RIFT ABANDONMENT IN ICELAND: THE WANING PHASE OF THE NORTHERN SNAEFELLSNES RIFT

The volcano-tectonic evolution of Iceland is characterized by the development and abandonment of rifts that represent the Mid-Atlantic Ridge on land. Rifts are progressively abandoned as they drift west in the hotspot reference frame and away from the Iceland hotspot. The most recently abandoned rift is the Tertiary Snaefellsnes rift in western Iceland. In the summer of 2003, a Keck Geology Consortium research project was conducted on the northern portion of the rift.

Samples were collected in Langadalsfjall, Vatnsdalsfjall, and Vididalsfjall (from east to west), with the latter being closest to the generalized axis of the rift. Ar/Ar ages bracket the volcanic stratigraphy of Langadasfjall from 8.6-7.1 Ma and Vatnsdalsfjall from 7.6-7.0 Ma. A lava in the volcaniclastic section of northern Vididalsfjall yielded an age of 8.6 Ma and a feldspar separate from a granite intruding the section yielded an age of 7.0 Ma. Intrusive and proximal volcanic rocks at ~7 Ma occur across at least 20 km perpendicular to the rift axis.

Most of the volcanic rocks are evolved tholeiitic basalts. Because they are not restricted to distally-derived lavas like other exposed Tertiary basalts, they might be expected to exhibit the variability observed in the active rift. However, these basalts have a much more restricted range in Zr/Nb. This may reflect melting of mantle not variably depleted by prior hotspot-influenced melting, the process that has been interpreted to account for variability at the active rift. The difference may be due to a less robust magmatic system after the rift drifted off the hotspot prior to abandonment.

A 7.8 Ma rhyolite lava was mapped in Langadalsfjall, and two rhyolite lavas were found in the interval between 7.6 and 7.3 Ma in Vatnsdalsfjall. The rhyolite in Langadalsfjall is associated with a coarsely porphyritic andesite. Field, textural, and chemical characteristics suggest that the eruption of the rhyolite was caused by injection of a plagioclase-ultraphyric basalt, and that the andesite is the product of mixing. Compositional trends suggest that the rhyolites may be the result of extreme fractionation of a basaltic parent rather than crustal melting. This contrasts with the rhyolites formed in the modern rift, and the difference may again reflect a less robust magmatic system after the rift drifted away from the hotspot.