THE GENERATION OF OCEANIC RHYOLITES BY CRYSTAL FRACTIONATION - THE BASALT-RHYOLITE ASSOCIATION AT VOLCAN-ALCEDO, GALAPAGOS-ARCHIPELAGO

Alcedo volcano is one of six shield volcanoes on Isabela Island in the western Galapagos Islands. Although Alcedo is dominantly basaltic, it is unusual in that it also has erupted similar to 1 km(3) of rhyolite. The rhyolitic phase marked a 10-fold decrease in the mass-eruption rate of the volcano, and the volcano has returned to erupting basalt. The basalts are tholeiitic and range from strongly to sparsely porphyritic. Olivine and plagioclase are the lipuidus Phases in the most primitive basalts. The MgO and Ni concentrations in the most primitive basalts indicate that they have undergone substantial differentiation since extraction from the mantle. The rhyolites contain the assemblage oligoclase-augite-titanomagnetite-fayalite-apatite and sparse xenoliths of quenched basalt and cumulate gabbros. Intermediate rocks are very rare, but some are apparently basalt-rhyolite hybrids, and others resulted from differentiation of tholeiitic magma. Several modeling approaches and Sr-, Nd-, and O-isotopic data indicate that the rhyolites resulted from similar to 90% fractionation (by weight) of plagioclase, augite, titanomagnetite, olivine, and apatite from the most primitive olivine tholeiite. The data are inconsistent with the rhyolites originating by crustal anatexis. The extreme Daly gap may be caused by the large increase in viscosity as the basaltic magma differentiates to intermediate and siliceous compositions; highly evolved magmas are eruptible only after they become saturated with volatiles by second boiling. The close association of the hybrid intermediate magmas and magmatic inclusions with the climactic plinian eruption indicates mixing between mafic and silicic magmas immediately before eruption. Rhyolite production was favored by the decrease in supply of basaltic magma as Alcedo was carried away from the focus of the Galapagos hot-spot. A three-stage model for the magmatic evolution of a Galapagos volcano is proposed In the first stage, the supply of basaltic magma is large. Basaltic magma continually intrudes the subcaldera magma chamber, buffering the magmas' compositional and thermal evolution. As the volcano is carried away from the basaltic source, the magma chamber is allowed to cool and differentiate, as exemplified by Alcedo's rhyolitic phase. Finally, the volcano receives even smaller influx of basalt, so a large magma chamber cannot be sustained, and the volcano shifts to isolated basaltic eruptions.