The In Ezzane volcanic field (Eastern Hoggar, Algeria) and its enigmatic circular structures

The In Ezzane region, located in eastern Algerian Sahara, close to Libya – Niger border, is marked by late Pliocene and Pleistocene volcanism and by a remarkable group of circular structures, 4–9 km in diameter, situated on a basement composed of Tassili Cambrian-Ordovician sandstones. Both manifestations are imbricated and can only be subcontemporaneous. In this study, first, we date and characterize the volcanism and second, we try to constrain the origin of the circular structures that were visited just before the region closed for security reasons. Two basaltic lavas were dated at 2.01 ± 0.05 Ma (Gelasian) and at 1.49 ± 0.03 Ma (Calabrian) by K-Ar method. Petrographic investigations show that most volcanic rocks are basanites composed of olivine and Al-Ti diopside phenocrysts with xenocrystic mantle olivine and orthopyroxene. Temperatures of eruptions (1200–1300 °C) and of primary picritic magma (1500–1600 °C, i.e. a depth of 120–150 km), along with major and trace elements, and Sr-Nd isotopes all point to an asthenospheric source along the western lithospheric margin of the Murzuq craton (Murzuq volcanic province). The In Ezzane counter-relief circular structures are nested and NNW-SSE aligned, with each structure being truncated by the next one to the south-south-east. Up to 500 m outside circular structures, Tassili sandstones are reddened, dissected by intense fractures that are filled with fine-grained brown material and newly formed acicular white quartz, and often show white, cm-size, vesicles. The rims of the circular structures are composed of densely brecciated sandstones with dark veins, whereas the central parts are crosscut by a loose network of dyke-like dark veins. Microscopic observations, SEM imagery, SEM-EDS determinations and laser-ICP-MS chemical analyses of hydrothermal breccia components point to large masses of fluids having moved rapidly at shallow depths and having induced hydrofracturing of the heated sandstones with the generation of veins composed mostly of iron oxide. The viability of various processes that may have induced these large circular structures, i.e. tectonic reliefs, igneous and associated hydrothermal phenomena, meteorite impact craters, are discussed and favour sandstone karst features.