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Vestiges of a fore-arc oceanic crust in the Western Mediterranean: Geochemical constraints from North-East Algeria

Abstract : The present day architecture of the Western Mediterranean mainly results from the interplay of different litho-spheric plates and the Cenozoic consumption of various branches of the Alpine Tethys and Neo-Tethys Oceans.Identifying relicsofthese oceanic domains inthe peri-Mediterranean belts enables pinpointing the earlieststagesof this evolutionary framework. In NE Algeria, the Kef Lakhal Complex (Edough Massif) is composed of amphib-olites and meta-gabbros metamorphosed under amphibolite facies conditions and thrust onto the northernAfrican margin during earliest Miocene. Geochemical analyses reveal that amphibolites and metagabbros havemajor and trace element signatures characteristic of tholeiitic basalts and gabbros and display N-MORB (La/SmN=0.7–0.9) toE-MORB (La/SmN=1.1–1.2) affinities. Enrichments in U and alkalis (Cs, Rb) and high87Sr/86Sriratios on bulk rocks and minerals (up to 0.70856) are typical of seawater alteration and indicate thatthe Kef Lakhal Complex represents a piece of altered oceanic crust. Parental magmas were derived from a De-pleted Mantle source (WRεNdi= 8.2–9.9 andεHfi= 10.4–14.4) that preserved some canonical MORB ratios(Ti/Eu = 5700–7600 and Y/Ho = 27.4–29.0), but which was contaminated by a component akin to recycled oce-anic crust and associated sediments (Ce/Pb = 10.5–22.2;206Pb/204Pbi=18.41–18.74;207Pb/204Pbi=15.49–15.59;208Pb/204Pbi=37.73–38.37). The sedimentary component is consistent with a volcaniclastic originand substantiatesanintra-oceanic setting.The geochemicalcharacteristics oftheKef Lakhalrocksare comparableto present day (Mariana, W Pacific Ocean) or fossil (Xigaze, SE Tibet) fore-arc oceanic crust. An overview of theWestern Mediterranean geodynamic indicates that the most likely period for initiating formation of a fore-arccorresponds either to the Early/Late Cretaceous or to the Middle Eocene.40Ar/39Ar dating on amphibole furtherindicate that after thrusting onto the African margin, the Kef Lakhal Complex cooled down toc.550 °C at18.14 ± 0.27 Ma (2σ).
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Laure Fernandez, Delphine Bosch, Olivier Bruguier, Dalila Hammor, Renaud Caby, et al.. Vestiges of a fore-arc oceanic crust in the Western Mediterranean: Geochemical constraints from North-East Algeria. Lithos, Elsevier, 2020, 370-371, pp.105649. ⟨10.1016/j.lithos.2020.105649⟩. ⟨hal-02894052v1⟩



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