GEOCHEMICAL CHARACTERISTICS OF BASALTS FROM THE TENDAHO AND DOBE GRABENS, CENTRAL AFAR, ETHIOPIA

The Afar Depression of Ethiopia, Eritrea, and Djibouti resides at the northern terminus of the East African Rift system where the Main Ethiopian Rift meets the on-land projections of the Red Sea and Gulf of Aden rifts. This region has been actively extending and volcanically productive since ~30 Ma. The on-land projections of the oceanic rifts, the Red Sea propagator and the Gulf of Aden propagator, form an overlap zone that manifests in NW-trending grabens such as the Dobe Graben. The NW-trending Tendaho Graben represents the southern extent of the Red Sea propagator. Both grabens displace and contain packages of ~2 Ma basaltic lavas, with basalts flooring the Tendaho Graben (TG) erupting as recently as ~35 ka. Moreover, the progressive younging of basalts toward the center of the TG suggests an active diking origin, an interpretation supported by magnetic and gravity data. This contrasts with the DG that appears to be deforming via mechanical stretching. Basalts spanning the documented age ranges and geophysical characteristics were collected from ~SW-NE and ~NW-SE transects in the DG and along an ~W-E transect across the TG. Compositionally these lavas all are evolved, high-Fe tholeiites, typical of the Afar “stratoid” basalts. No simple correlated spatial-temporal-geochemical variations are observed, suggesting that broadly similar sources and processes were involved in the generation and evolution of central Afar basaltic magmas over the past ~2 Ma. This holds within- and between grabens, although two basalt sub-groups are defined on TiO₂ content. Low- TiO₂ (2.5-3 wt%) and high-TiO₂ (~3.5 wt%) basalts are present in the TG and DG, both illustrating a range in SiO₂ from ~47-52 wt%, but only those with low-Ti have MgO ranging to > 6 wt%. Element-element plots involving Si, Mg, K, P, Ba, and Zr suggest two general basalt differentiation mechanisms and paths; closed system fractional crystallization, and fractional crystallization accompanied by assimilation of felsic crustal materials or melts. The K/P ratio in basalts is particularly sensitive to felsic input during differentiation and this ratio is constant across the TG and shows systematic increase with degree of basalt differentiation in the SW-NE DG transect. These petrogenetic issues will be further evaluated using additional trace element and Sr isotope data.