TEMPORAL AND GEOCHEMICAL EVOLUTION OF SUBGLACIALLY DERIVED DETRITAL ZIRCON ALONG THE ÖRÆFI VOLCANIC BELT, ICELAND

Iceland is a unique natural laboratory to investigate processes that lead to generation of abundant (~10-15%) silicic material at the intersection of a mid-ocean ridge and mantle hot spot. The coincidence of these tectonomagmatic features yields crust that undergoes repeated transformation as the active spreading centers relocate to remain coupled with the hot spot. Thus, at any given time, Icelandic rift segments are in various stages of activity, from nascent to established to waning. The ~100 km-long Öræfi volcanic belt (ÖVB) in eastern Iceland is generally considered a nascent rift, but detailed petrologic analysis of its petrogenetic and eruptive activity—apart from notable historic eruptions (1362 CE, 1727 CE) of Öræfajökull volcano at the southern end of the ÖVB—is complicated because the ÖVB is almost entirely obscured by Vatnajökull, Iceland’s largest glacier. Detrital zircons from subglacially derived river sediments are a critical tool for investigating the timing and petrogenetic histories of ÖVB-related magmas generated under Vatnajökull. Zircon’s physical and chemical robustness, combined with its common occurrence in silicic magmas—and absence in basaltic ones—provide a unique window into timing and nature of petrogenetic processes. Trace element analyses combined with U-Pb and U-Th ages on detrital zircons were measured on the Stanford-USGS SHRIMP-RG, supplemented with sparse outcrop and tephra derived zircon from multiple locations along the northern and southern extremes of the ÖVB. Results indicate younging of silicic units toward the south (grains are 1.2-2.5 Ma in the northern ÖVB, and <0.5 Ma in the south). Trace element concentrations are typical of Iceland zircon, but show variation in Ti, Hf, and REE along the ÖVB—in the north ÖVB, zircons have avg. Ti <10 ppm and Hf concentrations ~7,000-12,000 ppm; in the south ÖVB, Ti averages >10 ppm and Hf ranges from ~6,000-11,000 ppm. Northern ÖVB zircons also have more restricted Sc/Yb, U/Yb, and Nb/Yb than southern ÖVB zircons. These data suggest a systematic shift in source magma conditions from more evolved in the north to less evolved in the south—a trend consistent with propagation of a rift from north to south, as has been suggested based on whole rock elemental and isotopic data for the ÖVB (e.g. Hards et al., 2000).