Alkaline Rocks and Carbonatites of the World

Setup during HiTech AlkCarb: an online database of alkaline rock and carbonatite occurrences

Boina (Dabbahu)


Occurrence number: 
Longitude: 40.5, Latitude: 12.63

The name Dabbahu is a general one for this volcanic region the upper part of which is called Boina. Barberi et al. (1975b), however, proposed that the term Boina be applied to the whole volcanic unit. The area is 750 km2 and bounded by the Alayta shield volcano to the north and east, basaltic lava fields to the south and by sedimentary formations to the west. The base comprises basaltic lavas that issued from fissures above which is a shield volcano comprising intermediate products and, as the final phase, peralkaline rhyolite flows and domes with subordinate pyroclastics. Brief petrographic descriptions of analysed rocks by Barberi et al. (1975b) indicate that the basalts comprise olivine (Fa6-40), plagioclase (An75-56), clinopyroxene and opaques. Although orthopyroxene is apparently not present all these rocks are hy normative. The intermediate rocks, referred to as andesine basalt and dark trachyte by Barberi et al. (1975b), are basaltic trachyandesites and trachyandesites. They grade into trachytes and quartz trachytes in which olivine, but more fayalitic, persists and plagioclase is more sodic and may be rimmed by alkali feldspar; some 50% of these rocks is glass. There is a continuous series from silica oversaturated trachytes through rhyolites and comendites to pantellerites. Many of the pantellerites, particularly those forming flows, are obsidians but alkali feldspar phenocrysts or crystallites may be present, as may green pyroxene; needles of aenigmatite occur and fayalite is found in comendites. The evolution of the rhyolitic rocks is discussed by Barberi et al. (1975a). Numerous chemical analyses, including a very full suite of trace elements, are given by Barberi et al. (1975b) and these demonstrate the continuous nature of the series.

Two obsidians from the upper part of the volcano gave fission track ages of 44,000 and 10,500 years (Barberi et al., 1972a). There are many fumaroles located in the central areas of the volcano.

BARBERI, F., BORSI, S., FERRARA, G., MARINELLI, G., SANTACROCE, R., TAZIEFF, H. and VARET, J. 1972a. Evolution of the Danakil Depression (Afar, Ethiopia) in light of radiometric age determinations. Journal of Geology, 80: 720-9. BARBERI, F., CHEDEVILLE, E., FAURE, H., GIGLIA, G., MARINELLI, G., SANTACROCE, R., TAZIEFF, H. and VARET, J. 1973b. Geology of Northern Afar (Ethiopia) (with a geological map, scale 1:500,000). Revue de Geographie Physique et de Geologie Dynamique, Paris, 15: 443-89.BARBERI, F., SANTACROCE, R. and VARET, J. 1975a. Silicic peralkaline volcanic rocks of the Afar Depression (Ethiopia). Bulletin Volcanologique, 38: 755-90.BARBERI, F., FERRARA, G., SANTACROCE, R., TREUIL, M. and VARET, J. 1975b. A transitional basalt-pantellerite sequence of fractional crystallization, the Boina centre (Afar Rift, Ethiopia). Journal of Petrology, 16: 22-56.

Fig. 3_72 The distribution of Ethiopia occurrences 10-23 (after Barberi et al., 1973b).
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