LATE ALKALINE LAVAS OF THE OB AND LENA SE AMOUNTS (CONRAD RISE, INDIAN-OCEAN) - ROCK CHEMISTRY AND CHARACTERISTICS OF MANTLE SOURCES

For the first time, the geochemical composition of extrusive rocks of the Ob and Lena seamounts (Conrad Rise) has been studied. The final ma gmatic products of these seamounts have been shown to represent the al kaline magmatism typical of the intraplate islands of the Indian ocean . Two rock series (basanite-tephritic and trachybasal-trachytic) have been recognized based on distinctive mineral and major-element composi tions of the rocks. In their major- and trace-element characteristics, the rocks of these series are similar to the alkaline rocks of the Ke rguelen Archipelago and Afanasy Nikitin Seamount. The values of (La/Ta )(p) and (Th/Ta)(p) tracer ratios and isotopic enrichment of the rocks under study are evidence that the mantle source of alkaline melts of the Ob and Lena seamounts was a metasomatosed subcontinental lithosphe re compositionally similar to the source of magmas of the Kerguelen Pl ateau and Afanasy Nikitin Rise, a feature that reflects the existence of a stable mantle anomaly in the vicinity of the ancient South Indian Ridge at 70-120 Ma.