40. Petrology and Geochemistry of Rocks from the Angola Basin Adjacent to the Walvis Ridge: Deep Sea Drilling Project Leg 75, Site 530

Deep Sea Drilling Project Leg 75 drilled into igneous basement in the Angola Basin at Site 530, located about 20 km north of the escarpment at the eastern end of the Walvis Ridge. Petrographic and geochemical characteristics of the basalts indicate that the basement is quite uniform, variations in chemical composition being related to weathering, crystal fractionation, and the modal abundances of individual minerals. These basalts are similar to those drilled at Sites 527 and 528 on the central section of the Walvis Ridge, but are less enriched in incompatible elements than basalts dredged from the eastern end of the Walvis Ridge. They show distinct compositional differences from mid-ocean ridge basalts, which suggests that the Walvis Ridge hot spot has influenced the basement of the southern part of the Angola Basin. Comparison of the chemistry of Site 530 basalts with those drilled during Leg 74 indicates that the implied variation in mantle source is not in accord with any simple along-ridge progression, but may be related to vertical zonation within the volcanic pile.