The Magmatic Evolution of Þingmúli Central Volcano: Insights into Magma Evolution and Petrogenesis in Iceland

Abstract

Þingmúli central volcano is an extinct, heavily eroded Miocene volcano, located in the East fjords, eastern Iceland, and is one of the key localities to study magma processes over a range of lithological units. It is generally accepted that magmatism in Iceland is the consequence of two mechanisms associated with rifting of the Mid Atlantic Ridge, supplying depleted basalts to active rift zones, and upwelling of the Icelandic plume, supplying enriched basalts to the flank zones of Iceland. Volcanism in Iceland is predominantly basaltic, with ~80% of the volcanic material in Iceland being basaltic, but also rhyolitic (~15% of deposits). Across Iceland, a compositional gap occurs with most volcanism being either basaltic or rhyolitic with few volcanoes erupting significant intermediate (andesites-dacites) compositions. To understand the petrogenesis of magmas it is key to explore a full sequence of lithologies such as that found at Þingmúli central volcano.

At Þingmúli, a complete differentiation from basalt to rhyolite occurs representing an opportunity to investigate a full magmatic differentiation sequence. This project investigates in detail the sequence, supported by field relationships and stratigraphic constrains to fully characterise it, and sheds additional insights to the main mechanism, or combination of mechanisms acting at Þingmúli. Results of this thesis suggest the magma type at Þingmúli is transitional alkaline, with basalts sourced from a PREvalent MAntle source at pressures of 1.5 GPa. Rhyolite lavas, previously thought to be the result of fractional crystallisation are re-interpreted as the products of partial melting, and intermediate lavas, the result of magma mixing.

Results of this study also suggest Þingmúli can be used as a case study of magma evolution in Iceland, with data mining of Icelandic samples showing intermediate and rhyolite lavas contain similar geochemical characteristics to basalts, which cannot be reproduced by standard fractional crystallisation models. An investigation into basaltic magmas in Iceland also reveals areas of rift-plume interaction and that this interaction produces basaltic lavas in Krafla and Askja, northern Iceland.

Date of Award	2025
Original language	English
Awarding Institution	Edge Hill University
Supervisor	JOAQUIN ALBERTO CORTES CARRILLO (Director of Studies), JOANNE EGAN (Supervisor) & Dave McGarvie (Supervisor)