ENIGMATIC ALKALINE VOLCANISM IN WESTERN SAUDI ARABIA: SUB-LITHOSPHERIC CHANNELING FROM THE AFAR MANTLE PLUME? (Invited Presentation)

The intraplate volcanic fields (harrats) of the western Arabian Peninsula are some of the world’s most voluminous, and best-preserved continental volcanic fields. However, the ultimate cause of volcanism remains unclear. Volcanism started at ~15 Ma and trends north as discrete harrats from the Red Sea along the Makkah-Madinah-Nafud (MMN) line. Larger harrats define the central axis of the MMN line, which is also underlain by anomalously hot asthenosphere that exhibits north-south shear wave splitting directions. Smaller fields lie on the periphery of the MMN line. The western margin of the Red Sea (African plate) has not experienced comparable volcanism.

We present results of an ongoing study of harrat volcanism, including over 50 new ⁴⁰Ar-³⁹Ar ages, major and trace elements, and Pb-Nd-Sr-He isotopic data. Lava compositions are dominated by alkali basalt, trachybasalt and basanite, and more evolved compositions also occur in larger harrats. Along the MMN line the earliest melting occurred within the garnet stability field, and beneath thick lithosphere (~80 km). However the compositions of more recent lavas suggests thinner lithosphere (<40 km) and higher degrees of melting. In contrast, the peripheral harrats are underlain by thicker lithosphere (>60-80 km) and volcanism is sourced from lower degree melts. Our results show that volcanism along the MMN line was accompanied by lithospheric thinning, probably driven by progressive northward movement of hot asthenosphere. Elevated ³He/⁴He, and Pb isotope compositions suggest involvement of a mantle plume source. We suggest this results from lateral flow of Afar plume material at depth northwards and oblique to the Red Sea margin, possibly in response to Mediterranean subduction.