Detrital zircon resolve longevity and evolution of silicic magmatism in extinct volcanic centers: A case study from the East Fjords of Iceland

Abstract

Breiðuvik and Kaekjuskorð are two neighboring extinct eruptive centers in the East Fjords of Iceland. Together, they compose the second-largest volume of silicic rock in the country (after Torfajokull, an active volcanic system in southern Iceland). We use ages and compositions of detrital zircon collected from two riv- ers, the Storaa and Krossa-Kaekjudalsa, to investigate the origins and longevity of silicic magmatism at Breiðuvik-Kaekjuskorð. Zircon populations from the two catchments have identical median U-Pb dates (12.9 Ma), O isotopes (d 18 O Vienna standard mean ocean water = 3.1‰ versus 3.3‰), and Hf isotopes ( I Hf = 14.7). We interpret coherence of zircon elemental and isotopic compositions to indicate that a signifi cant volume of relatively uniform silicic material was produced in close temporal and spatial proximity between 11.2 ± 0.7 Ma and 15.0 ± 0.9 Ma (all errors are 1 i ), dominated by assimilation–fractional crystallization processes. To test the robustness of this longevity estimate, we applied Monte Carlo modeling to the Breiðuvik-Kaekjuskorð detrital zircon results and found the age span to be statistically resolvable at ³2.8 m.y. While this lifespan is comparable to those of large mafi c-silicic volcanic systems that have described in other settings glob- ally, it is the longest reported estimate for any Icelandic volcano, where typical longevity is thought to be ~0.5–1.5 m.y. The ³2.8 m.y. lifespan we present for Breiðuvik-Kaekjuskorð is a conservative assessment, because the dates used in this study only represent the zircon-saturated period of magmatic activity. This study demonstrates that detrital zircon analysis of volcanigenic sediment pro- vides an effi cient and powerful tool that can illuminate histories of zircon-satu- rated magmatism at targeted volcanic centers and systems. This approach can be particularly valuable in dominantly mafi c provinces where silicic material is subordinate (e.g., ocean islands, fl ood basalt provinces), where glaciation, ero- sion, or alteration has transformed the landscape, or in areas that are inaccessi- ble (e.g., obscured by glacial ice)