Origin and signficance of basic and ultrabasic outcrops from Northeastern Algeria (Edough massif)

International audienceThe Maghrebian, Betics and Apenninic chains outcrop around the mediterranean basin and constitute dismembered fragments of the Alpine orogen. Conflicting geodynamic models have been proposed in order to explain the different paleogeographic settings from which these fragments derived.Crust-mantle interactions following subduction of Jurassic oceanic crust and collision-related tectonic events of Eocene age in relation to the northward motion of Africa have been demonstrated by numerous works. The incorporation of mafic/ultramafic rocks into the basement is evidenced in various peri-Mediterranean areas, in particular at c. 22 Ma. This work is focused on the basic and ultrabasic rocks from the Easternmost internal part of the Maghrebides. An extensive petrological and geochemical study has been performed on three distinct outcrops, i.e. Bou Maiza gabbros, amphibolitesfrom La voile Noire and Sidi Mohamed peridotites. Peridotites display a primitive character (Mg number >85), but slightly enriched trace elements patterns (1 to 10 times CHUR) characterized by negative Nb anomalies and flat to slightly LREE-depleted patterns. Associated isotopic constraints suggest a possible continental contamination of the peridotites by the surrounding gneisses. These ultrabasic rocks are interpreted as parts of the lithospheric mantle incorporated into the continental crust during a late Burdigalian extensional event that opened the Algerian basin. The Bou Maiza gabbros and La Voile Noire amphibolites show complementary trace elements spectra suggesting derivation from a common MORB source reservoir, but without filiation with the Sidi Mohamed peridotites. Such affinities suggest they represent a fragment of the Neothetys lithosphere obducted onto the North African margin during Miocene times

Reworking of intra-oceanic rocks in a deep sea basin: example from the Bou-Maiza complex (Edough massif, eastern Algeria)

International audienceMetagabbros and amphibolites exposed in the Bou-Maïza area of the Edough massif (northeast Algeria) are described in detail. Field and petro-structural observations point to the syn-sedimentary emplacement of gabbros as clasts, blocks and lenses of polymictic gabbroic breccias. Associated amphibolites display fine-scale parallel sedimentary bedding and represent mafic epiclastites, litharenites and mafic greywackes. The mafic beds and lenses are intercalated with aluminous pelitic schists of continental origin, quartzite and marble. It is concluded that all mafic rocks from this locality derive from the erosion of an oceanic plutono-volcanic complex of MORB affinity that was reworked in a block matrix mélange and emplaced as turbidites and debris flows during the Mesozoic. We propose a convergent plate margin setting for these formations connected with the subducted Calabrian branch of the Tethyan slab

Geochemical study (major, trace elements and Pb-Sr-Nd isotopes) of mantle material obducted onto the North African margin (Edough Massif, North Eastern Algeria): Tethys fragments or lost remnants of the Liguro-Provencal basin?

International audienceThe Maghrebides, Betics and some parts of the Calabrian, NE Sicilian and Tuscan allochtons constitute dismembered fragments of the Alpine belt that resulted from the Cenozoic collision between Africa and Eurasia and the opening of the Western Mediterranean basin. Mineral and whole-rock geochemical analyses have been performed on three distinct outcrops of mantle material from the Edough Massif of NE Algeria, namely the Bou Maiza (BM) gabbros, the La Voile Noire (LVN) amphibolites and the Sidi Mohamed (SM) peridotites. In all samples, Sr isotopes are largely affected by seawater alteration (87Sr/86Sract. > 0.70384 and up to 0.70888) and cannot be used to evaluate the nature of the source reservoirs. SM peridotites display variable depleted mantle Nd isotopic signatures (εNdact. from + 7.0 to + 12.2) and geochemical features suggesting no significant chemical depletion as a result of partial melting and melt extraction (Mg# + 7.9) and are likely cogenetic, but without filiation with the SM peridotites. Pb isotopes indicate a contribution of sediments in the source reservoir, which is attributed to contamination solely by hydrous fluids released from a sedimentary component. This observation, together with a LILE-enrichment, suggests a back-arc basin environment. These results indicate that BM and LVN units were obducted onto the North African margin and subsequently fragmented, probably during doming and exhumation of the lower continental crust of the Edough massif. Doming resulted in opposite movements of the overlying oceanic units, southward for the BM units and northward for LVN amphibolites

Discovery of metamorphic diamonds in garnets from the Edough massif (Northeastern Algeria): LA-ICP-MS U-Pb constraints

International audienceInclusions in a garnet megacryst (≥ 5 cm) from the Edough Massif (NE Algeria) have been studied and we report for the first time the discovery of diamond inclusions (5-30 micrometer in size), identified by Raman spectroscopy and the characteristic sharp band at 1332 cm-1 for crystalline diamond. The garnet is adjacent to actinolite and ultramafic boudins and slices of marbles that are inserted within a major mylonite-ultramyloniteband. This tectonic contact sharply delineates the Kef Lakhal oceanic unit from the granite-gneiss core below. This 1 km thick slab of amphibolites and pyrigarnites, derived from layered gabbros of MORB affinity, rest above the granite-gneiss units from the core complex. Garnet-forming reactions and dehydration melting are the oldest metamorphic stages recognized in the Kef Lakhal mafic complex (≥ 800 ◦C, ≥ 17 Kbar). The diamond-bearing garnets display a dense rutile acicular network interpreted as exsolutions. Zircons inclusions have been also observed in this garnet megacryst. Diamond crystals, up to 50 micrometer across, were identified first by optical microscopy and later by Raman spectroscopy. Major and trace elements show a gradual but significant zonation from core to rim, and a sharp increase in grossular component in the rim. Trace element analyses of prismatic rutile inclusions up to 300 micrometer in size indicate that the host metamorphic rock was a mafic protolith of MORB affinity. The Zr-in-rutile thermometry indicates a temperature range of 724-778 ◦C that we relate to rutile growth, either during prograde metamorphism or under peak UHP metamorphic conditions of ≥ 3.6 GPa that were reached during subduction of the UHP-rock precursors. In situ U-Pb dating obtained on rutile inclusions yield a 32.4+/ 3.3 Ma interpreted as the age of the UHP metamorphic event. LA-ICP-MS U-Pb ages on zircon provide a 20.9 +/- 2.2 Ma age attributed to the thrusting onto the North-African margin. We suggest that the mafic protolith of the analyzed garnet megacryst originates from the subducted retreating Tethyan slab, which broke or tore, and which fragments were dragged upward and thrust onto the North African margin along with the Kef Lakhal unit, shortly followed by building up of the Edough dome and opening of the Algerian basin

Metamorphic diamonds in a garnet megacryst from the Edough Massif (northeastern Algeria). Recognition and geodynamic consequences

International audienceWe report for the first time the discovery of diamond inclusions (5–30 μm in size), identified by Raman spectroscopy and the characteristic sharp band at 1332 cm− 1 for crystalline diamond, in a garnet megacryst (≥ 5 cm) from the Edough Massif (NE Algeria). The garnet is adjacent to actinolite and ultramafic boudins and slices of marbles that are inserted within a major mylonite–ultramylonite band. This tectonic contact sharply delineates the Kef Lakhal oceanic unit from the granite-gneiss core below. The host garnet is almandine-dominant and is rich in exsolution of acicular rutile needles. Major and trace elements show a gradual but significant zonation from core to rim, and a sharp increase in grossular component in the rim. Trace element analyses of prismatic rutile inclusions up to 300 μm in size indicate that the host metamorphic rock was a mafic protolith of MORB affinity. The Zr-in-rutile thermometry indicates a temperature range of 724–778 °C that we relate to rutile growth, either during prograde metamorphism or under peak UHP metamorphic conditions of ≥ 3.6 GPa that were reached during subduction of the UHP-rock precursors. We suggest that the mafic protolith originates from the subducted retreating Calabrian branch of the Tethyan slab, that broke or tore, and which fragments were dragged upward and thrust onto the North African margin along with the Kef Lakhal unit, shortly followed by formation of the Edough dome and opening of the Algerian basin

Age of UHP metamorphism in the Western Mediterranean: Insight from rutile and minute zircon inclusions in a diamond-bearing garnet megacryst (Edough Massif, NE Algeria)

International audienceDiamond-bearing UHP metamorphic rocks witness for subduction of lithospheric slabs into the mantle and their return to shallow levels. In this study we present U–Pb and trace elements analyses of zircon and rutile inclusions from a diamond-bearing garnet megacryst collected in a mélange unit exposed on the northern margin of Africa (Edough Massif, NE Algeria). Large rutile crystals (up to 300 μm in size) analyzed in situ provide a U–Pb age of interpreted as dating the prograde to peak subduction stage of the mafic protolith. Trace element analyses of minute zircons (≤30 μm) indicate that they formed in equilibrium with the garnet megacryst at a temperature of 740–810 °C, most likely during HP retrograde metamorphism. U–Pb analyses provide a significantly younger age of attributed to exhumation of the UHP units. This study allows bracketing the age of UHP metamorphism in the Western Mediterranean Orogen to the Oligocene/early Miocene, thus unambiguously relating UHP metamorphism to the Alpine history. Exhumation of these UHP units is coeval with the counterclockwise rotation of the Corsica–Sardinia block and most likely resulted from subduction rollback that was driven by slab pull