Geology and petrogenesis of lavas from San Cristobal Island, Galapagos Archipelago

Volcanism became emergent on San Cristobal Island 2.3 m.y. B.P. in the eruption of floods of alkali basalt. Since then eruption continued until 0.6 m.y. B.P. in a central shield volcano. From 0.9 m.y. B.P. to almost historical time, activity moved to the NE and was characterized by a fissure-fed eruption. Three distinct magma series occur on San Cristobal: an alkaline series, a series of MORB-like tholeiites and a series of incompatible-element-enriched tholeiites typical of oceanic islands. Each series is represented by very primitive basalts and more-evolved lavas resulting from a simple fractional crystallization process. The isotope and trace-element data suggest that the different series are generated by different degrees of partial melting and mixing of both a depleted and an enriched source.