Mesozoic volcanism in the Middle East: geochemical, isotopic and petrogenetic evolution of extension-related alkali basalts from central Lebanon

Abstract

Mesozoic picritic and alkali basalts from central Lebanon represent a significant part of an extension-related Upper Jurassic to Upper Cretaceous discontinuous volcanic belt which occurs throughout the Middle East. Volcanism was associated with an episode of intraplate extension that followed a period of continental break-up, where Mesozoic micro-continental blocks separated from Gondwana as the Neotethys ocean opened in Jurassic times. This volcanic episode produced mafic lava flows ranging in thickness from 5 to 20 m, along with some minor pyroclastic flows. These flows are stratigraphically intercalated with thick carbonate platform deposits. The basalts are made up of about 15–20% olivine (Fo_78–91), 30–35% clinopyroxene (salite), 40–50% plagioclase (An_56–71) and opaque Fe–Ti oxides (∼5%). Geochemically, the rocks exhibit a relatively wide range of SiO₂ (40.4 to 50.5 wt%) and MgO (5.1 to 15.5 wt%) contents, are relatively enriched in TiO₂ (1.7 to 3.7 wt%) and vary in composition from alkali picrite and basanite to alkali basalt. The Mg numbers range from 0.56 to 0.70, with an average of 0.63. The rocks are enriched in incompatible trace elements such as Zr (86–247 ppm), Nb (16–66 ppm) and Y (19–30 ppm). Such compositions are typical of those of HIMU-OIB and plume-related magmas. The REE patterns are fractionated ((La/Yb)N = 11), LREE enriched, and are generally parallel to subparallel. Elemental ratios such as K/P (1.1–4.7), La/Ta (11–13), La/Nb (0.57–0.70), Nb/Y (0.68–1.55) and Th/Nb (0.20–0.36) suggest that crustal contamination was minor or absent. This may be related to a rapid ascent of the magma, in agreement with the nature (mafic, oceanic-like) and the small thickness (about 12 km) of the Mesozoic crust of the Eastern Mediterranean region. The ¹⁴³Nd/¹⁴⁴Nd isotopic compositions of the lavas range from 0.512826 to 0.512886, and ⁸⁷Sr/⁸⁶Sr from 0.702971 to 0.703669, suggesting a HIMU-like mantle source. Trace element compositions indicate a melt segregation depth of 100–110 km, well within the garnet lherzolite stability field. The geochemical characteristics of the rocks are typical of within-plate alkali basalts, and suggest that the magmas were derived from a fertile, possibly plume-related, enriched mantle source. Petrogenetic modelling indicates that the magmas were produced by very small degrees of batch partial melting (F = 1.5%) of a primitive garnet-bearing mantle source (garnet lherzolite).