40Ar39Ar chronology of tertiary magmatic activity in Southern Yemen during the early Red Sea-Aden rifting

The early opening of the Red Sea rift was accompanied by magmatism that formed lava flows, plutons and dyke swarms. Some of these formations were sampled in Southern Yemen during a preliminary field survey and were investigated by geochemistry and 40Ar39Ar geochronology. Ten plateau ages and more disturbed age spectra on plagioclase, alkali feldspar, biotite and whole rocks are discussed in detail, and allow us to establish the first chronological succession of magmatic events in this area immediately adjacent to the well-studied Afar region. An approximately 2000-m-thick section from the middle part of the traps gives ages ranging from 28.9 ± 0.1 Ma at the bottom to 26.5 ± 0.8 Ma at the top (lower part of rhyolitic lava flows). These ages are concordant with the oldest precisely dated alkaline lava flows from the harrats (Harrat Hadan) of Saudi Arabia. The majority of the analysed samples from two distinct dyke swarms mainly trending N120–150 °E give ages of around 25.5 Ma and 16–18.5 Ma, respectively. These dyke swarms do not reflect any drastic change in stress regime in this region during this period, but show, in the vicinity of the Gulf of Aden, the existence of volcano-tectonic trends similar to those of the Red Sea Rift in Saudi Arabia. mical analysis (trace element and isotopic ratio) of these samples show a break at 21–22 Ma contemporaneous with a major magmatic event of tholeiitic affinity occurring over nearly 1700 km along the Red Sea Rift in Saudi Arabia.