Alkaline Rocks and Carbonatites of the World

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Pagalu (Annobon)

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Occurrence number: 
051-00-002
Country: 
Equatorial Guinea
Location: 
Longitude: 5.62, Latitude: -1.43

Pagalu, formerly known as Annobon, is the southernmost and smallest (17 km2) of the chain of volcanic islands in the Gulf of Guinea. Pico de Fogo the highest point on the island (813 m) is a bare volcanic cone in which there is a crater lake. The island is the upper part of a large stratovolcano sited on oceanic crust, the oldest part of which consists of palagonitic breccias intruded by basaltic dykes; these outcrop around much of the coast. The basaltic dykes are basanites rich in pyroxene and olivine phenocrysts. The second stage of the volcano consists of basanite flows, although Fitton (1987) also reports the presence of hy normative basalts, which cover most of the island. These are cut by benmoreite and trachyte plugs and are overlain in the south and northwest by younger basanite flows the most northerly one of which is rich in peridotite nodules (Cornen and Maury, 1980). The basanites (ca. 42-44% SiO2; 7-12% ne) contain olivine, clinopyroxene and more rarely magnetite phenocrysts in a matrix of these minerals plus plagioclase, ilmenite, apatite, glass and rare sanidine. A variant of these rocks, called potassic hawaiite by Cornen and Maury (1980), has more plagioclase and abundant phenocrysts of kaersutite, magnetite, apatite and titanite in a groundmass with alkali feldspar. The benmoreite of the plugs is a potassic type, referred to as tristanite by Cornen and Maury (1980) and others, and consists of phenocrysts of plagioclase, abundant kaersutite, magnetite, apatite and titanite in a groundmass of clinopyroxene, magnetite, ilmenite, plagioclase and alkali feldspar. The trachytes are silica oversaturated with phenocrysts of mica, abundant alkali feldspar, magnetite, apatite and titanite in a groundmass of alkali feldspar, tridymite, magnetite and ilmenite. Microprobe analyses of a range of minerals and rock analyses are given by Cornen and Maury (1980); rock analyses including REE and other trace elements are in Liotard et al. (1982). Rb-Sr, Sm-Nd, U, Pb and O isotopic data for an alkali basalt will be found in Halliday et al. (1988), and Hf, Sr, Nd and Pb isotope data on two further samples in Ballentine et al. (1997). Earlier work is summarised by Mitchell-Thome (1970).

Age: 
Piper and Richardson (1972) report K-Ar ages of 19.2 and 17.5 Ma for the base of the succession and 2.8 and 2.4 Ma for one of the younger northern flows. Cornen and Maury (1980) give ages of 5.35(0.25 Ma for a dyke and 3.9(0.2 Ma for tristanite.
References: 
BALLENTINE, C.J., LEE, D.-C. and HALLIDAY, A.N. 1997. Hafnium isotopic studies of the Cameroon line and new HIMU paradoxes. Chemical Geology, 139: 111-24.CORNEN, G. and MAURY, R.C. 1980. Petrology of the volcanic island of Annobon, Gulf of Guinea. Marine Geology, 36: 253-67.FITTON, J.G. 1987. The Cameroon line, West Africa: a comparison between oceanic and continental alkaline volcanism. In J.G. Fitton and B.G.J. Upton (eds), Alkaline igneous rocks. 273-91. Geological Society of London Special Publication 30.HALLIDAY, A.N., DICKIN, A.P., FALLICK, A.E. and FITTON, J.G. 1988. Mantle dynamics: a Nd, Sr, Pb and O isotopic study of the Cameroon Line volcanic chain. Journal of Petrology, 29: 181-211.LIOTARD, J.M., DUPUY, C., DOSTAL, J. and CORNEN, G. 1982. Geochemistry of the island of Annobon, Gulf of Guinea. Chemical Geology, 35: 115-28.MITCHELL-THOME, R.C. 1970. Geology of the south Atlantic islands. Beitrage zur Regionalen Geologie der Erde, 10. Gebruder Borntraeger, Berlin. 367 pp.PIPER, J.D.A. and RICHARDSON, A. 1972. The palaeomagnetism of the Gulf of Guinea volcanic province, West Africa. The Geophysical Journal of the Royal Astronomical Society, 29: 147-71.
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