ON THE NATURE OF RIFT VOLCANISM IN THE AR EA OF ROMANCHE-CHAIN FAULT (EQUATORIAL ATLANTICS)

The study is based on chilled glass and subordinate basaltic samples c ollected in 13-th cruise of ''Academician Nikolai Strakhov'' ship in 1 992 and in 5-th route of ''Academician Boris Petrov'' ship in 1986. It is shown that the magmatic rocks to tile north of Romanche fault are differing from that of the southern part of North Atlantic ridge (0-2 degrees S). Magmatic rocks of Chain fault area are corresponding to th oleiites of TOR-2 type. Small variations of melt composition from depl eted up to slightly enriched in lithophyles are observed. The latter a re characteristic of the Chain fault intersection with the rift valley . Magmatic rocks of Romanche area are more variable, Melts enriched in Na, Si, P and depleted in Fe are mostly abundant. These melts could h ave been formed in the process of melting of low depleted primary mant le at low pressure (similar to 6 kbar) and temperature and low melting extent. Subordinate members of magmatic series are presented by thole iites corresponding to TOR-2 and alkaline melts. The latter presumably reflect the highest melting extent of similar composition at greater depths. According to Ce/Yb and Nb/Zr ratios the melting could have inv olved some portion of partly metamorphised (amphibolitic) mantle close to San-Paolo mantle.