Petrogenesis and evolution of Miocene-Quaternary continental basalts at Harrat Ash Shaam (Al Harrah), NW Saudi Arabia

The geochemical compositions of Miocene-recent basalts from Harrat Ash Shaam (HAS) contain a record of the temporal and spatial evolution of the subcontinental lithospheric mantle in response to regional tectonics in northern Saudi Arabia. HAS lavas show two distinct episodes of magmatism: (1) widespread Oligocene/early-Miocene through mid-Miocene effusive, plateau-forming subalkaline basalts and (2) Late-Miocene through historic primitive alkaline basalts erupted from monogenetic cinder cones superimposed on the existing volcanic plateau. Geochemical evidence indicates a change in the source region mineralogy from spinel peridotite/pyroxenite with minor amounts of hydrous phases (amphibole/phlogopite) in the early Miocene to garnet peridotite/pyroxenite with abundant amphibole and phlogopite in the Quaternary. Melt equilibration pressures and temperatures in Quaternary (1 to $>$5 GPa, $\approx$1300–1500 ${}^{\circ }$C) and Miocene/Pliocene ($<$1 to 3 GPa, $\approx$1250–1400 ${}^{\circ }$C) lavas, plus REE evidence (e.g., Tb/Yb_n = 3.7 in the Quaternary vs. 2.6 in the Miocene), suggest a deepening of the melt region through time. Olivine ³He/⁴He ratios range from 6 to 9 R_A and do not require contributions from a deep-seated mantle plume; we suggest that genesis of the older Miocene basalts involved contributions of melt from the asthenosphere. Petrogenesis of Cenozoic HAS lavas occurred in two stages. Oligocene through mid-Miocene magmas formed from adiabatic melting of lithospheric and sub-lithospheric mantle associated with movement along the Azraq-Sirhan Graben during widespread extension across the Arabian Peninsula. Late-Miocene through present day magmatism was likely triggered by foundering of unstable (dense, metasomatized) lithosphere into the underlying asthenosphere, triggering lithospheric and asthenospheric melting subsequent to and coeval with tectonic movement along the Dead Sea Fault.