Cenozoic volcanism in the Middle East: petrogenesis of alkali basalts from northern Lebanon

Abstract

The Cenozoic volcanic field of the Akkar region in northern Lebanon consists of a thick succession (200 m) of basaltic lava flows, erupted at the junction between a restraining bend (the Yammouneh transform fault) and its northern extension (the Ghab transform) in Syria. Both faults are part of the Dead Sea transform fault system, which represents the boundary between the Arabian and African plates and the Levantine subplate. The lavas are made up of about 15–25 vol. % olivine (Fo_79–84), 30–40 % clinopyroxene (salite), 40–50 % plagioclase (An_58–67), and opaque Fe–Ti oxides (∼ 5 %). Geochemically, they exhibit a narrow range of SiO₂ (44.6 to 47.0 wt %), and MgO (2.9 to 7.5 wt %), are relatively enriched in TiO₂ (2.0 to 2.9 wt %), and are classified as alkali basalts. Mg-numbers range from 0.32 to 0.59, with an average of 0.47. The rocks are enriched in incompatible trace elements such as Zr (98–184 ppm), Nb (16–39 ppm) and Y (25–34 ppm). The REE patterns are fractionated ((La/Yb)_N=8.2), and are generally parallel to subparallel. Such compositions are typical of those of HIMU-OIB and plume-related magmas. Elemental ratios such as K/P (2.9), La/Ta (21.8), La/Nb (0.80), Nb/Y (0.92) and Th/Nb (0.35), and the low average SiO₂ content (46.1 wt %) suggest that the magma was subjected to minimal crustal contamination. This may be related to a rapid ascent of the parental magma, in agreement with the nature (mafic, oceanic crust-like) and the thickness (only about 12 km) of the crust of the Eastern Mediterranean region. Cenozoic volcanism in this region is interpreted to have occurred in association with an episode of localized extension, particularly at the junction between the Yammouneh restraining bend and the Dead Sea–Ghab Transform (that is, in a transtensional tectonic regime). The ¹⁴³Nd/¹⁴⁴Nd isotopic composition of the basaltic rocks of northern Lebanon ranges from 0.512842 to 0.512934 (ε_Nd=4.0 to 5.8), and ⁸⁷Sr/⁸⁶Sr from 0.703317 to 0.703579, suggesting a HIMU-like mantle source. Modelling indicates that the magma was produced by a small degree of partial melting (F=2 %) of a primitive, garnet lherzolitic mantle source, possibly containing a minor spinel component.