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The Arabian continental alkali basalt province: Part I. Evolution of Harrat Rahat, Kingdom of Saudi Arabia

¹ Directorate General of Mineral Resources, P.O. Box 345, Jiddah, 21191 Saudi Arabia

The Harrat Rahat lava field forms a major component of an extensive but poorly known continental alkali basalt province extending from Yemen in the south, through Saudi Arabia and Jordan, to Syria in the north. The continental intraplate volcanism which produced the Arabian lava fields (harrats) was contemporaneous with opening of the Red Sea, collision of the Arabian and Eurasian plates, and uplift of the Afro-Arabian dome.

Harrat Rabat has evolved over the past 10 m.y. It contains two prominent lateritic disconformities which separate three stratigraphic units: the Shawahit (10-2.5 Ma), Hammah (2.5-1.7 Ma), and Madinah (1.7-Recent) basalts, comprising 68%, 19%, and 13% of the total harrat volume, respectively. The Shawahit basalt is composed of coarse-grained, dictytaxitic olivine transitional basalt (OTB) and minor (10%) alkali olivine basalt (AOB) which erupted through scoria cones and shield volcanoes. In contrast, the Hammah basalt is dominated by equal volumes of fine-grained, intergranular-to-intersertal AOB and hawaiite which erupted mainly through scoria cones and relatively few shield volcanoes. Sparse domes and flows of mugearite and benmoreite first appear in the Hammah basalt. The Madinah basalt spans the full compositional range of Harrat Rahat; 103 analyzed samples comprise OTB (8%), AOB (47%), hawaiite (32%), mugearite (4%), benmoreite (8%), and trachyte (2%). Here scoria cones dominate, whereas shield volcanoes are sparse and occur only in the lower Madinah basalt; domes and associated pyroclastic deposits are common. The final events of harrat volcanism include 11 "post-Neolithic" eruptions (<4,500 yr old) and two historical eruptions (in 641 and 1256 A.D.).

Arabian harrat magmatism may have been initiated by partial melting of garnet perido-tite in the asthenosphere at a depth > 100 km. Accumulation of this partial melt at the crust-mantle boundary (37-44 km) may have resulted in Miocene uplift of the Afro-Arabian dome. Harrat Rahat volcanism began shortly after significant initial uplift with the voluminous extrusion of Shawahit OTB lava. Chemical and petrographic data suggest that these OTB magmas ascended rapidly, with little crustal residence time. The data support a model of open-system fractionation for AOB and hawaiite lavas of the younger Hammah and Madinah basalts. In this model, some of the potential Shawahit OTB was trapped in crustal chambers during magma ascent. Crystal fractionation in these chambers was accompanied by their periodic replenishment by rising OTB magma. This resulted in the mixing of magma types and the development of AOB and hawaiite magmas which are anomalously enriched in highly incompatible elements. The mugearite, benmoreite, and trachyte lavas are the products of advanced fractionation in these crustal chambers with little or no magma replenishment.

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