Petrology of the Volcanic Rocks of Saint Helena Island, South Atlantic

Detailed field mapping of the two deeply eroded shield volcanoes of Saint Helena has permitted a reconstruction of their growth from initiation of sub-aerial activity approximately 14.3 ± 1.0 m.y. ago to its cessation some 7 or 8 m.y. later. Rocks of the alkali basalt-trachyte-phonolite assemblage forming the volcanoes demonstrate progressive mineralogical, textural and chemical variations from basic to salic ends of the series. Major oxide contents and a number of trace element abundances have been studied for 70 specimens of known stratigraphic position. The intermediate and trachytic rocks are believed to be derived from a mildly nepheline-normative parental basalt, dominantly by processes of crystal fractionation. Volatile transfer and migration of alkali-rich late-crystallizing fluids probably played the major role in the derivation of late, markedly undersaturated phonolitic rocks. The composition of the parental basalt remained nearly constant throughout the history of the volcanoes, and intermediate and salic rocks increased in volume and frequency of occurrence during their growth. It is suggested that on Saint Helena the increasing production with time of derivatives from the parental basalt was dependent upon the development of a high level magma reservoir during the course of sub-aerial volcanism.