Abstract
Fantale (Lat. 8° 58′ N., Long. 39° 54′ E) is a typical Quaternary silicic strato-volcano situated on the floor of the northern part of the Main Ethiopian Rift. Following the growth of the main central cone, a voluminous ash-flow tuff was erupted in association with the development of the 4 km summit caldera. On the upper parts of the volcano the tuff is virtually restricted to three major eruptive pathways descending from the caldera. The tuff is more extensive on the lower flanks and forms a continuous sheet surrounding the volcano.
The tuff is welded throughout, even when it is less than 1 m thick. It shows typical vitroclastic texture and is markedly eutaxitic in the intensely welded sections. The degree of welding appears to he largely independent of the total thickness of the tuff. Microscopic observations provide confirmatory evidence of postdepositional vesiculation of the tuff and also suggest that recrystallisation has not induced the development of cavities in the tuff.
The detailed analytical data presented suggest that the chemical composition of the tuff was modified by recrystallisation which produced relative depletion in SiO2, total Fe, Cl, and Na2O and enrichment in Al2O3, K2O and CaO. The trace elements Ba, La, Nb, Rb, and Zr were not affected by this process. Study of serial samples from three sections through the tuff has led to the identification of five of the constituent flow units of the tuff. The units are approximately homogeneous in composition but differ from each other. The earliest unit is more silicic and pantelleritic, whereas the later flow units are more trachytic in composition. This data is interpreted in terms of the progressive emptying of a small but strongly zoned high-level magma chamber.
The presence of primary laminar flowage structures, postdepositional vesiculation effects, intense welding in thin units and high initial dips suggests that the Fantale tuff was deposited from a series of dense, perhaps partially fluidized, magma pulses.
Similar content being viewed by others
References
Ewart, A.: Mineralogy and Petrogenesis of the Whakamuru ignimbrite in the Maraetai area of the Taupo volcanic zone, New Zealand. New Zealand J. Geol. Geophys. 8, 611–677 (1965).
Gibson, I. L.: Preliminary account of the volcanic geology of Fantale, Shoa, Ethiopia. Bull. geophys. Obs. Addis Ababa, 10, 59–67 (1967).
—, Tazieff, H.: Additional theory on the origin of fiamme in ignimbrites. Nature (Lond.) 215, 1473–1474 (1967).
Katsui, Y.: Evolution and magmatic history of some Krakatoan calderas in Hokkaido, Japan. Hokkaido Univ. Fac. Sci., ser. 4, Geol. and Mineral. 11, 631–650 (1963).
Lacroix, A.: Les roches hyperalcalines du massif du Fantale et du col de Balla. Mem. Soc. Geol. France 14, 89–102 (1930).
Lipman, P. W.: Mineral and chemical variations within an ash-flow sheet from Aso Caldera, southwestern Japan. Contr. Mineral. and Petrol. 16, 300–327 (1967).
- Christiansen, R. L., O'Conner, J. T.: A compositionally zoned ash-flow sheet in southern Nevada. U.S. Geol. Surv. Profess. Paper 524-F, 47 p. (1966).
Nicholls, J., Carmichael, I. S. E.: Peralkaline acid liquids; a petrological study. Contr. Mineral. and Petrol. 20, 268–294 (1969).
Noble, D. C.: Gold flat member of the Thirsty Canyon Tuff — a pantellerite ash-flow sheet in southern Nevada. U.S. Geol. Surv. Profess. Paper 525-B, 85–90 (1965).
Ramsay, J. G.: Folding and fracturing of rocks. 568p. McGraw-Hill (1967).
Ratte, J. C., Steven, T. A.: Magmatic differentiation in a volcanic sequence related to the Creede Caldera. U.S. Geol. Surv. Profess. Paper 476-D, D49-D53 (1964).
Rohleder, H. P. T., Stanfield Hitchen, C.: Volcanologische Beobachtungen längs der Bahnlinie Adis Abebe — Djibouti (Abessinien). Z. Vulk. 12, 269–289 (1930).
Ross, C. S., Smith, R. L.: Ash-flow tuffs — Their origin, geologic relations and identification. U.S. Geol. Surv. Profess. Paper 366, 81 p. (1961).
Schmincke, H. U., Swanson, D. A.: Laminar viscous flowage structures in welded ash-flow tuffs from Gran Canaria, Canary Islands. J. Geol. 75, 641–664 (1967).
Smith, R. L.: Ash-flows — a review. Geol. Soc. Am. Bull. 71, 795–842 (1960a).
—: Zones and zonal variations in welded ash-flows. U.S. Geol. Surv. Profess. Paper 354-F, 149–159 (1960b).
— Bailey, R. A.: The Bandelier Tuff, a study of ash-flow eruption cycles from zoned magma chambers. Bull. volcanol. 29, 83–104 (1966).
Tazieff, H., Gibson, I. L.: Sur la genèse de l'ignimbrite pantelléritique de Fantalé. Compt. Rend. 265, 950–953 (1967).
Teillhard de Chardin, P., Lamare, P.: Le canon de L'aouache et le volcan Fantale. Mem. Soc. Geol. France 14, 13–20 (1930).
Walker, G. P. L.: Tertiary welded tuffs in eastern Iceland. Quart. J. Geol. Soc. (London) 118, 275–293 (1962).
Williams, H.: The geology of Crater Lake National Park, Oregon, with a reconnaissance of the Cascade Range southward to Mt. Shasta. Carnegie Inst. Publ. 540, 162p. (1942).
Zavaritsky, A. N.: The ignimbrites of Armenia. Akad. Nauk SSSR Izv., Ser. Geol. No. 3, 3–18 (1947).
Author information
Authors and Affiliations
Rights and permissions
About this article
Cite this article
Gibson, I.L. A pantelleritic welded ash-flow tuff from the Ethiopian Rift Valley. Contr. Mineral. and Petrol. 28, 89–111 (1970). https://doi.org/10.1007/BF00404992
Received:
Issue Date:
DOI: https://doi.org/10.1007/BF00404992