Alkaline Rocks and Carbonatites of the World

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Boseti

stripes

Occurrence number: 
054-00-038
Country: 
Ethiopia
Region: 
Main Ethiopian Rift
Location: 
Longitude: 39.48, Latitude: 8.58

The Boseti volcanic complex is about 20 km in diameter and comprises the two adjacent volcanoes of Gudda and Bariccia both of which are over 2000 m high. Activity has been divided into two cycles (Brotzu et al., 1974 and 1980) with initial volcanism apparently centred on the Gudda centre. Products of the first cycle at Gudda were basalt flows that were followed by flows of trachyte, pantelleritic trachyte, pantellerite and comendite and these in turn by more voluminous activity including rhyolitic pumice flows, comenditic lavas and pantelleritic obsidians transitional to pumiceous rocks. The end of this cycle is marked by pumiceous fall deposits which correlate with the formation of a caldera. The second cycle at Gudda opened with massive flows of porphyritic to aphyric pantellerite and pantelleritic trachyte followed by various flow and ash units principally of pantellerite but with some pantelleritic trachytes and alkali rhyolites. The birth of the Bariccia volcano was probably contemporaneous with development of the post-caldera second cycle at Gudda (Brotzu et al., 1980). An older cycle opened with thick flows of trachyte and alkali trachyrhyolite which were followed by pumice flows that are sometimes welded and these alternate with trachytic comendites and rare alkali rhyolites; it closed with explosive activity that gave rise to a pumice cone. The younger cycle consists of various flows of comenditic composition and activity closed with expulsion of long flows of mugearite and benmoreite between the two centres. The early basalts contain phenocrysts of olivine, labradorite, augite and an opaque phase in a matrix of a similar composition. There is a complete series from the basalts through hawaiites to mugearites and benmoreites in which plagioclase trends to andesine and alkali feldspar increases in abundance. The trachytes are characterised by oligoclase, augite, rare olivine, opaque and alkali feldspar phenocrysts in a groundmass of oligoclase, alkali feldspar, clinopyroxene, opaques and a little quartz. These rocks grade into alkali rhyolites with an increase in quartz and the presence in some of a reddish-brown amphibole. Comendites contain phenocrysts of alkali feldspar, fayalitic olivine, augite, opaques and sometimes brown amphibole, whereas the pantellerites of Gudda have phenocrysts of aenigmatite, alkali feldspar, clinopyroxene and rare amphibole. Rock analyses are given by Brotzu et al. (1974) and further analyses, including abundant trace element data, by Brotzu et al. (1980). A detailed study of the influence of magma composition and oxygen fugacity on pyroxenes throughout the basalt-pantellerite series was made by Carbonin et al. (1991), who also give modal and compositional details of phenocrysts in the 18 specimens they investigated.

Age: 
K-Ar on a specimen from Gudda gave 1.6(0.3 Ma (Morbidelli et al., 1975) but fumaroles on Bariccia suggest recent activity.
References: 
BROTZU, P., MORBIDELLI, L., PICCIRILLO, E.M. and TRAVERSA, G. 1974. Petrological features of Boseti Mountains, a complex volcanic system in the axial portion of the main Ethiopian rift. Bulletin Volcanologique, 38: 206-34.BROTZU, P., MORBIDELLI, L., PICCIRILLO, E.M. and TRAVERSA, G. 1980. Volcanological and magmatological evidence of the Boseti volcanic complex (main Ethiopian rift). In Geodynamic evolution of the Afro-Arabian rift system. Accademia Nazionale dei Lincei. Atti dei Convegni Lincei. 47: 317-66.CARBONIN, C., DAL NEGRO, A., GANEO, S. and PICCIRILLO, E.M. 1991. Influence of magma composition and oxygen fugacity on the crystal structure of C2/c clinopyroxenes from a basalt-pantellerite suite. Contributions to Mineralogy and Petrology, 108: 34-42.MORBIDELLI, L., NICOLETTI, M., PETRUCCIANI, C. and PICCIRILLO, E.M. 1975. Ethiopian south-eastern plateau and related escarpment: K/Ar ages of the main volcanic events (main Ethiopian rift from 8°10' to 9°00' lat. north). In A. Pilger and A. Rosler (eds) Afar Depression of Ethiopia. Inter-Union Commission on Geodynamics. Scientific Report 14: 362-9. E. Schweizerbart'sche
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