Geochemistry and petrology to understand the evolution of the Stratoid and younger volcanism in Afar

In rift settings, magmatism and tectonics play a key role during continental breakup leading to oceanic spreading. A control by mantle plumes is commonly invoked for the flood basalt volcanism taking place during rifting. However, given the possible mixing of melts derived from different mantle reservoirs and the spatial and temporal variability of the magmatic activity in rift setting, depicting the nature and variation of the magmatism in the rifting cycle is challenging. Furthermore, the transition from widespread continental flood basalt to localized rift volcanism during rift migration and localization is still a matter of debate. The Afar depression (Ethiopia) is a rift-rift-rift triple junction located at the intersection of the Red Sea, Gulf of Aden and East African Rift System (EARS), where the continent-ocean transition is currently represented by the ongoing activity of the magmatic segments. It therefore represents an excellent place to study magmatism and its relation with rift evolution from continental breakup to oceanic spreading. In my thesis I investigated the petrology of the Stratoid flood basalt and younger volcanism (4.5-0.6 Ma; i.e., the activity preceding the current magmatic segments) associated with the Red Sea branch in Afar, by means of field observations and petrographic, geochemical, isotopical and microanalytical studies. In particular I studied the characteristics of the mantle source and the reservoir/s involved during the partial melting to identify the relation between magmatic activity and the continent-ocean transition in Southern and Central Afar. I then studied the evolution of the magmatic system during the continental breakup in Central Afar. Lastly, I studied the magmatic activity close to the Afar boarded faults to investigate the development of rift segments at the margin of the depression and their relation with the silicic central volcanoes. The systematic geochemical study carried out in this work reveals for the first time the variable petrological nature of the Stratoid Series. This work indicates that the temporal distinctions between Lower and Upper Stratoid Series (Kidane et al., 2003) is related to changes in the conditions of primitive magma genesis and also points out inner variability within the Series. Differences in garnet-compatible elements indicate a shallower melting column for the oldest and youngest products (4.5-2.6 Ma Lower Stratoid Series; 1.1-0.6 Ma Gulf Series) with respect to a deeper melting column for the products erupted at 2.6-1.1 Ma (Upper Stratoid Series). Accordingly, the isotopic signature indicates a more depleted mantle source and a major involvement of the lithospheric component for the Lower Stratoid and Gulf Series with respect to a more plume-like mantle source for the Upper Stratoid. The geochemical study is integrated with independent geophysical and stratigraphic evidence, explaining the results with rift relocalization and focusing: the Pliocene rift phase of localized extension in Southern Afar leads to melting of the shallow mantle reservoirs and, consequently, to the observed shallow melting column for the Lower Stratoid. At 2.6 Ma the Pleistocene rift jumped to Central Afar under a less-extended lithosphere leading to a major involvement of the deep Afar plume for the Upper Stratoid. Subsequently, the stretching of the lithosphere and rift progressively focusing in Central Afar along the Tendaho graben led to the shallowing of the melting column and to the major involvement of the shallower mantle reservoirs as observed for the Afar Gulf Series. In Central Afar the microanalytical study revealed a lower crustal, moderately zoned, melt-rich magmatic system for the Upper Stratoid with magma rising mostly directly to the surface, similar to the models of flood basalt magmatic systems. This phase was followed by the formation of several shallow silicic magma chambers of the central volcanoes and, subsequently, by the development of a lower crustal, polybaric, interconnected, crystal-rich magmatic system for the Afar Gulf. I interpreted these variations as related to an increase of the intruded material during the Afar Gulf activity and to the development of crystal mushes in the crust during rift focalization with respect to the Upper Stratoid.