THE ONGOING SEARCH FOR RHYOLITE AT KVERKFJÖLL (ICELAND) VIA DETRITAL SEDIMENTS

Iceland’s magma and rock compositions are compositionally diverse. Approximately 85–90% of the Icelandic crust is mafic (SiO₂ < 52 wt% SiO₂), while ~10–15% of the bedrock has higher silica contents (SiO₂ ≥ 65 wt%; ‘silicic’). These silicic rocks commonly form and are concentrated at central volcanoes. The purpose of this research is to better understand the distribution and formation of elusive rhyolites at generated Kverkfjöll volcano. The Kverkfjöll double-caldera volcanic system is largely covered by Vatnajökull ice cap, although parts are accessible at the ice periphery. It is relatively understudied compared to other, more accessible, volcanoes in the Northern Volcanic Zone like Askja and Krafla. It also has a strong and active hydrothermal system. After a search of moraine clasts via pXRF yielded no rhyolites that would be viable for further study (i.e., a thin section, mineral separation, etc.), nine sediment samples were collected from ~7.5 km² north of the volcano from glacial outwash streams to search for zircon and apatite—accessory minerals common in silicic rocks but far less so in mafic and intermediate rocks. Approximately 10 L of sediment were ultimately processed for mineral separation, but zircon and apatite were sparse (n<10) and too small to analyze (<10 µm). Pyrite was abundant in dense mineral separates, likely due to pervasive phyllic (aka quartz sericite pyrite (QSP)) alteration. QSP alteration is common in silicic rocks subjected to high-T hydrothermal environments, as is common at Kverkfjöll. This suggests that rhyolites were potentially present in at least a small volume, but perhaps have since been altered and eroded due to environmental conditions. Zircon is physically and chemically robust and should survive such processes—why didn’t the sediments contain detrital zircon? We explore several options, including lack of zircon saturation due to low Zr and high temperature in rhyolitic magma at Kverkfjöll; a unique lack of volumetrically significant rhyolite at this central volcano; our choice of sampling locations; and comparison to nearby systems such as Askja.