Heterogeneity of Rift Magmatism in the Equatorial Province of the Mid-Atlantic Ridge (15o N to 3o S)

New data on the compositions of chill glasses, which were sampled in the central part of the Romanche Fracture Zone during the Russian-Italian cruise of the R/Y Gelendzhik in 1996, and the first analyses of the concentrations of 28 lithophile elements in glasses from the Equatorial Mid-Atlantic Ridge (EMAR) made it possible to identify distinctive features of magmatism in this zone, which was earlier recognized as a specific tectono-magmatic province of MAR. The ratios of lithophile elements and Sr, Nd, and Pb isotopes suggest that the enrichment character of tholeiites from two areas of the province (the 14°-15° N anomaly in the north and the 2° N-1° S anomaly in the south) is similar. The enriched melts are noted for high 206Pb/204Pb = 20.0, 207Pb/ 204Pb = 15.6, 208Pb/204Pb = 39.5, and 87Sr/86Sr = 0.7030 at a 143Nd/ 144Nd ratio as low as 0.5128. In this sense, the melts are similar to the alkaline magmas in islands of the Cameroon line. Conceivably, the development of the geochemical anomalies in the north and south of EMAR in the process of melting involved the subcontinental and suboceanic mantle, which had been metasomatized by the intrusion and capture of enriched melts early during the opening of the Atlantic Ocean. Similarities and differences were determined for the magma generation conditions in the southern and northern provinces. It was established that the conditions of magma derivation in the Mid-Atlantic Ridge (MAR) near the Romanche Fracture Zone had been unstable over a long period of time (at least 40 m.y.), which resulted in the origin of lavas of different compositions and geneses within a single area. The lavas ranged from normal tholeiites (of the TOR-2 type) to tholeiites whose parental melts were enriched in Na and Si (Na type), alkaline melts, and lavas of mixed types variously enriched in incompatible elements and radiogenic isotopes. Copyright © 2002 by MAIK " Nauka/Interperiodica" (Russia).