Alkaline Rocks and Carbonatites of the World

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Bericcio, Bora And Tullu Moye' (Tulle Moje)

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Occurrence number: 
054-00-041
Country: 
Ethiopia
Region: 
Main Ethiopian Rift
Location: 
Longitude: 39.1, Latitude: 8.15

The adjacent volcanoes of Bericcio and Bora are located between the Zwai (Ziway) and Koka lakes on the floor of the rift valley (Di Paola, 1973 and 1977), and immediately northwest of the Chilallo volcano (No. 42). Bora rises about 350 m above the rift floor and has a summit crater 1.5 km in diameter. Bericcio is about 500 m high and forms a more regular cone; it has a small summit crater. There are numerous other volcanic centres in the vicinity, particularly to the east and southeast, a significant one being Tullu Moye' which has a 700 m diameter summit crater, while several kilometres north of Tullu Moye' are several large obsidian flows that form domes the largest (Jano or Giano) about 3 km in diameter. Apart from the obsidian domes most of the volcanic structures are built predominantly of silicic pyroclastics including unwelded pumice, cinerites and ashes but there are also domes and flows of aphyric and porphyritic obsidians, perlites and lavas. Bora consists of alternating pumice and ash layers some of which are poorly welded; there are subordinate rhyolite lavas. Bericcio comprises unwelded pumices while Tullu Moye’ is also made of ash and pumice but overlying a base of trachytic and rhyolitic lavas. Ignimbrites are strongly welded close the eruptive vents and usually related to caldera collapse (Trua et al., 1999). Compositions of alkali feldspar and glass in a pantellerite from this locality are in Trua et al. (1999). Few petrographic data seem to be available but Di Paola (1977) indicates that most rocks are pantelleritic while Metrich and Clocchiatti (1979) describe a specimen of pantellerite from the 'Bora-Bereccio' complex as comprising phenocrysts of quartz, alkali feldspar, aegirine-augite and aenigmatite in a glassy mesostasis with a spherulitic texture. A general account of the mineralogy of the Tullu Moye’ area is that of Bizouard and Di Paola (1978) but they include under this heading adjacent localities. Compositions of alkali feldspar and glass in a pantellerite and seven analyses of pantellerite and comendite from Tullu Moye’ are in Trua et al. (1999).

Age: 
The Giano obsidian domes are less than 200 years old, according to local folk lore (Di Paola, 1977).
References: 
BIZOUARD, H. and DI PAOLA, G.M. 1978. Mineralogy of the Tullu Moje active volcanic area (Arussi: Ethiopian Rift valley). In E.-R. Neumann and I.B. Ramberg (eds) Petrology and geochemistry of continental rifts 87-100. D. Reidel, Dordrecht.DI PAOLA, G.M. 1973. The Ethiopian rift valley (between 7°00' and 8°40' lat. north). Bulletin Volcanologique, 36: 517-60.DI PAOLA, G.M. 1976. Geological map of the Tullu Moje' volcanic area (Arussi: Ethiopian rift valley). Consiglio Nazionale delle Ricerche, Italy. (unpublished).METRICH, N. and CLOCCHIATTI, R. 1979. Contribution à la connaissance des conditions de cristallisation et de mise en place d'une coulée pantelléritique du Rift Ethiopien: étude des inclusions vitreuses. Comptes Rendus des Séances de l'Académie des Sciences. Paris. 289D: 57-60.TRUA, T., DENIEL, C. and MAZZUOLI, R. 1999. Crustal control in the genesis of Plio-Quaternary bimodal magmatism of the Main Ehiopian Rift (MER): geochemical and isotopic (Sr, Nd, Pb) evidence. Chemical Geology, 155: 201-31.
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