A NEW INSIGHT INTO THE TRANSPORT AND STORAGE OF MAGMA IN SUBVOLCANIC SYSTEMS: A CASE STUDY FROM THE REYDARA LACCOLITH, SE ICELAND

The movement of magma and how it accumulates as igneous intrusions in subvolcanic systems require an understanding of parameters such as; intrusive geometry, magma flow patterns, the rate of construction, and the physical behavior of magma-host rock dynamics. The true complexity of such variables has often been misrepresented in previous studies as the available exposure of igneous intrusions has limited the amount of three-dimensional structural information that can be gathered. The Reydara Laccolith in SE Iceland thus provides an ideal setting to investigate such parameters owing to its excellent three-dimensional exposure and well exposed host rock contact relations. The 10 km², ~350m thick, Reydara Laccolith consists of granophyric and granitoid rocks emplaced at a depth of ~ 1.0 km below the Palaogene surface of the Icelandic rift zone into flat-lying flood basalts that were later tilted by up to 10º together with the Laccolith. This study presents new data arising from comprehensive field mapping, anisotropy of magnetic susceptibility (AMS) and, U-Pb zircon ID CA TIMS analysis on the Reydara Laccolith. The intrusion defines a dome shaped geometry exploiting the gently tilted layering of the basaltic host rock where generally, the layers are reoriented to gently dip perpendicularly away from the contact. The preferential orientation of the K1 magnetic lineation in AMS analysis suggests variably inclined, multi-directional, magma transport in several magma lobes down the structural section of the intrusion. Consistent U-Pb age dates of 7.40-7.41 Ma (+0.04) down a structural transect of the intrusion suggest inferred magma lobes were emplaced in rapid succession relative to one another. These data will provide a basis for a more comprehensive dataset for magma emplacement mechanisms, particularly in relation to how ambient and magmatic stresses, magma differentiation, rate of emplacement, and the methods by which space is progressively created, affects shallow-level laccolith construction.