GEOLOGY AND PETROLOGY OF FLOREANA-ISLAND, GALAPAGOS ARCHIPELAGO, ECUADOR

Floreana is the sixth largest of the Galapagos Islands and lies at the extreme southern margin of the Galapagos Platform. In sharp contrast to the other major islands, Floreana lacks a well-developed volcanic center, but instead is dominated by pyroclastic vents and derivative ash and cinder deposits. The majority of exposed lavas are xenolith-bearing alkali-olivine basalts that constitute a coherent suite referred to as the Main Series. The Flank Series comprises a group of isolated vents on the southern flank of the island which erupted xenolith-poor basaltic lavas that are generally poorer in incompatible trace elements and richer in plagioclase. Paleomagnetic measurements, supported by K-Ar age determinations, record a relatively limited timespan for subaerial volcanism. Shield development reached maximum dimensions by 1.0 Ma, with the final phases of pyroclastic activity at an end by 0.3 Ma. Main Series rocks are phenocryst-poor and typically contain clusters of xenocrysts in an extremely fine-grained. granular matrix. Petrogenetic modelling indicates they represent products of partial melting of a LREE-enriched spinel-lherzolite source, little modified by subsequent fractional crystallization. Porphyritic Flank Series lavas experienced rather different ascent histories and may have resided in shallow-level magma chambers prior to eruption. Isotopic, geochemical and petrographic data demonstrate that Flank Series lavas are not cogenetic in any simple way with Main Series flows. Floreana lavas contain a diverse suite of ultramafic xenoliths. Al-augite group megacrysts are likely to be cognate and comagmatic with host alkali-olivine basalts. Transition metal contents of megacrysts and host lavas, however, preclude clinopyroxene fractionation as a mechanism of magmatic differentiation. It is proposed that megacrysts were products of passive crystallization and were never effectively separated from host magmas. Cr-diopside group nodules may have crystallized at very high temperatures from advanced partial melts and are less likely to bear any direct genetic connection to host basalts.