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 lithospheric plates and the Cenozoic consumption of various branches of the Alpine Tethys and Neo-Tethys Oceans. Identifying relics of these oceanic domains in the peri-Mediterranean belts enables pinpointing the earliest stages of this evolutionary framework. In NE Algeria, the Kef Lakhal Complex (Edough Massif) is composed of amphibolites and meta-gabbros metamorphosed under amphibolite facies conditions and thrust onto the northern African margin during earliest Miocene. Geochemical analyses reveal that amphibolites and metagabbros have major and trace element signatures characteristic of tholeiitic basalts and gabbros and display N-MORB (La/SmN = 0.7–0.9) to E-MORB (La/SmN = 1.1–1.2) affinities. Enrichments in U and alkalis (Cs, Rb) and high 87Sr/86Sri ratios on bulk rocks and minerals (up to 0.70856) are typical of seawater alteration and indicate that the Kef Lakhal Complex represents a piece of altered oceanic crust. Parental magmas were derived from a Depleted Mantle source (WR εNdi = 8.2–9.9 and εHfi = 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 oceanic 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 origin and substantiates an intra-oceanic setting. The geochemical characteristics of the Kef Lakhal rocks are comparable to present day (Mariana, W Pacific Ocean) or fossil (Xigaze, SE Tibet) fore-arc oceanic crust. An overview of the Western Mediterranean geodynamic indicates that the most likely period for initiating formation of a fore-arc corresponds either to the Early/Late Cretaceous or to the Middle Eocene. 40Ar/39Ar dating on amphibole further indicate that after thrusting onto the African margin, the Kef Lakhal Complex cooled down to c.550 °C at 18.14 ± 0.27 Ma (2σ).