Evolution of a silicic magma reservoir in the upper crust : Reyðarártindur pluton, Southeast Iceland

Abstract: Field observations of extinct and exposed magma reservoirs shed light on processes operating in the roots of presently active volcanoes. The Reyðarártindur pluton, Southeast Iceland is an example of a fossil shallow magma reservoir that fed eruptions. The different chapters in this thesis examine the accumulation of magma, and processes occurring during the development and evolution of the magma reservoir from different methodological perspectives. A final model for the evolution of the Reyðarártindur pluton is then presented.The majority of the pluton consists of one voluminous rock unit, the Main Granite, that formed by rapid magma emplacement. However, a local zone of geochemically distinct, but related further Granite Enclaves and Quartz Monzonite Enclaves attest to variations in the composition of the underlying source reservoir. Space for the ca. 2.5 km3 of magma in the pluton was made by piecemeal floor subsidence, which began with multiple dykes that then propagated laterally to form flat-roofed intrusions at different depths. During the first stages of magma emplacement, shattering, sintering and sanidinite-facies contact metamorphism affected a ca. 10 m thick zone of the basalt host rock at the magma reservoir roof. The resulting hornfels was stronger than the original altered basalt, and contained zero porosity and permeability. It thus formed a ‘cap-rock’ to the magma reservoir, limiting heat, volatile and fluid transfer until fractured and faulted at a later stage. The magma reservoir erupted at least once, causing local subsidence of the roof, which would have been observable at the Earth’s surface. Recharge of the magma reservoir by the same Quartz Monzonite and further Granite as exposed in the Reyðará River led to overpressure and eruption. We envisage that cooling and sealing of the piecemeal subsidence network preceded eruption, causing overpressure with magma recharge. The eruptive lifetime of the magma reservoir was limited to ca. 1000 years. This timeframe is much less than the duration of silicic magmatism in a typical Icelandic central volcano, or at other rhyolite-erupting volcanoes worldwide, which is in the order of hundreds of thousands to millions of years. Hence, the Reyðarártindur pluton likely represents a small, ephemeral part of a wider magmatic plumbing system that feeds a central volcano.The results from these studies can provide volcano-monitoring personnel with scenarios for magma emplacement, and processes leading to eruption, which they can then use as a framework for interpreting detectable signals of magma movement and volcanic unrest.