Volcanic evolution of the marginal and interarc basins

Volcanic evolution of the interarc and marginal basins is analysed using the available data on volcanics from the presently existent and ancient back-arc basins of the western Pacific and Mediterranean. It is shown that in early (pre-spreading) stages of back-arc rifting, the character of volcanism is determined by “maturity” of the adjacent island arc. It is predominantly alkaline or mildly alkaline for back-arc basins related to the island-arcs with high-potash calc-alkaline and shoshonitic volcanism. The back-arc alkaline and mildly alkaline basalts strongly differ from the continental and oceanic rift volcanoes by constantly lower Ti, Nb and Zr contents. Because of these features these basalts are akin to the basaltic members of the island-arc volcanic series. As the latter, they are generally strongly enriched in K₂O and LIL elements, whereas Na₂O reveals comparatively small variability. With initiation of spreading a sharp depression of K₂O, LIL and light REE occurs in the axial basalts of back-arc basins, that progressively approach the MORB composition. But even tholeiites from the most evolved basins that underwent a considerable spreading reveal slight but detectable geochemical peculiarities, indicating their island-arc affinities. Origin of the low-Ti alkaline basaltic magmas of the active continental margins is discussed.