The Oligo-Miocene alkaline volcanism of Bahariya (Western Desert, Egypt)

Late Oligocene-Early Miocene volcanic rocks around the Bahariya Oasis in the Western Desert, Egypt occur as basaltic lava flows, subvolcanic sills and necks in an area of approximately 1800 km2. Five new K-Ar age cluster in a relatively narrow range from 23.9 to 20.7 Ma. Bahariya rocks are mildly alkaline basic in terms of silica content (SiO2 ranging from 48.4 to 51.4 wt.%), with total alkali content ranging from 4.9 to 5.9 wt.%, and variable amounts of normative nepheline. The samples are essentially sodic (Na2O-K2O > 2) and can be classified as alkali basalts, hawaiites or mugerarites. Only the samples from one locality (Qala Siua) can be classified as K-trachybasalts. Samples are usually porphyritic, with phenocryst assemblage made by olivine + clinopyroxene + plagioclase, and the same minerals along with alkali feldspar, ilmenite, Ti-magnetite, and eventually intersertal glass in the groundmass. Samples from Qala Siua can be distinguished also by their petrography, being plagioclase present only in the groundmass. All the samples have slightly fractionated REE patterns with (La/Yb)N ≈ 10-18, and no Eu anomaly. Primitive mantle-normalized patterns are bell-shaped, with positive Nb and Ta anomalies, which are more pronounced of Qala Siua samples, and small troughs for Zr and Hf. The 143Nd/144Nd initial isotopic ratios are high and constant (≈ 0.51295 for all the samples), whereas some variations are revealed by 87Sr/86Sr ratios, which are around 0.7028 for two samples, and 0.7036 for the most evolved sample of Baharyia volcanic field. The 206Pb/204Pb isotope ratios vary from 19.19 to 19.31 and are negatively correlated with the SiO2 content. Summing up, all the petrographic, geochemical and isotopic characters of studied rocks, are typical of within plate magmas, sourced in a depleted asthenospheric mantle unaffected by any subduction imprint. Their composition fully overlap the field of the other rocks of the Circum-Mediterranean anorogenic Cenozoic igneous rocks (CiMACI Province). Their relationships with coeval extensional faulting, characterized by NE directed faults, may indicate that these magmas were erupted in an incipient intracontinental rift, with no geochemical evidence for the involvement of any deeper mantle sources.