Alkaline Rocks and Carbonatites of the World

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Fantale

stripes

Occurrence number: 
054-00-036
Country: 
Ethiopia
Region: 
Main Ethiopian Rift
Location: 
Longitude: 39.9, Latitude: 8.97

Fantale is a strato-volcano 600 m high extending over about 100 km2 and composed predominantly of pantelleritic trachytes and obsidians in which tuffs are subordinate (Gibson, 1970). The summit area is dominated by a 3.5 km diameter caldera with a depth of 300 m. The main cone was initiated by extrusion of a series of plagioclase-fayalite-ferro-augite trachyte lavas which were followed by lavas of anorthoclase-fayalite-ferrohedenbergite trachyte and rhyolite, with the youngest flows of the main cone strongly pantelleritic rhyolites and obsidians, most of which contain fayalite and some of them aenigmatite. Activity was then concentrated on three satellite centres two of which produced anorthoclase-aenigmatite pitchstones and the third an obsidian free of phenocrysts. There was then erupted the pantelleritic Fantale ash-flow tuff which led to the development of the summit caldera. Post-caldera formation activity comprised eruption of trachytic flows within the caldera and subsequent extrusion of pantellerites in the caldera and from a number of vents on the flanks of the volcano. The only basaltic volcanism is to the southwest of Fantale where a 1 km diameter tuff-ring predates the ash-flow tuff and a series of basaltic flows emanated from a fissure system. However, Gibson (1975) considers these to be unrelated to the Fantale centre. The Fantale ash-flow tuff is estimated by Gibson (1970) to have extended over 100 km2 to an average depth of 20 m. On the higher slopes of the volcano the tuff is confined to three pre-existing valleys, where it is only several metres thick, but over the plains it may be 25 m or more in thickness. Five units were identified and are described and illustrated in considerable detail by Gibson (1970). The rock comprises pumice (fiamme), shards, dense glass and rock and crystal fragments and is welded to varying degrees. Variable vesiculation led to the formation of most unusual 'blisters' which may be up to 100 m across and several metres high. Phenocrysts of aenigmatite, sodic ferro-hedenbergite and anorthoclase, and in the upper flow units only fayalitic olivine, comprise less than 5% of the tuff. Numerous rock analyses of the Fantale ash-flow tuff are given in Gibson (1970) and analyses of 20 pantelleritic pitchstones, including both pre- and post-caldera samples, by Gibson (1972); earlier chemical data on Fantale pantellerites are collected in Macdonald and Bailey (1973). Sr87/Sr86 data are given by Dickinson and Gibson (1972) who also discuss the evolution of the volcano in terms of feldspar fractionation. Glass inclusions in phenocrysts from a late glassy pantellerite flow were studied by Webster et al. (1993) in order to constrain the pre-eruptive composition of the magma.

Age: 
Although a poorly defined Rb-Sr isochron indicated an age of 2.6 Ma (Dickinson and Gibson, 1972) the authors consider this to be in error and the volcano to be less than one million years old.
References: 
DICKINSON, D.R. and GIBSON, I.L. 1972. Feldspar fractionation and anomalous Sr87/Sr86 ratios in a suite of peralkaline silicic rocks. Bulletin, Geological Society of America, 83: 231-40.GIBSON, I.L. 1970. A pantelleritic welded ash-flow tuff from the Ethiopian rift valley. Contributions to Mineralogy and Petrology, 28: 89-111.GIBSON, I.L. 1972. The chemistry and petrogenesis of a suite of pantellerites from the Ethiopian rift. Journal of Petrology, 13: 31-44.GIBSON, I.L. 1975. A review of the geology, petrology and geochemistry of the volcano Fantale. Bulletin Volcanologique, 38: 791-802.MACDONALD, R. and BAILEY, D.K. 1973. The chemistry of peralkaline oversaturated obsidians. United States Geological Survey, Professional Paper, 440-N-1: 1-37.WEBSTER, J.D., TAYLOR, R.P. and BEAN, C. 1993. Pre-eruptive melt composition and constraints on degassing of a water-rich pantellerite magma, Fantale volcano, Ethiopia. Contributions to Mineralogy and Petrology, 114: 53-62.
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