Discovery tablemount basalt: Petrology and geochemistry

Dredged porphyritic basalt from the Discovery Tablemount, South Atlantic Ocean, has been described and analysed, and the rare earths determined. The basalt is a quartznormative tholeiite but mineralogically and chemically (including trace elements) has affinities with alkali basalt; the pyroxene trend falls between those of alkali basalts and Hawaiian tholeiites. In addition to reverse zoned labradorite phenocrysts, corroded feldspar xenocrysts are present, ascribed to pressure fluctuations. The basalt has an alteration rind showing chemical variations which in general agree with other published data; surprisingly, however, the rind shows a loss of potash. There is noticeable enrichment in the lighter rare earth elements and an unusually high [La/Sm]_E.F ratio; when taken together with the tablemount's position near known hotspots, this evidence leads to the suggestion that the basalt derived from a small mantle plume. The age of the basalt capping is 25 m.y., compared with 67 m.y. for the nearest magnetic anomaly. It is suggested that, unless the tablemount was intrusive, it may have drifted to its present postion from a point over a fixed mantle hotspot with which, however, it remained in contact through a thermal and hydrodynamical channel.