Thinning of Refertilized Sub-Continental Lithospheric Mantle (SLCM) beneath the Main Ethiopian Rift During Tertiary Rifting: Petrologic and Thermal Constraints from (Garnet)-Spinel Peridotite Xenoliths (Mega, Ethiopia).

Friday, 19 December 2014
Alessio Casagli, University of Siena, Siena, Italy, Maria Luce Frezzotti, University Milan Bicocca, Department of Earth and Environmental Sciences, Milan, Italy, Angelo Peccerillo, University of Perugia, Perugia, Italy, Massimo Tiepolo, CNR - IGG - Pavia, Pavia, Italy and Gianfilippo De Astis, National Institute of Geophysics and Volcanology, Rome, Italy
The East African Rift System (EARS) represents a key locality for the knowledge of the nature and evolution of SCLM during continental rifting processes, traditionally ascribed to ascending mantle plumes. We report petrological and geothermobarometric data from mantle xenoliths in Quaternary alkali-basalt lava flows and scoria cones at Mega (Sidamo Region; EARS) in the southern Main Ethiopian Rift (MER), that give evidence for refertilization of SCLM and for thinning during Tertiary rifting. Studied samples consist of seven lherzolites, five harzburgites and one olivine-websterite that contain spinel-pyroxene symplectites, interpreted as products of garnet breakdown reactions. These rocks were analyzed for major (whole rock and minerals) and trace elements (pyroxenes). Major element data have been used to reconstruct original garnet composition (pyrope). Equilibration temperatures range from 985 ± 40°C in the garnet facies (2.9-2.2 GPa) to 960 ± 55°C in the spinel facies (1.3 GPa). Xenoliths consist of depleted and fertile peridotites. Five lherzolites have up to 4 wt% of CaO, high CaO/Al2O3 (1.42-4.46), and the most fertile are more enriched than primitive mantle. Variations of major oxides in bulk rocks and minerals are consistent with variable degrees of melt extraction. Evidence for modal and cryptic metasomatism is given by addition of clinopyroxene ± phlogopite, and by LILE and LREE enrichment in clinopyroxene. Refertilization process appears to have been induced by sub-lithospheric volatile-rich melts at high melt/rock ratio, and were followed by cooling. To account for the geodynamic evolution of SCLM beneath the southern MER, which implies a temperature gradient from 50-60 to ∼ 90 mW/m2, we propose that thinning of the base of fertile SCLM from 90-95 to ∼45km depth and associated magmatism occurred along a normal-mantle adiabat above an upwelling asthenosphere (i.e., decompression melting) without the need for significant heat sources.