SILICIC VOLCANISM AT REYKJARFJÖRÐUR, NORTHWESTERN ICELAND

The Mid-Atlantic ridge at Iceland drifts westward in relation to the Iceland mantle plume. Instead of separating entirely, a new rift develops, centered over the plume, and the old rift is abandoned. The Árnes central volcano in the Westfjords of Iceland erupted at about 11 Ma in the Skagi-Snaefellsnes rift zone and represents a transitional period between the 15 Ma formation of the rift and its 7 Ma abandonment. This project focuses on the silicic lava flows north of Reykjarfjörgur in the southern portion of the Árnes central volcano complex. XRF and petrographic analysis were carried out on 25 representative samples, revealing three distinct rock types. The first unit (~300m thick) consists of flood basalts (48-52 wt% SiO2) characterized by intergranular groundmasses with mostly zoned plagioclase phenocrysts and few olivine phenocrysts. The next unit above the basalts consists of numerous rhyolite flows (70-74 wt% SiO2) characterized by clinopyroxene and plagioclase phenocrysts in very fine-grained cryptocrystalline groundmasses. Numerous textures, including spherulites, lithophysae and flow banding on the outcrop scale, were found in these devitrified rhyolites. The third unit consists of a distinct andesite flow (57 wt% SiO2) which has sparse phenocrysts and an extremely fine-grained groundmass. Above the andesites are more basalts, similar to the lower unit. Major and trace element analysis suggests that the andesites where formed by fractionation of a basaltic melt (near liquidus), as evidence by non-linear trends in P2O5, Sr, FeO, and TiO2, plotted against SiO2 and MgO. Further major and trace element analysis will reveal whether the formation of the rhyolites formed at a near liquidus environment (extreme fractionation of a basaltic melt) versus a near solidus environment (partial melting of a basaltic crust), or a combination of these. This work, along with the other field work carried out during the 2003, 2004, and 2007 seasons, will provide insight into the formation of silicic lavas in northwest Iceland during the history of the ridge-plume interaction.