Principe

Having a maximum length of 19 km and width of 15 km the island of Principe rises to 948 m at the Pico do Principe. Much of the island is heavily forested which, together with an extensive laterite cover in places, makes it difficult to distinguish geological contacts. However, Neiva (1956) mapped the island and demonstrated that the northern half, which is generally low lying, is basaltic whereas the more mountainous southern half consists of phonolites and tephrites. The island basement consists of sub-horizontal basaltic lava flows which Fitton and Hughes (1977) subdivide into an older series of mildly alkaline basalts and hawaiites that are overlain by a younger series of basanites and nephelinites. The older series is cut by numerous dykes which are petrographically and chemically similar to the basaltic flows. The basaltic rocks are cut by spectacular plugs of phonolite, trachyte and tristanite and, mainly in the south, overlain by flows of phonolite. The petrography has been described in detail by Barros (1960), work which is summarised in full by Mitchell-Thome (1970). The older series of flows and dykes contains phenocrysts of titanaugite, plagioclase, titanomagnetite and in all but the most evolved hawaiites, olivine; apatite, hornblende and biotite phenocrysts also occur in the more evolved types. The groundmass comprises plagioclase, augite and titanomagnetite. Basanite flows with abundant phenocrysts of olivine and titanaugite and fewer of plagioclase set in a groundmass of the same minerals plus alkali feldspar, titanomagnetite and nepheline are the most prominent members of the younger series (Fitton and Hughes, 1977). There are also olivine nephelinites consisting of phenocrystal olivine set in a matrix of olivine, titanaugite, titanomagnetite and nepheline. Phonolite, which forms both plugs and flows, comprises phenocrysts of sanidine, aegirine-augite, nepheline and, in some rocks, sodalite and hornblende, set in a matrix of the same minerals. Trachytes are sanidine- and hornblende-phyric rocks with fine-grained matrices of sanidine. The rock of a prominent plug in the southeast of the island has been classified as tristanite (Fitton and Hughes, 1977), and contains phenocrysts, up to 2 cm in diameter, of anorthoclase overgrown by potassic oligoclase and then sanidine and smaller phenocrysts of aegirine-augite, hornblende, hexagonal pseudomorphs, probably after nepheline, and rare titanite, titanomagnetite and apatite in a groundmass of felted sanidine laths and aegirine-augite. Numerous rock analyses are given by Barros (1960) and Fitton and Hughes (1977) the latter including chemical data on pyroxenes. Rb-Sr, Sm-Nd, U, Pb and O isotopic data for seven rocks will be found in Halliday et al. (1988), and Hf, Sr, Nd and Pb isotope data on a further three basalts in Ballentine et al. (1997). A detailed palaeomagnetic study has been made by Piper and Richardson (1972) who found both normal and reversed magnetism in the lavas.