Petrogenesis of the largest intraplate volcanic field on the Arabian plate (Jordan): a mixed lithosphere-asthenosphere source activated by lithospheric extension

Miocene to Recent volcanism in northwestern Arabia produced the largest intraplate volcanic field on the Arabian plate (Harrat Ash Shaam, Jordan). The chemically and isotopically diverse volcanic field comprises mafic alkali basalts and basanites. The magmas underwent limited fractional crystallization of ol +/- cpx +/- plag and rare samples have assimilated up to 20% of Late Proterozoic crust en route to the surface. However, there are subtle Sr-Nd-Pb isotopic variations (Sr-87/Sr-86 = 0.70305-0.70377, Nd-143/Nd-144 = 0.51297-0.51285,Pb-206/Pb-204 = 18.8-19.2), which exhibit marked correlations with major elements, incompatible trace element ratios and abundances in relatively primitive basalts (MgO >8.5 wt %), and cannot be explained by fractional crystallization and crustal contamination alone. Instead, the data require polybaric melting of heterogeneous sources. Semi-quantitative melt modelling suggests that this heterogeneity is the result of small degree melts (2-5%) from spinel- and garnet-facies mantle, inferred to be shallow Arabian lithosphere, that mixed with smaller degree, melts (<1%) from a predominantly deep garnet-bearing asthenospheric(?) source with ocean island basalt characteristics. The latter may be a ubiquitous part of the asthenosphere but is preferentially tapped at small degrees of partial melting. Volcanism in Jordan appears to be the result of melting lithospheric mantle in response to lithospheric extension. With time, thinning of the lithosphere allowed progressively deeper mantle (asthenosphere?) to be activated and melts from this to mix with the shallower lithospheric mantle melts. Although Jordanian intraplate volcanism is isotopically similar to examples of Late Cenozoic volcanism throughout the Arabian peninsula (Israel, Saudi Arabia), subtle chemical and isotopic differences between Yemen and Jordan intraplate volcanism suggest that the Afar plume has not been channelled northwestwards beneath the Arabian plate and played no role in producing the northern Saudi Arabian and Jordan intraplate volcanic fields.