Volcanics from the Sierra Leone Rise

-THE Sierra Leone Rise, located in the east equatorial Atlantic. forms a discontinuous chain of seamounts as shallow as 2 km extending with a general NE-SW trend from near the Sierra Leone Coast of Africa, to the St Paul fracture zone near the Mid-Atlantic Ridge (Fig. 1). The origin of this feature has remained a topic of discussion. Sheridan et al.' have hypothesised that the Sierra Leone Rise is a volcanic structure formed at the beginning of the opening of the Atlantic in the early Cretaceous per iod . \~he twin features of the Sierra Leone and the Ceara Rises are probably of oceanic origin and were created 80 Myr ago or later in their present-day position with respect to Africa and South ~ m e r i c a ' . The Atlantic ocean exhibits several similar aseismic structures which appear symmetrically oriented with respect to the mid-oceanic ridge. such as the Walvis-Rio Grande Rise and the Iceland Faeroes-lceland Greenland Ridges. These structures arc volcanic edifices having a composition similar to that found in their associated islands' '. Deep sea drilling of the C'eara ~ i se" . ' penetrated a basaltic hasement of the upper Cretaceous period (Maestrichtian) (Leg 39, Site 354). Similarly. a DSDP hole (Leg 4 1. Site 366) on the Sierra Leone Rise, penetrated sedimentsof the same period. without reaching hasement"'. We report here the discovery of alkali-rich volcanics in an area of the Sierra Leone Rise. The sediment overlying the rock fragments is aged -45 Myr. During the Romancha cruise of the RV Jean Charcot in December 1977 we studied the south eastern slopes of the Sierra Leone Rise. near DSDP Site 366. with the objective of sampling crustal material (Fig. 1). From a detailed bathymetric survey, using a multichannel narrow beam echo-sounder, it was possible to locate the best existing outcrops for dredging. However, despite this. our dredging operation proved unsuccessful, mainly