Isotopic and geochemical evidence for the source of volcanism at Harrat Rahat, Kingdom of Saudi Arabia

Abstract

Pleistocene and Holocene basalts, hawaiites, mugearites, benmoreites, and trachytes from the northern part of the Harrat Rahat volcanic field, Kingdom of Saudi Arabia, were analyzed for Sr, Nd, Hf, and Pb isotopic compositions. Evolved trachytes with Mg number <0.1 (Mg# = Mg/[Mg+Fe 2+ ], molar) have relatively radiogenic Sr isotopic compositions indicating that they were influenced by contamination probably in the upper crust. Volcanic rocks with Mg# >0.1, consisting chiefly of alkali basalts but encompassing hawaiites, mugearites, and benmoreites, show a limited range in Hf, Nd, Sr, and Pb isotopic compositions. Although the total Pb isotope variation is only 1 percent, the Pb isotope values correlate with Mg#, where the least radiogenic Pb is in samples with the lowest Mg#. The trend formed in Pb isotope space points toward an unradiogenic Pb composition that is similar to the Pb isotopic composition of lower crust of the Precambrian Arabian-Nubian Shields, as well as to feldspars and galena in the upper crust of the western Arabian Shield. This trend is interpreted as progressive but overall minor (no more than 5 weight percent) assimilation of shield rocks, or their partial melts, during fractional crystallization. Isotopic compositions of the least evolved northern Harrat Rahat magmas are most similar among analyzed Arabian harrats to depleted spreading-ridge basalts of the active Red Sea rift, but isotopic values are displaced toward those of spreading-ridge basalts of the Gulf of Aden that are proximal to the site of the Afar mantle plume. The Pb isotopic compositions very near the Northern Hemisphere Reference Line indicate no discernable lithospheric contribution to yield the parental basalts of northern Harrat Rahat, and their isotopic compositions are consistent with derivation predominantly from depleted Northern Hemisphere asthenosphere with a subordinate (20–30 weight percent) component from the Afar mantle plume. Trace-element variations show that appreciable portions of melting were in the garnet stability field, confirming the sub-lithospheric origin of the magmas, and that melting extents were low, accounting for the alkalic, trace-element-enriched character of the suite. The presence of possible Afar mantle beneath the western part of the Arabian Shield and its absence beneath the Red Sea rift may result from capture and channelized flow along high-relief structures incised into the base of the sub-continental lithosphere, as revealed by geophysical images.