First [formula omitted] dating on early pliocene to plio-pleistocene magmatic events of the Afar — Republic of Djibouti

Abstract

Many detailed geochemical, geophysical and structural studies have been made in the Afar — Republic of Djibuti, but its geologic evolution remains poorly constrained in time. To improve the understanding of the timing of significant geologic events, we analyzed by the 40Ar 39Ar stepwise heating method twenty-one samples (plagioclases, sanidines and whole rocks) carefully selected from each main volcanic formation. Fifteen plateau ages were obtained, whereas the other samples revealed alteration processes and/or excess argon components. The oldest reliable age is obtained on an altered lava flow from the Ali Sabieh formation, giving a plateau age at 23.6 ± 0.5 Ma. The dyke swarm of the Ali Sabieh area gave homogeneous ages of 17-16 Ma on dykes of different compositions (rhyolitic and basaltic) and orientations (N-S, N140–160 °E and N35 °E), similar to those obtained on some dyke swarms of Southern Yemen. Two plateau ages obtained on two lava flows from the bottom and the top of the Mablas series seem to indicate a period of activity slightly shorter (15.4 ± 0.3 to 11.8 ± 0.6 Ma) than that deduced from conventional K Ar data (16.3 to 9.7 Ma). Plateau ages from 2.0 ± 0.2 to 1.00 ± 0.09 Ma were obtained on lava flows from different locations of the Stratoid series. They are concordant with previous conventional K Ar data obtained by Courtillot and co-workers, but ages corresponding to the earliest part of the 4-1 Ma period, generally accepted for the whole Stratoid series, were not found. Precise plateau ages ranging from 1.03 ± 0.03 Ma to 0.25 ± 0.02 Ma were obtained on lava flows and intrusions from the Asal and Manda Inakir rifts and the Moussa Ali volcano. These new data slightly modify parts of the generally accepted period of emplacement of the main volcanic units of Afar, and show that some previous conventional K-Ar ages may be affected by argon loss (alteration) and/or excess argon.