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

Petrographic, minerochemical, and geothermobarometric data are reported for a suite of 19 spinel-lherzolite pargasite xenoliths hosted in a Quaternary basanitic lava flow from 20 the North-Western Ethiopian Plateau (Injibara, Lake Tana Province). Protogranular to 21 porphyroclastic (deformed) rocks show evidence of a modal metasomatism, represented 22 by a Cl-rich pargasitic amphibole, coupled with criptic enrichment in Fe and Al. 23 Equigranular rocks (granular) record a further cryptic metasomatism, represented by 24 enrichment in Fe, Al, Na, and depletion in Ni, Cr and Cl. Some xenoliths (transitional) 25 show intermediate textural and compositional characters, indicating that the granular 26 samples represent an evolution of the deformed ones. All xenoliths give the same P–T 27 equilibration conditions for Opx-Cpx pairs (947–1015 C and 1.3–2 GPa), but in 28 granular samples, recrystallised olivine and spinel record T about 100 C higher. Two 29 distinct metasomatic processes, probably connected with the emplacement of the Afar plume, are proposed. The first one is a pervasive modal metasomatism produced by 2 water-rich fluids. The latter is a non-pervasive cryptic metasomatism, probably con- 3 nected to migration of melts. The comparison among the mantle beneath the Ethiopi- 4 an Volcanic Plateau, the southern Main Ethiopian Rift and the central Main Ethiopian 5 Rift suggests a spatial heterogeneity in mantle processes, during asthenospheric 6 upwelling