Tectono-volcanic activity at the axial zone of the Mid-Atlantic Ridge between the Fifteen Twenty and Mercurius fracture zones, central Atlantic

The materials recovered during the R/V Akademik Nikolai Strakhov Cruises 9 and 15 are used to analyze the major-element chemistry of basalts and the c haracter of their spatial and structural distribution at the crest zone of the Mid-Atlantic Ridge between the Fifteen Twenty and Mercurius fracture zo nes. Most of the basalts, including those obtained from far outside the rif t, are typical oceanic tholeiites, which can be subdivided into the followi ng varieties: depleted MORE of the N type, intermediate, and enriched T- an d P-MORB. The enriched basalts are localized within regions with elevated l ithospheric permeability: at zones where the rift segment between the Fifte en Twenty and Mercurius fracture zones are transected by the fracture zones and at uplifts of the rift valley floor between the fracture zones, where the floor is cut by a system of northwest-trending structures that are obli quely oriented with respect to the rift. These areas display traces of the cyclic character of volcanism. Within each cycle, the composition of the pa rtial basalt melts varies from depleted to enriched, and each of the cycles lasted for 2-2.5 m.y., with the enriched basalts derived at similar to 25 Ma, i.e., simultaneously with the origin of the Researcher Ridge. In additi on to the main group of basalts, there are two groups that have a lower sil icity but are richer K2O, TiO2, P2O5, L- and MREE than the enriched tholeii tes. These low-silicity basalts compose structures that can be assigned, ba sed on a complex of their morphological, geological, and petrographic featu res, to central volcanoes that developed beyond the limits of the rift. The rest of the low silicic basalts also occurs outside the rift and affiliate s with depleted varieties. Their compositional signatures suggest that the rocks are the products of tholeiitic magma differentiation in large interme diate chambers with the participation of orthopyroxene and Fe-Ti minerals. The distinctive features of volcanism in the MAR segment may probably be ex plained by the ascent of a mantle plume near the Fifteen Twenty Fracture Zo ne and the partial melting of peridotites that occurred in the tectonically active magma generation zone.