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The island of São Tomé is 47 km long and covers an area of 854 km2; the summit of Pico S. Tomé is at 2023 m. The southern and western parts of the island are rugged with a more gentle topography in the north and east; there are impressive cliffs along much of the coast. There are many volcanic cones and craters while spectacular plugs and necks of phonolite and trachyte rise from the rain forest in the southern part of the island, as illustrated with photographs by Carvalho (1921). The oldest rocks exposed occur in stream sections in a small area on the eastern side of the island. They are conglomerates, sandstones and shales which may be Cretaceous in age. Basalts are the predominant rocks but there is a continuous series through trachybasalt to trachytic and phonolitic rocks with no apparent compositional gap (Fitton, 1987). Submarine eruptions are represented by palagonitic tuffs and pillow lavas. The basalts are essentially alkaline with basanites and tephrites predominant but hy normative types are also represented. The mineralogy is essentially olivine, augite and magnetite in a groundmass of labradorite-bytownite, augite, iron oxides and accessory titanite; analcime is abundant and nepheline occurs in the groundmass of some rocks. More evolved rocks comprise phonolite and trachyte the former including varieties with haüyne and nepheline. The haüyne-bearing phonolites, which are comparable with tahitites, contain phenocrysts of andesine, albite-oligoclase, aegirine-augite, hornblende, haüyne, titanite and iron oxides in a feldspathic groundmass. Some varieties are characterised by K2O > Na2O; these rocks grade into trachytes. Petrographic details are given by Mitchell-Thomé (1970), based principally on earlier accounts of Carvalho (1921) and Assunção (1957), who also cites much other work. Whole rock analyses are given in these papers and by Fitton (1987) demonstrating the continuous nature of the series. There are photomicrographs in Carvalho (1921). Rb-Sr, Sm-Nd, U, Pb and O isotopic data for five rocks will be found in Halliday et al. (1988), and Hf, Sr, Nd and Pb isotope data on a further basalt in Ballentine et al. (1997). A detailed palaeomagnetic study has been made by Piper and Richardson (1972).
ASSUNÇÃO, C.T. de 1957. Alguns aspectos da petrografia da Ilha de S. Tomé. Garcia de Orta, Junta das Missões Geográficas e de Investigações do Ultramar, 5: 497-515.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.CARVALHO, A.F. de 1921. As rochas da ilha de S. Tomé. Publicações do Museu Mineralógico e Geológico da Universidade de Coimbra. Memórias e Notícias, 1: 9-24.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.FITTON, J.G. and DUNLOP, H.M. 1985. The Cameroon line, West Africa, and its bearing on the origin of oceanic and continental alkali basalt. Earth and Planetary Science Letters, 72: 23-38.GRUNAU, H.R., LEHNER, P., CLEINTUAR, M.R., ALLENBACH, P. and BAKKER, G. 1975. New radiometric ages and seismic data from Fuertaventura (Canary Islands), Maio (Cape Verde Islands), and Sao Tome (Gulf of Guinea). In G.J. Borradaille, A. R. Ritsema, H.E. Rondeel and O.J. Simon (eds) Progress in geodynamics. (Proceedings of the National Symposium on Geodynamics held in Amsterdam, April 3-4, 1975). 90-103. North-Holland, Amsterdam.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.MITCHELL-THOMÉ, R.C. 1970. Geology of the South Atlantic islands. 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.