PETROGENESIS OF THE EARLY SKAGI-SNAEFELLSNES RIFT, WESTFJORDS, ICELAND

A Keck Geology Consortium project in 2004 focused on Tertiary lavas on the south flank of the Hrafnfjordur central volcano in the Westfjords of northwestern Iceland. These lavas reflect early activity on the Skagi-Snaefellsnes rift, which initiated as a plume-centered rift at 15 Ma and was abandoned after it had drifted off the plume at 7 Ma.

In the Westfjords there were three projects at Geirsfjall, Seljafjall, and Leirufjordur (from west to east). At Geirsfjall the stratigraphy of three mountains comprises numerous subalkaline, olivine-normative basalt (48.2-49.7 wt% SiO₂) flows as well as tuff at the top of the section. A ⁴⁰Ar/³⁹Ar age of the lowermost exposed basalt puts an upper limit of 14.20 +/- 0.17 Ma on the rocks of this study. At Seljafjall, an extensive (12 km²) basaltic andesite to andesite unit (55-61 wt% SiO₂) is at least 200 meters thick and stratigraphically equivalent to the upper Geirsfjall basalt. Continuing east, at about the same stratigraphic position as the Seljafjall lava, is a thinner andesite to dacite (60-66 wt% SiO₂) lava exposed along the southern edge of Leirufjordur. A dacite lava dome at Trollafell (67 wt% SiO₂) overlies the andesite/dacite flow.

On an AFM diagram, the rocks follow a tholeiitic trend, and the andesites are geochemically similar to the high Fe, low Al “Icelandite”. Major and trace element variations relate all units, except the Trollafell dacite, through a fractional crystallization sequence of olivine, plagioclase, clinopyroxene, Fe-Ti oxides, and apatite. The Trollafell dacite may reflect significant crustal input.

Basalts in the Westfjords are enriched in incompatible elements relative to N-MORB and are similar to E-MORB but have lower LILE. Zr/Y vs. Nb/Y shows that the Westfjords basalts are similar to other Icelandic basalts, but distinct from N-MORB. Zr/Nb ranges from 11.5 +/- 2.1 (1 s.d., n=14) in the Westfjords basalts to 8.0 +/- 1.4 (n=41) in Skagi Peninsula (north-central Iceland) basalts which were erupted in the waning stages of the rift. The difference may be due to slightly higher degrees of partial melting when centered on the plume. Both areas lack the extreme variability of Zr/Nb seen in rocks from the neovolcanic zone that has been attributed to melting of a plume source depleted by prior melt extraction.