Miocene magmatism of sinai related to the opening of the red sea

Abstract

Extension associated with the initial (early Miocene) stages of opening of the Red Sea resulted in intrusion of a widespread system of dikes and smaller bodies, primarily along the northeastern margin of the Red Sea/Gulf of Suez axis. Dikes, some up to more than 800 km in length, were emplaced dominantly parallel to the Red Sea/Gulf of Suez from the Sinai Peninsula to Yemen. Intrusive rocks are mainly basaltic in composition, but minor granophyres are also present. In the Sinai, this magmatic event is about 20 m.y. old. Dikes of the southern and southeastern Sinai crop out in Precambrian terrane. In the central and northern Sinai, they occur in Phanerozoic terrane, where they may be associated with linear grabens and cryptovolcanic features. Basaltic dikes of the Sinai underwent variable degrees of alteration. Alteration phases include seriate, quartz, smectite, chlorite, and serpentine. X-ray diffraction and petrographic analysis indicate that, in spite of extensive alteration, primary minerals still control the major-element compositions of the rocks. Basalts are tholeiitic in composition, with SiO 2 = 48.5 to 50%, and Na 2O + K 2O = 3 to 3.5%. Rb and Sr concentrations are probably dominated by alteration and by introduction of secondary material (primarily carbonate). Zr/TiO 2, which is relatively unchanged during alteration, substantiates that the basaltic rocks are subalkaline. Initial 87Sr/ 86Sr, determined after leaching of samples, is approximately 0.7043–0.7063, much higher than Red Sea axial trough basalts ( < 0.703). We infer that these Sinai basalts equilibrated at depths of 35–50 km, possibly within an enriched mantle. Whether the main trough of the Red Sea is underlain by extended and modified continental crust or by oceanic crust is controversial. Since Miocene dikes were intruded along both coasts (but primarily the Saudi Arabian coast) of the Red Sea/Gulf of Suez axis in a zone up to 150 km wide, they may also underlie the main trough of the Red Sea and result, at least partially, in the discontinuous magnetic and gravity anomalies observed there. Possibly the abundance of dikes beneath the main trough is much greater than adjacent to the Red Sea, giving rise to properties intermediate between those of continental and oceanic crust. Downwarping and widespread intrusion of dikes along the Red Sea 18–26 m.y. ago probably signified crustal dilation and the initiation of a continental rift. Because tholeiitic basalts of the Sinai are higher in 87Sr/ 86Sr than Red Sea axial trough basalts, we infer that the conditions of their origin must also be very different. We suggest that the Sinai basalts may have been generated within but near the base of subcrustal continental lithosphere , perhaps indicating thermal thinning of lithospheric mantle above upwelled asthenosphere associated with continental rifting. This inference, if correct, would indicate that the lithosphere was at least 35–50 km thick during the early Miocene phase of rifting but possibly not much thicker. Since dikes farthest from the Red Sea are apparently identical in composition to those closest, asthenospheric upwarping must have been broad enough that all magmas were derived from approximately the same depth. In the Yemen-Ethiopia areas, volcanism began earlier (about 30 m.y. ago) and was much more voluminous than elsewhere along the Red Sea/Gulf of Suez axis, suggesting the presence of underlying mantle upwelling accompanying rifting of the Red Sea. In spite of differences in age and volume, the presence of large volumes of tholeiites among the Miocene basalts of the Yemen-Ethiopia areas suggests that the lithosphere was similarly thinned along the entire length of the Red Sea.