ALKALIC VOLCANISM OF THE GORRINGE BANK, ATLANTIC-OCEAN

Petrologic data on the alkali basalts of the Gorringe Bank allowed us to distinguish three main rock groups: lamprophyre, phonolite, and pot assic trachyte. Each of the groups shows characteristic petrographic a nd petrologic features, and transitional rock varieties are essentiall y missing. The available data suggest that the lamprophyres and phonol ites were formed by the melting of a metasomatized mantle. Xenoliths o f such ultrabasic rocks were found in the lamprophyres. Mantle melting under high water pressure could have resulted in the formation of two immiscible melts, leucocratic and melanocratic. The potassic trachyte s could be derived either by the low-degree melting of the altered man tle at shallow depths, or by partial contamination with potassium-bear ing buried sediments. The alkali basaltoids show Sr-86/Sr-87 isotopic ratios much higher than those of the depleted mantle, but similar to t he values typical of sea water.