The Red Sea rift system represents a key case study of the transition from a continental to an oceanic rift. The Red Sea rifting initiated in Late Oligocene to Early Miocene (24-23 Ma) and was accompanied by extensive magmatism throughout the rifted basin, from Afar and Yemen to northern Egypt. Here, we present a petrological and geochemical study of two gabbro bodies and associated basalts from the Tihama Asir igneous complex, which formed at 24-20 Ma within the rifted Arabian-Nubian Shield (ANS). The Tihama Asir is therefore an ideal location to study the initial phase of syn-rift magmatism and its influence on the geodynamic evolution of the Red Sea rift system. The most primitive olivine gabbros present modal, bulk and mineral compositions consistent with formation from Mid-Ocean Ridge Basalt (MORB)-type parental melts, whereas the evolved olivine-free gabbros and oxide-bearing gabbros show saturation of phlogopite and a crystal line of descent diverging from fractional crystallization trends. In detail, whole-rock and mineral compositions in the most evolved lithologies show high Light over Middle Rare Earth Elements (LREE/MREE) ratios (La-N/Sm-N = 0.89-1.31) and selective enrichments in Sr, K and highly incompatible elements (Rb, Ba, U, Th). We relate these geochemical characteristics to a process of progressive assimilation of host continental crust during the emplacement of the gabbroic plutons. Interestingly, high LREE/MREE ratios (La-N/Sm-N = 1.45-4.58) and high Rb, Ba, Th and U contents also characterize the basaltic dike swarms associated to the gabbros. Incompatible trace element compositions of these basalts approach those of the melts that formed the most hybridized gabbros. Therefore, we propose that the dike swarms represent melts partially contaminated by assimilation of continental crust material, extracted from the underlying gabbroic crystal mush. Our results suggest that early syn-rift magmatism led to the partial replacement of the thinned continental crust by MORB-type gabbroic bodies, in turn suggesting that oceanic magmatism started prior to continental break-up. Extensive syn-rift magmatism is consistent with the interpretation of the southern Red Sea rift system as a volcanic rifted margin. One possible implication of this study is that extensive but diffuse syn-rift magmatism possibly hampered continental break-up, leading to a protracted rifting stage.
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