Layering mechanisms, compaction, and liquid migration in the layered nepheline syenites of the Ilímaussaq complex, South Greenland

Abstract

Understanding the mechanics of mineral layering and subsequent modification within igneous intrusions is of fundamental importance for magmatic differentiation, the accumulation of crystal-poor melts, and the formation of ore deposits. Layered igneous intrusions offer a unique opportunity to study these broad processes as they commonly preserve multiple stages of their crystallisation history. The Ilímaussaq complex, South Greenland, is of particular interest as it displays clear modal layers that host the world’s second most valuable rare earth element (REE) deposit.

This thesis investigates three key processes at Ilímaussaq: (1) the initial development of mineral layers; (2) if and how compaction occurred in the layered mush; and (3) late-stage liquid migration through the mush and how this enhanced or diminished REE concentration. This is achieved by quantifying and recording rock fabrics and textures through combined stratigraphic logging, rock magnetic, petrographic, and crystallographic preferred orientation methods.

Detailed stratigraphic logs, magnetic fabrics, and CPO reveal sub-horizontal silicate mineral foliations across all igneous layers, consistent with layer formation through gravitational settling of crystals. These foliations are often oblique to the modal layer contacts, suggesting post-cumulus modification, likely due to compaction. Rock magnetic fabrics also show sub-vertical patterns orthogonal to igneous layering, indicating vertical liquid migration through the mush.

The data presented in this thesis support a new model for REE enrichment in layered igneous intrusions, where igneous layering forms through gravitational settling of crystal mats, followed by mechanical compaction. During compaction, a saline, hydrous liquid migrates vertically through the crystal pile. The trapping and movement of this late-stage liquid through impermeable and permeable layers plays a key role in REE concentration. This study highlights the value of structural and textural analysis in understanding layered intrusions, especially when paired with robust field context. The methods and insights from Ilímaussaq are broadly applicable to layered intrusions worldwide.

Date of Award	2 Dec 2025
Original language	English
Awarding Institution	University of St Andrews
Supervisor	William McCarthy (Supervisor) & William Hutchison (Supervisor)