RHYOLITE PETROGENESIS AT THE TERTIARY ARNES CENTRAL VOLCANO, NORTHWESTERN ICELAND

The Tertiary Arnes central volcano in the Westfjords of northwest Iceland is a product of the now abandoned Skagi-Snaefellsnes rift system. At Iceland, the unique expression of the Mid-Atlantic Ridge above sea-level and the intersection of this ridge with the Iceland hotspot, rift abandonment occurs as the extant rift drifts away from the hotspot until an incipient rift propagates over the hotspot while the old rift dies out. Paleorifts such as the Skagi-Snaefellsnes are recognized by regional synclines produced by the isostatic response to crustal loading of lava along concentrated volcanic zones. Nearly all of the 10% of silicic or intermediate Icelandic volcanism originates at central volcanoes in the axial rift zones. The mechanisms of petrogenesis of silicic and intermediate magmas remain equivocal.

Field and petrographic analyses of the northern portion of the Arnes central volcano on the Krossnes Peninsula have yielded seven units. Basalts compose the base of the section, followed by five intermediate-silicic flows, capped by another basalt unit. The silicic-intermediate flows may be genetically related by variable degrees of partial melting, fractional crystallization, and magma mixing.

Integrating these observations with pending geochemical analysis, this project will characterize silicic and intermediate magma petrogenesis of the Arnes central volcano. Major element geochemistry will show compositional differences and similarities across units. Trace element data will be used to link genetically related magmas to enable petrogentic analysis. Data from Arnes will be compared with data from other central volcanoes erupted from the same rift during its evolution to characterize the spatial and temporal effects of rift-drift on magma genesis with applications to present-day Icelandic volcanism.