Temporal and Structural Interactions Between Silicic Volcanic Centers and the Slaufrudalur Pluton, Southeast Iceland.

Magma intrusion and the generation of silicic magmas are fundamental processes controlling volcanic behaviour and continental crust formation. Exposed plutons provide key constraints on these processes; however, the geological context of silicic plutonic systems, particularly in oceanic rift environments, remains poorly resolved. Southeast Iceland hosts exceptionally exposed large Miocene plutonic complexes, due to a combination of higher erosion and crustal accretion rates. This setting provides a unique insight into the evolution of Iceland’s geodynamics, making it a prime natural laboratory for investigating silicic magma generation within anomalously thick (>40 km) basalt-dominated, plume-influenced, oceanic crust setting.

Recent geological mapping in Southeast Iceland has revealed four previously unknown, and unmapped, silicic formations preserved as caldera-bounded pyroclastic successions, conduit facies, and shallow intrusions: Kvosir, Þórgeirsstaðurdalur, Kapaldalur, and Hvammsheiði These formations surround the 8–10 km³ Slaufrudalur granite pluton and bound it to the south, east, and northwest, appearing structurally linked to its emplacement. This discovery is significant because the Slaufrudalur pluton is the largest granitic intrusion exposed in Iceland, yet its relationship with surrounding rocks remains poorly constrained.

The aim of our research is to constrain the processes that govern the petrogenesis and temporal evolution of large silicic volcanic centres in Iceland. We integrate high-resolution field mapping, photogrammetry, structural analysis, petrography, geochemical and geochronological data on key stratigraphic units to examine how these volcanic formations relate to pluton emplacement and shallow (<5 km) crustal magma storage.

Preliminary zircon U-Pb ages of both plutonic and the newly identified silicic caldera formations indicate that silicic magmatism in the area spans a near continuous 5 million-year period, from 9–<4 Ma, starting with the Hvammsheiði formation. Within the pluton, zircon crystallisation ages reveal a resolvable age difference between an older roof unit and younger units beneath it. This is consistent with previous studies that suggest a top-down magma batch emplacement (Carmody, 1991; Burchardt et al.  2012; Quintela et al. 2025). The data further reveal a complex temporal and structural interplay between pluton growth and surrounding volcanism:

(1) Early magma injections forming the pluton roof are broadly coeval with the largest silicic centre, the Kvosir caldera, whose bounding fault straddles the pluton.
(2) Þórgeirsstaðadalur bimodal volcanism predates pluton emplacement, potentially creating a structural weakness that localized granitic magma intrusion. This lineament remained a focus of magmatic injection after pluton construction.
(3) The Kapaldalur formation represents a younger explosive center emplaced along a pluton wall fault. A pyroclastic unit containing plutonic lithics records two zircon populations: one coeval with, and one younger than, the pluton.

These findings offer new insights into possible geodynamic scenarios in Iceland during the Miocene. This work also contributes to a broader understanding of crustal accretion and crustal recycling processes in long lived silicic volcanic centers. This framework provided also opens for new constraints on silicic magma generation and pluton emplacement in oceanic rift environments and has implications for understanding the formation of continental crust.