Sub-continental lithospheric mantle deformation in the Yerer-Tullu Wellel Volcanotectonic Lineament: A study of peridotite xenoliths

Abstract

Volumetrically, the lithospheric mantle comprises the bulk of the continental lithosphere, yet the mechanisms by which the lithospheric mantle is deformed during rifting are unresolved. Stretching and thermo-mechanical erosion are often cited mechanisms for facilitating lithospheric deformation during continental rift development; however, the infiltration of melt into the lithosphere during rift development also results in chemical alteration of the lithospheric mantle. The purpose of this study is to test the potential mechanisms by which the continental lithospheric mantle is chemically altered during rift development. Here we present a study of mantle xenoliths derived from the lithospheric mantle in Ethiopia, which has been deformed during rifting. The data collected suggest that the lithospheric mantle beneath this region exhibits evidence of focused magma-lithosphere interaction, resulting in four distinct types of peridotite xenoliths: a) deformed xenoliths representing the ancestral lithospheric mantle; b) granular xenoliths representing overprinted lithospheric mantle; c) replacement dunite xenoliths that are evidence of pervasive melt-lithosphere interaction; and d) cumulate xenoliths representing remnants of a metasomatic agent. The deformed xenoliths have a high Mg# (>89) and exhibit little, if any, interaction with melt. The remaining xenolith groups exhibit lower Mg# (<89), suggestive of magma-lithosphere interaction. The high Ni content in olivine and depleted incompatible elements concentrations in orthopyroxene of the granular xenoliths are inconsistent with simple metasomatic enrichment. Furthermore, the presence of dunite containing olivine with low Ca and Sc, and spinel with a Cr# of ~60 is suggestive of a replacement dunite. These data support a melt channeling model over a pervasive chromatographic metasomatism event. Given that the xenoliths examined in this this study are derived from a lithospheric shear zone, we conclude that melt channeling is likely facilitated by the focusing of melts along topography at the lithosphere-asthenosphere boundary, and shear-induced porosity.