| Summary: | A petrological and geochemical study of basalts from Iceland's neovolcanic zones and Tertiary lava piles was carried out in order to investigate volcanic processes associated with the development of the Iceland platform, an anomalously elevated segment of the Mid-Atlantic Ridge (MAR). The Northeast and Southwest axial rift zones are dominated by olivine tholeiites and tholeiites, whereas the Southeast, Skagi, and Snaefellsnes flank zones are characterized by alkali basalts and mildly alkaline FeTi basalts. Phenocryst assemblages and textures are more diverse in flank zone basalts than in axial rift zone basalts, suggesting mixing of more diverse magma compositions and longer residence times in flank zone magma chambers. Highly variable REE patterns in axial rift zone basalts from the same volcanic system suggest variable degrees of melting over a wide depth range, whereas more uniform REE patterns in flank zone basalts from the same volcanic system suggest less variable melting conditions. The SE volcanic zone, which changes in nature from an axial rift zone in central Iceland to a flank zone in southern Iceland, exhibits distinct mineralogical, geochemical, and geophysical gradients. These gradients are interpreted in terms of systematic variations in melting and crystallization conditions associated with a propagating rift. Geochemical trends observed in 2 to 14 m.y. old basalts from eastern and western Iceland indicate a complex and varied source region for Iceland magmas, dependent on both mantle plume activity and the maturity of rift zones. Decreasing {La/Sm}(,E.F.) and ('87)Sr/('86)Sr ratios over the last 14 m.y. suggest decreasing mantle plume activity. Shorter episodes (2 to 5 m.y.) of declining {La/Sm}(,E.F.) and ('87)Sr/('86)Sr are explained by the development of new rift zones caused by repeated shifts in the location of the spreading axis. On the average, Tertiary basalts from eastern and western Iceland are chemically more similar to young (< 0.7 m.y.) basalts from immature flank zones than to young basalts from the axial rift zones, implying that the Tertiary basalts were erupted along immature rift zones. Geochemical cycles, lasting 0.1 to 0.3 m.y., in the Borgarfjordur lava pile suggest progressive melting of mantle diapirs, rising from the garnet stability zone where melting begins.
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