Petrogenesis of evolved basalts and rhyolites at austurhorn, Southeastern Iceland: The role of fractional crystallization

The Austurhorn intrusive complex in southeastern Iceland represents the evolved hypabyssal remains of an eroded Tertiary (6-7 Ma) central volcano. The complex consists of a layered gabbro intrusion, a composite granophyric stock, and abundant mafic and felsic dikes. Mineralogical and geochemical trends among contemporaneous, compositionally diverse liquids from the complex provide insight into the genesis of evolved basalts and rhyolites in Iceland that are difficult to infer from studies of only lavas. Mafic and felsic samples have comparable ranges in incompatible trace element ratios (Ba/La and P/Ce) and Sr- and Pb-isotopes (O'Nions and Pankhurst, 1973; B. Hanan, pers. comm., 1988), suggesting derivation from a common parental composition. Major and trace element variations throughout the Austurhorn suite are consistent with fractional crystallization of the observed phenocrysts. Quartz-normative basalts were derived from parental basalt containing 7.8 wt.% MgO by extensive low-pressure crystallization of olivine, augite, plagioclase, magnetite, and ilmenite. The fractionating assemblage is consistent with the observed mineralogy of associated gabbro. Moreover, the cumulus mineralogy of the gabbro provides evidence for fractionation processes in a compositional interval not represented by dikes and sills (i.e., 54-63 wt.% SiO₂).Diversity among the mafic dikes reflects several additional factors: (1) crystallization under conditions of variable oxygen fugacity; (2) separate mantle melting events that produce different Ce/Yb values; (3) contamination of some mafic dikes at depth, presumably by interaction with felsic magmas.Major and trace element trends among most felsic samples can be modeled by fractionation of the observed mineral phases: plagioclase, K-feldspar, clinopyroxene, ilmenite, apatite, allanite, and zircon. Although crustal melting has been proposed for generating Icelandic rhyolites, most Austurhorn felsic samples are unlike liquids derived by melting of hydrated basalts. In particular, apatite and zircon have controlled the abundances of Zr, Hf, and the REE in the felsic rocks, but they are unlikely to be residual phases during partial melting of basalt. One felsic dike, interpreted as a melt of an evolved source, shows petrographic evidence of in situ anatexis and also has anomalous trace element abundances and unusually high ₂₀₆Pb/₂₀₄Pb.