Geochemistry, Mineralogy and Magmatic Evolution of the Basaltic and Trachytic Lavas from Gough Island, South Atlantic

The Gough Island lavas range from picrite basalt through to sodalite-bearing aegirine-augite trachyte. The basaltic lavas are predominantly nepheline normative alkali basalts, although a group of hypersthene normative tholeiitic basalts does occur. The oldest lavas on the island, represented by the Lower Basalt series, are approximately 1·0 m.y. old and the youngest are the Upper Basalts with an age of ˜ 0·13 m.y. Relatively coherent variations are described by the basaltic and trachytic lavas with respect to both bulk rock major and trace element geochemistry and mineral chemistry, and quantitative petrogenetic modelling suggests that most of the variation can be attributed to crystal fractionation/accumulation processes acting on a number of geochemically distinct parental magmas. The Upper Basalts and Lower Basalts have (within the limits of sampling) a relatively restricted composition compared to the Middle Basalt series lavas, with the latter ranging from picrite basalt through to trachyandesite. The picrite basalts and coarsely pyroxene-olivine phyric basalts represent partial cumulates with varying proportions (up to 40 wt. per cent) of accumulated olivine and clinopyroxene. In contrast, the moderately phyric and aphyric/finely porphyritic lavas represent the products of crystal fractionation with the most evolved lavas having experienced at least 40 per cent fractional crystallization of clinopyroxene, olivine, plagioclase and minor Fe-Ti oxides and apatite. The detailed abundance variations in these lavas indicate that a number of parental magma compositions have fractionated to produce the overall variations in basalt geochemistry, and some of the magmas have interacted through mixing processes. The trachytic lavas show a large range in trace element abundance, but have only a limited major element variation. Most of this variation can be attributed to extensive (up to 70 per cent) fractional crystallization of predominantly alkali feldspar with minor clinopyroxene, olivine, biotite, titano-magnetite and apatite. A number of genetically distinct trachytes can be recognized which are probably not related to each other by any simple fractional crystallization process. The composition of the least evolved trachytes can be adequately accounted for by relatively extensive (up to 60 per cent) fractionation of the more evolved Middle Basalt series lavas. The trace element and isotopic characteristics of primitive Gough Island basalts support the concept that the source region(s) giving rise to these lavas is extremely enriched in highly incompatible elements relative to primordial or ‘undepleted’ mantle of bulk earth composition. It is unlikely that the lavas have sampled undepleted mantle as might be suggested by the similarity of the Sr and Nd isotopic ratios to ‘bulk earth’ values. Rather, a model is favoured whereby the lavas are derived from previously enriched sub-oceanic mantle which was subsequently invaded and further enriched, at some time prior to partial melting, by melts or fluids highly enriched in incompatible elements. The enrichment could have occurred as veining by small degree partial melts or by infiltration of metasomatic fluids.