EXAMINING THE MAGMATIC HISTORY OF THE OXNADALUR BIMODAL VOLCANIC COMPLEX IN NORTH-CENTRAL ICELAND

Iceland is dominated by basaltic volcanism, but many volcanic centers host substantial amounts of felsic volcanic rocks. One such center is the Oxnadalur volcanic complex in north Iceland. This complex hosts multiple basaltic lava flows as well as a gabbro intrusion. Capping many of the ridges is a rhyolitic ignimbrite~70m thick. This ignimbrite is cut by many felsic and mafic dikes and is bounded to the north by a flow banded rhyolite and an obsidian layer that may represent the central vent of the complex. Early eruptions in the complex alternated between basaltic flows, rhyolite flows and an isolated felsic surge deposit. This succession was followed by an intermediate lava flow, also capped by basalt. The youngest remaining layer in the complex is a rhyolitic ignimbrite, comprising ~5% of the complex by volume. The Oxnadalur complex differs from the several other bimodal complexes by hosting a distinct intermediate lava flow that preserves disequilibrium mineral textures. The intermediate layer shows evidence for hybridization, with two distinct generations of plagioclase phenocrysts: one euhedral and unaltered, the other resorbed and with a honeycomb texture. Infrared spectroscopic measurements of water concentrations in flow banded glasses of rhyolite dikes cutting the rhyolite yielded an average concentration of 3.0-3.5wt%. These concentrations are much higher than those measured by Tuffen and Castro (2009) of ~0.37wt% on a rhyolite dike in the Krafla complex and those measured by Tuffen et al (2008) of ~0.50wt% in a subglacial rhyolitic eruption at Torfajokull. The greater abundance of water in the Oxnadalur magmas may have enhanced the hybridization process and the explosivity of the rhyolitic eruptions.