Asymmetrical magma chamber beneath the Red Sea system controlled Cenozoic alkaline magmatism on the western margin of the Arabian Shield

The western margin of the Arabian Shield along the Red Sea between Jordan and Yemen is littered with numerous (mostly mid-Miocene to Quaternary) basalt fields known as harrats. Harrat Uwayrid consists of a NW–SE-oriented elongated field that extends for c. 230 km in NW Saudi Arabia. The magmatic activities include older alkaline lava flows of transitional basalt, alkali basalt, hawaiite and basanite, together with younger scoria basanite that erupted through the central segment of the older lavas. Based on chondrite-normalized Sm/Yb v. La/Yb and Tb/Yb v. La/Sm diagrams, we show that the older transitional lavas were generated from spinel lherzolite, whereas the younger lavas were generated from garnet lherzolite. The primitive-mantle-normalized patterns are similar to those of ocean island basalts. 40 Ar/ 39 Ar plateau dating yields eruption ages of 10.15 ± 0.09 to 9.87 ± 0.04 Ma ( n = 3) for the older alkaline basalts and 0.96 ± 0.03 to 0.16 ± 0.03 Ma ( n = 5) for the younger basanites. The 143 Nd/ 144 Nd ratios for both the older lavas and the younger scoria are 0.51285–0.51292 and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="IM1" overflow="scroll"> <mml:mi>ε</mml:mi> </mml:math> Nd (4.0–5.4) is positive; the ratios of 87 Sr/ 86 Sr are low (0.7032–0.7039). The range of 206 Pb/ 204 Pb (18.5305–19.3701) values and the 143 Nd/ 144 Nd v. 87 Sr/ 86 Sr ratios overlap those of FOcal ZOne (FOZO)-type mantle; we interpret this FOZO signature to indicate metasomatism of the asthenosphere from slabs subducted during the construction of the 900–550 Ma Arabian–Nubian Shield. Harrat Uwayrid records (possibly plume-related) potential temperatures of 1400–1490°C and pressures of 2.0–4.4 GPa ( c. 70–140 km depth), which are within the range of the Afar plume. Some Harrat Uwayrid scoria and lava also fall within the isotopic range of basalts in the Red Sea trough, Yemini flood basalts and Afar plume basalts. After the collision of Arabia–Eurasia in the Miocene, the subsequent eastern movement of the Arabian Shield caused asymmetrical drag of the upwelling asthenosphere (and possibly also the Afar plume) and the magma chambers to a location beneath the western limb of the Arabian Shield.