Rift Valley in the Atlantic Ocean near 36°50′N: petrology and geochemistry of basaltic rocks

Abstract

Dredged rocks from an area of about 15 km 2 within the inner floor and on the adjacent walls of the Rift Valley were collected. Based on petrographic and chemical data, four types of basaltic rocks were recognized: (1) picritic basalts with olivine xenocrysts, TiO 2 < 0.6%, K 2O < 0.1%, (2) olivine basalts with olivine megacrysts, TiO 2 = 0.8–1.5%,K 2O = 0.1–0.2%, (3) highly phyric and moderately phyric plagioclase basalts with megacrystic plagioclase, TiO 2 < 1.3%, K 2O < 0.3%, and (4) pyroxene basalts with pyroxene > plagioclase, TiO 2 = 0.8–1%,K 2O = 0.2–0.4%. The Cr and Ni having high partition coefficients show different variation trends for each type of rock and their values decrease continuously as crystallization proceeds within each type of basalt. It is speculated that two different magmas have given rise to the above-mentioned rocks. One has yielded the picritic basalts and subsequently the olivine basalts after a separation of the olivine cumulates; the other gave rise to the plagioclase basalts.