Petrogenesis of mafic lavas from Harrat Uwayrid, Saudi Arabia: geochemical characteristics and tectonic implications of lower lithospheric melting

Abstract:

Continental basalts provide an opportunity to study the interactions between tectonic and igneous processes. The Arabian Plate is an ideal area to study the genesis of continental basalts as it is dotted with Cenozoic basalt fields and surrounded by complex tectonic regimes: to the north is the Bitlis-Zagros Suture Zone, to the south is the Afar Plume and diffuse rifting of the Red Sea, and to the northwest is the Dead Sea Transform Fault. Source mineralogy and melt mechanisms for these basalts are poorly constrained. Combining new and published data on xenoliths and lavas, we document melting of metasomatized pyroxenite and peridotite beneath Harrat Uwayrid, NW Saudi Arabia, within the last 7 Ma (Kaliwoda et al., 2007, 2008; Altherr et al., 2019). Harrat Uwayrid basalts are distinct from the nearby Makkah-Medinah Line basalts, which derive by shallow melting of a peridotite source (Zn/Fe < 11, Zn/Mn < 0.07, Tb/Ybn < 2.2) with negligible hydrous phases (Rb/Sr < 0.04, Ba/Rb < 30). In contrast, our samples exhibit characteristics of both peridotite and pyroxenite (Zn/Fe = 50-75, 10000*Co/Fe = 4-15) that have undergone hydrous metasomatism as indicated by elevated Rb/Sr and Ba/Rb. Values of 87Sr/86Sr decrease from 7 Ma to present, approaching typical “MORB-like” (0.703) values. Although REE signatures suggest melting in the spinel-garnet transition zone (Tb/Ybn = 1.5-2.5), olivine-liquid equilibrium calculations suggest much deeper melting (90-220 km). Petrographic indicators, such as juxtaposed zoned and exsolved minerals and dissolution features at grain boundaries, suggest ascent was rapid following a period of magma mixing. Diffusion modelling of the Fe-Mg and Al-Cr systems in chromian spinels further supports a short timescale for magma mixing and ascent. These timescales and the bulk geochemical data can be reconciled through a model wherein a “package” of dense, metasomatized lithosphere melts during descent into the mantle. We suggest that tectonic adjustment involving the Dead Sea Fault and diffuse rifts of the Red Sea margin destabilized the anomalously dense, metasomatized lower lithosphere, which sank as a coherent package before progressively melting as it traveled deeper through the mantle.