Composition and thermal structure of the lithosphere beneath the Ethiopian plateau: evidence from mantle xenoliths in basanites, Injibara, Lake Tana Province

Abstract

Petrographic, minerochemical, and geothermobarometric data are reported for a suite of spinel-lherzolite pargasite xenoliths hosted in a Quaternary basanitic lava flow from the North-Western Ethiopian Plateau (Injibara, Lake Tana Province). Protogranular to porphyroclastic (deformed) rocks show evidence of a modal metasomatism, represented by a Cl-rich pargasitic amphibole, coupled with cryptic enrichment in Fe and Al. Equigranular rocks (granular) record a further cryptic metasomatism, represented by enrichment in Fe, Al, Na, and depletion in Ni, Cr and Cl. Some xenoliths (transitional) show intermediate textural and compositional characters, indicating that the granular samples represent an evolution of the deformed ones. All xenoliths give the same P–T equilibration conditions for Opx-Cpx pairs (947–1015 C and 1.3–2.0 GPa), but in granular samples, recrystallised olivine and spinel record T about 100 C higher. Two distinct metasomatic processes, probably connected with the emplacement of the Afar plume, are proposed. The first one is a pervasive modal metasomatism produced by water-rich fluids. The latter is a non-pervasive cryptic metasomatism, probably connected to migration of melts. The comparison the mantle beneath the Ethiopian Volcanic Plateau, the southern Main Ethiopian Rift and the central Main Ethiopian Rift suggests spatial heterogeneity of the mantle and variable mantle processes during asthenospheric upwelling.