ALKALINE VOLCANO OF PALEOCENE AGE ON THE SOUTHERN GUINEAN MARGIN - MAPPING, PETROLOGY, AR-40-AR-39 LASER PROBE DATING, AND IMPLICATIONS FORTHE EVOLUTION OF THE EASTERN EQUATORIAL ATLANTIC

A segment of the Southern Guinea Plateau Margin (S.G.P.M.) was surveye d during the 1988 Equamarge II cruise of the R/V Jean Charcot. Detaile d Seabeam mapping reveals a huge 3000 m high volcano which was called the Nadir seamount, culminating at 840 m below sea level, and 6 advent ive cones. Volcanics dredged between 2100 and 1300 m of water depth co nsist of hyaloclastite breccias and subaerial alkali basalts recovered as both massive blocks and eolian pebbles with a desert varnish. The data are consistent with the growth of a volcano up to emergence and, thereafter, its dismembering and subsidence. Volcanological, petrograp hic and geochemical features (REE) of the volcanics present similariti es with those observed (1) in early basalts from Fuerteventura (Canary Islands) and Maio (Cape Verde Islands), (2) in lamprophyric volcanism from the Sierra Leone Rise and (3) in volcanics from the Gorringe ban k. Ar-40-Ar-39 laser probe dating was performed on two biotite phenocr ysts from a massive basalt, using step heating and spot fusion procedu res. Both experiments concordantly display a plateau-age of 58.6+/-0.3 Ma and an integrated age of 58.6+/-0.7 Ma, respectively. The Nadir se amount provides the third evidence of Paleocene alkaline volcanism in the Eastern Atlantic after the Gorringe bank and the northern Sierra L eone Rise (Krause seamount). These volcanoes are presumed to have been controlled by Fractures Zones. The ages and compositions of the Nadir and Krause seamounts, respectively, suggest a connection between the volcanic chains of both the Sierra Leone Rise and Guinea F.Z. which ma y have recorded the same hotspot track.