Ar/Ar dating of alkaline and tholeiitic magmatism of Saudi Arabia related to the early Red Sea Rifting

⁴⁰Ar/³⁹Ar plateau-ages at 27–28 Ma obtained on feeder dykes and one lava flow of the alkaline massif of Harrat Hadan are in agreement with the assumed age partly deduced from the conventional K/Ar data of the early activity of other alkaline volcanic massifs from Saudi Arabia. This magmatic activity is spatially distributed over a large area, along the western edge of the Arabian plate, and their N-S to NW-SE volcano-tectonic directions are similar to those of the future Red Sea Rift.Preliminary results obtained on tholeiitic lava flows, dykes and plutons gave 17 plateau-ages which, combined with 6 ages deduced from more disturbed age-spectra, display a main histogram peak from 24 to 21 Ma, much narrower than that obtained with conventional K/Ar ages on the same formations. Therefore, a strong tholeiitic activity affected a narrow linear area following the actual eastern Red Sea coast, over nearly 1700 km, during a brief period of time, and without showing any apparent migration. The main volcano-tectonic features of the future Red Sea are strongly outlined during this event. Such brief magmatic episodes related to continental rifts have also been documented by precise ⁴⁰Ar/³⁹Ar analyses on the British Tertiary Igneous Province, the Deccan traps and the eastern Central Atlantic.