Giant Dykes and Deep Time Tectonics; the Role of Dyke Swarms in Shaping the Early Earth

Magma plumbing systems are commonly dominated by vertical dykes, which deliver magma from depth into shallow-level reservoirs or to be erupted at the surface (Magee et al., 2018a). In active volcanic settings, we track subsurface dyke intrusions to monitor and assess volcano hazards. We also analyse the petrology and chemistry of erupted rocks to shed light on the processes within magma reservoirs that control eruption and hazard style; these processes can also drive the formation of economically important ore deposits. However, within these systems, dykes are typically assumed to be relatively simple pathways that link areas of magma storage to eruption sites. Yet there is a growing awareness that physical and chemical magma processes within dykes can very over small-scales in time and space, giving rise to domains within dykes where magma behaves differently (e.g., Magee et al., 2016; Holness et al., 2017). For example, dykes may be expected to convect, focusing magma ascent in some areas (Holness et al., 2017), whereas other parts of dykes may stagnate and allow layering to occur (Upton & Thomas, 1980). Magma processes within dykes are therefore more complex than many previous studies and models consider. Critically, we do not know how such complexity within dykes could impact the location of eruption sites, the distribution of minerals (including ore deposits), or the evolution and rheology of magma during transport. Addressing this knowledge gap in our understanding of dyking will provide fundamental insights crucial to volcano hazard assessment and mineral exploration.

This project will examine the ~1.1 billion year old Tugtutôq Giant Dykes of SE Greenland, which represent a voluminous phase of Proterozoic rift magmatism (Upton & Blundell, 1978). Giant dykes, like those in Tugtutôq, are extremely wide (10's-100's m) and often contain zones of mineral layering (e.g., Upton & Thomas, 1980), which are known to form important mineral and metal ore deposits (e.g., Chaumba & Musa, 2020). These giant dykes represent an excellent natural laboratory to study magma processes within dykes because they contain discrete domains (e.g. layered pods and areas of presumed flow), where magma behaved differently, which can be easily sampled. Previous mineralogical and geochemical studies provide a broad understanding of the Tugtutôq Giant Dykes petrogenic history and associated Ti-Fe deposits (see Steenfelt et al., 2016). However, the mechanics of dyke injection, the formation of pristine mineral layering, and the interaction between domains within these intrusions remain unstudied.

This PhD will specifically examine physical and chemical records of magma movement and crystallisation within the Tugtutôq Giant Dykes, in order to determine the controls on the: (1) localisation of magma flow, which may represent a key influence on the location and distribution of eruption sites; and (2) accumulation and evolution of minerals within stagnating magma, which will shed light on how magma chemistry is modified during dyking and formation of associated ore deposits. The resulting data will help the community to pinpoint how magma moves through and stalls within dykes, and how these processes impact magma evolution, mineralisation, and eruption style.