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

Abstract:

Although a great deal is known about crustal extensional processes in the East African Rift System (EARS), questions remain as to the impact of extension on the continental lithospheric mantle. The northernmost portion of the EARS is the Main Ethiopian Rift (MER), which is divided into three sectors: the Northern, (NMER), Central (CMER), and the Southern Main Ethiopian Rift (SMER). The NMER-CMER transition coincides with the Yerer-Tullu Wellel volcanotectonic lineament (YTVL), and may represent a terrain boundary along which extension was directed during the Cenozoic. Here we present petrographic data from peridotite xenoliths contained within ~6 Ma lavas recovered along the western portion of the YTVL near Nekempte, Ethiopia to examine the characteristics of the lithospheric mantle, and how peridotite has responded to extensional strain. Our preliminary results show that the Nekempte xenoliths are deformed spinel-bearing lherzolites, which previous studies have constrained to the spinel lherzolites field at 1.1 GPa in the lithospheric mantle. Petrographic examination of these xenoliths reveals two generations of crystals. These generations are defined by both size and texture; the first generation (coarse crystals) being up to 4 mm in diameter and the second generation (fine crystals) being less than 1 mm in diameter. The samples contain two generations of olivine (Ol I and Ol II) and orthopyroxene (Opx I and Opx II), one generation of clinopyroxene (Cpx I), spinel (Spl I), amphibole (Amp), and many fluid inclusion trails. We compare these xenoliths to samples from Injibara, Lake Tana Province, approximately 200 km north of Nekempte on the Ethiopian Plateau, a region with less focused extensional strain. Xenoliths from Injibara are also spinel-bearing lherzolites, however they exhibit less strain and metasomatic textures in comparison to xenoliths from along the YTVL. Based on the petrographic and textural analyses performed, we propose that the larger scale recrystallization of mineral phases and metasomatic textures seen in the Nekempte xenoliths are a result of more focused extensional strain in the continental lithospheric mantle.