recorded along the Atlantis Bank detachment fault system, enhancing reaction-softening mechanisms, and
localizing deformation in zones with high fluid influx.

Palavras Chave

Atlantis Bank; mylonitic gabbros; microstructural characterization; oceanic core complex.