TEMPORAL EVOLUTION OF MANTLE SOURCES DURING CONTINENTAL RIFTING - THEVOLCANISM OF DJIBOUTI (AFAR)

Magmatism occurred almost continuously over the past 25 m.y. in the Re public of Djibouti. Lavas are mainly basic to intermediate with some r hyolites. Large chemical and isotopic variations among the volcanic se ries are interpreted in terms of mantle source heterogeneity. Crustal contribution is only evidenced in the oldest rhyolites emplaced during the initial stages of rifting. Excluding these old rhyolites, a clear evolution through time of the mantle sources is observed in relation to rifting. Three sources were involved in the genesis of these lavas: (1) an old subcontinental lithospheric component (Sr-87/Sr-86 almost- equal-to Sr-86 almost-equal-to 0.706, Pb-206/Pb-204 almost-equal-to 17 .9), mainly observed in the oldest lavas (25 to 10 Ma), (2) an HIMU (h igh U/Pb ratio)-type reservoir, and (3) a depleted mantle. As rifting goes on, there is an increasing contribution of an HIMU-type mantle so urce. It is attributed to the influence of a mantle diapir (Afar plume ) thermally eroding the subcontinental lithosphere. The geochemical ch aracteristics of 9 to 1 Ma old lavas, erupted after the strong increas e of spreading rate in Afar, reflect this evolution of mantle sources. The influence of the mantle plume is most prominent in the northern y oungest lavas (<1 Ma), particularly Manda, characterized by the strong est HIMU signature (Sr-87/Sr-86 almost-equal-to 0.7035, Pb-206/Pb-204 almost-equal-to 19.2). The contribution of the depleted mantle compone nt originating from the asthenosphere is best recognized in the young (<4 Ma) lavas, particularly Tadjoura and Asal lavas (3 to 1 Ma). The e volution of Djibouti lava sources through time may be accounted for by the recent models developed for plume structure.