Volcanism at the edge of the Hawaiian plume: Petrogenesis of submarine alkalic lavas from the North Arch volcanic field

Submarine lavas erupted onto the Hawaiian arch 200-400 km north of Oahu sho w that the areal extent of Hawaiian volcanism is much larger than previousl y recognized nle North Arch volcanic field comprises 25 000km(2) of similar to 0.5-1.15 Ma, volatile-rich, olivine-phyric alkalic lavas (alkalic basal t to nephelinite). These lavas are similar in composition to rejuvenated-st age lavas such as the Koloa Volcanics (Kauai) and Honolulu Volcanics (Oahu) . North Arch lavas that encompass the compositional extremes have similar S r, Nd and Pb isotopic ratios. Olivine accumulation and fractionation was th e major post-melting process that affected the compositions of North Arch l avas. After correction for these processes; the inferred primary magma comp ositions show that they were derived by variable, factor of four, and relat ively lore, extents of melting of garnet peridotite. Garnet and olivine wer e important residual phases during partial melting; in contrast to the Hono lulu Volcanics, there is little evidence for residual hydrous phases, sulfi des or Fe-Ti oxides. The mantle source for the North Arch lavas had Sr, Nd and Pb isotopic ratios intermediate between those of Pacific Ocean lithosph ere and the inferred range for Hawaiian plume components.;These data are co nsistent with a mixed lithosphere-plume source. Although the plume-derived component was probably from the Hawaiian plume, an alternative hypothesis i s that during the middle Cretaceous, South Pacific lithosphere was contamin ated by plumes that formed large oceanic plateaux (e.g. Ontong Java). This mixed source was subsequently partially melted as it passed near the Hawaii an plume.