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An ash-flow tuff of 3.3 meters thickness cored from a seamount at the southwest extremity of the Walvis Ridge (~ 1000 km east of the Mid-Atlantic Ridge) has trachytic bulk composition (SiCh 62.5 wt %; total alkalis 12 wt %) and contains abundant alkali feldspar (avg. AmAbsyOns>), clinopyroxene (FsisEn37Wo*j), biotite, and Fe-Ti oxides in a dusty, partly devitrified matrix. Moderately compacted pumice lapilli are abundant, but no original shard structures remain. The relative freshness and absence of graded bedding and marine material indicate that the tuff was emplaced subaerially. Fossils in overlying sediments indicate an open-marine environment about 40 m.y. in age. Within those sediments are three ash layers, 3.6,4.0, and 4.6 meters above the tuff. The ash is composed of highSiθ2 glass (~69 wt %) and contains alkali feldspar, hedenbergite, and Fe-Ti oxides. The feldspar in the trachytic ash flow gives a K-Ar age of 40.1 ±1 m.y.. The ash-flow tuff is compositionally similar to trachytes on the nearby islands of Tristan da Cunha and Gough (moderately alkalic volcanic provinces). The seamount it caps probably represents a volcanic center that originated on or near the Mid-Atlantic Ridge at least 50 m.y. ago (based on spreading rates averaging 2 cm/yr and its location between magnetic anomalies 21 and 31), differentiated from basalt to trachyte, and migrated northeastward. Overlying open-marine sediments about the same age as the tuff indicate that the tuff was submerged shortly after its emplacement; this probably happened by collapse of a summit caldera. The ash layers in overlying sediments indicate that intermediate to silicic volcanism persisted in the region of the seamount until at least late Eocene.

Petrology and K-Ar Age of Volcanic Tuff and Ash from the Walvis Seamount Province, DSDP Site 359, Leg 39