A DECLINING TERTIARY RIFT ZONE: NORÐURFJÖRÐUR, ICELAND

Arnes central volcano, located in the Westfjörds of NW Iceland, was erupted about 11 Ma in the now-extinct Tertiary Skagi-Snaefellsnes rift zone, which was active between 15 and 7 Ma. Regional dips are 2–4º east, consistent with the area being west of the former spreading center. This study aims to define the crustal magmatic processes of the rift in the mid-point of its evolution. We present results from a detailed field study of the Arnes central volcano at Norgurfjörgur, as well as major and trace element analyses of samples from lava flows mapped in a ~4 km2 field area. Multiple compound flows occur in the region, with rhyolitic flows located higher in the stratigraphic column. Most lavas from the upper part of the stratigraphic column have 3-4% phenocrysts, mostly feldspar and clinopyroxene, whereas lower lavas contain 1-2% of the same phenocrysts. Feldspar phenocrysts are notably embayed, possibly the result of multiple phases of crystallization and injection of new magma into a fractionating magma chamber. Glomerocrysts of feldspar and clinopyroxene are also prevalent, as is altered olivine. Ferroaugite is found in some of the more evolved material. Lavas in the area range from basalt to rhyolite, and systematically increase in SiO2 content with increasing stratigraphic height, with the exception of a few anomalous samples likely displaced by faulting. The lavas generally define a single differentiation trend, although several of the intermediate rocks diverge from the main trend. All lavas are light REE enriched, consistent with compositions observed in modern Icelandic rift zones. Minimal variations in incompatible trace element ratios indicate that most of the lavas were derived from similar parental magmas. Compositions suggest that the lavas are related by extensive fractional crystallization of plagioclase, augite, olivine, FeTi oxides, apatite, and zircon. The most evolved rhyolite can be generated from the most primitive basalt (7.2 wt.% MgO) by ~85% fractional crystallization. There is little chemical evidence for assimilation of hydrothermally altered crust. These results suggest a magmatic system already in decline, in which the rhyolites are derived not from melting of the crust, as may be the case in the modern plume-centered system, but from increasing amounts of fractional crystallization of a waning magma supply.