PETROLOGY AND P-T-T EVOLUTION OF THE SOUTH WEST-INDIAN RIDGE PERIDOTITES - A CASE-STUDY - EAST OF THE MELVILLE FRACTURE-ZONE AT 62-DEGREES-E

Mineral and bulk rock chemistry of serpentinites recovered from the Southwest Indian Ridge (SWIR) at 62-degrees-E, east of the Melville Fracture Zone, are consistent with an origin as residual harzburgite products of low degrees of partial melting of a parental peridotite suitable as a source of MORB's. The peridotites originated in the upper mantle within the spinel-lherzolite stability field. The lack of equilibrium in the plagioclase-peridotite stability field suggests a relatively slow ascent, without the introduction of exotic melts or mixing between residue and melt fraction. The harzburgites ascended after extraction of the basaltic melts. The evolution of the harzburgites is characterized by four stages: decompression, rising and emplacement of the residue in the upper mantle-lower crust, syn-tectonic deformation and post-tectonic recrystallization. During ascent of the harzburgites into the upper mantle-lower crust area, tensional stresses and transform activity prevailed east of the Melville Fracture Zone. The studied samples suggest pressure and temperature conditions at the beginning of deformation-mylonitization to have been less than 800-degres-C and 7 kb. Tectonic activity persisted during cooling through the low amphibolite facies until nearly 300-degrees-C and 2 kb, which corresponds to depths close to the uppermost mantle-lower crust depths. Comparisons of the 62-degrees-E peridotites with those from the Melville and Atlantis Fracture zones imply that two large and different provinces of the SWIR are characterized by mixing between mantle sources to the south and by a homogeneous more fertile source to the north. The transform faults east of the Melville fracture zone have played a role in the mechanical increase of mantle ascent rates in the absence of a hot spot signature.