Melanie Siegburg (U. of Southampton)

Geological Evolution of Boset-Bericha Volcanic Complex, Ethiopia, northern East African Rift

Continental rifting and break up is accompanied by tectonism and magmatism that varies over time in the Main Ethiopian Rift (MER) at the northern part of the East African rift. The Boset-Bericha volcanic complex (BBVC) is one of the largest stratovolcano complex in the MER, but little is known about the eruptive history, despite the fact that 4 million people live within 100 km of the volcano. The BBVC consists of the Gudda Volcano and the younger Bericha Volcano, two silicic eruption centres along the rift axis. The main volcanic activity of the BBVC is estimated to have occurred over the last ~2 Ma. Here I present a detailed
relative eruption chronology for the BBVC from morphometric analyses and mapping using high resolution LiDAR data. Petrographical and whole rock analyses provide new constrains on magmatic evolution at the BBVC.
The formation of the BBVC can be differentiated into 5 main eruption stages, subdivided in at least 12 phases with in total 128 mapped lava flows. Cross-cutting relationships of lava flows and eruption phases provide a relative chronology of the eruption history of the BBVC, starting with pre-BBVC rift basalts, pre-caldera and caldera activity and two post-caldera phases at
the Gudda Volcano. The Bericha Volcano formed during two main phases, followed by another post-caldera phase at Gudda Volcano. At least four fissure eruption phases recurred on the rift axis temporally in between the main eruptions phases. Morphometric analyses indicate a total corrected volume of eruptive material at the BBVC of ~36 km3, of which 68 % is
felsic, in particular erupted by the pre- and first post-caldera phase of the Gudda Volcano, 25 % is basaltic and 7 % has an intermediate composition. The total area of the BBVC is ~870 km2, of which ~62 % are rift basalts. Magma evolution is spatially and temporally complex shown by a bimodal distribution of rift floor basalts as well as peralkaline trachytic and rhyolitic lavas for the Gudda and Bericha Volcano, respectively, with rare intermediate lavas from fissure eruptions. Main compositional correlations indicate that fractional crystallisation may have played a major role in the development of magmas at the BBVC. Different FeO, MnO and Al2O3 contents at the Gudda and Bericha lavas may suggest different parental magmas
and/or a separate feldspar crystallisation due to changed conditions in the magma chamber of the Gudda volcano. Detailed differentiation of processes for magmatic evolution will be investigated by trace element and isotopic analyses. The composition and morphometry of lava flows changed over relative time from mafic rift floor basalts covered a large area, to explosive large – volume activity at silicic centres, which demonstrates a complex development of the BBVC.