GEOLOGY AND GEOCHEMISTRY OF A 800-M SECTION THROUGH YOUNG UPPER OCEANIC-CRUST IN THE NORTH FIJI BASIN (SOUTHWEST PACIFIC)

We report the results of geological and structural observations made d uring one dive of the French submersible Nautile during the STARMER Cr UiSe in 1989 in the vicinity of the 16-degrees-40'S triple junction in the North Fiji Basin. This dive provided a spectacular, 800 m continu ous section of oceanic crust exposed along the northern wall of a 3000 m deep basin at the eastern branch of the triple junction. A number o f volcanic facies have been encountered including massive and pillowed lavas, sheeted dikes and spectacular columnar-jointed massive lavas. Pillowed lavas have been observed only at the base and at the top of t he section. The ratio between massive and pillowed basalts is high sug gesting that the crust in this area grew mostly during a stage of high eruptive rate. Dikes have been observed in the middle part of the sec tion. They probably do not represent the top of a classical ''dike com plex'' but an isolated set of sheeted dikes, possibly the feeders of o verlying flows. The abundance of vertical tectonic breccias observed d uring the first part of the dive confirms that the basal part of the w all can be regarded as a major tectonic boundary along which occurred significant vertical and strike-slip motions. Micropaleontological dat a from sedimentary rocks collected during the dive bring new constrain ts to the evolution of the triple junction. The oceanic crust in the s urveyed area is at least as old as 1.9-1.3 Ma, based on the age of sed imentary rocks collected in talus at the base of the wall and observed at the summit of the section. The collected basalts include (1) N-typ e MORBs, (2) E-type MORBs and (3) BABBs. This emphasizes the heterogen eous nature of the mantle beneath the central NFB. Some portions of th e depleted mantle underlying the central NFB triple junction area have recorded contamination from melts or fluids from an ancient subductio n zone (probably the New Hebrides during the opening of the NFB), wher eas adjacent mantle areas show the influence from an alkali-enriched s ource. Such alkali-enriched characters are reported from recent basalt s of the northern NFB (South-Pandora ridge-Rotuma island).