Geodynamic Controls on Basaltic Volcanism in the Arabian Peninsula: Evolution of Harrat Uwayrid, Saudi Arabia

Basaltic lavas from Harrat Uwayrid, Saudi Arabia, record the evolving magmatic and tectonic context of the Arabian Peninsula from at least the mid-Miocene to the present day. New 40Ar/39Ar ages spanning from the mid to late Miocene reveal that mid-Miocene mafic volcanism formed a large, subalkaline volcanic plateau parallel to Red Sea rifts. Subsequent volumetrically subordinate late Miocene-Quaternary alkaline volcanism erupted monogenetic cinder cones roughly orthogonal to the earlier volcanic field. The source region for all samples was affected by both fluid and silicate metasomatism; inferred mantle mineral assemblages include amphibole for mid-Miocene lavas and phlogopite for late Miocene-Quaternary samples. Calculated melting depths become shallower with time across the Miocene volcanic episode (similar to 20-15 Ma) but become deeper in the late Miocene to Quaternary (similar to 10-0 Ma), indicating melting pressures and temperatures significantly higher than those recorded in Miocene lavas despite progressive lithospheric thinning. We offer a two-stage model for the formation of Harrat Uwayrid: (a) Early- and mid-Miocene rifting associated with the Red Sea opening facilitated adiabatic melting of uppermost mantle lithosphere to form the early volcanic plateau and (b) Plate motion changes in the mid- and late-Miocene initiated the Dead Sea Fault and destabilized a dense pyroxenitic lower lithosphere leading to foundering or lithospheric drip beneath Harrat Uwayrid that allowed deep lithospheric melting and formed the young volatile-rich eruptives. Mafic volcanism at Harrat Uwayrid records the geodynamic evolution of the Arabian plate margin since the OligoceneMio-Pliocene lavas formed from adiabatic melting as a result of lithospheric extension associated with spreading of the Red SeaVolatile-rich Quaternary alkaline basalts reflect melting of hydrated metasomatized lithosphere