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

岩浆管道系统通常由垂直岩脉主导，垂直岩脉将岩浆从深处输送到浅层储层或在地表喷发（Magee et al., 2018a）。在活火山环境中，我们跟踪地下岩脉侵入以监测和评估火山危害。我们还分析了喷发岩石的岩石学和化学特征，揭示了岩浆储层内部控制喷发的过程和危险类型；这些过程还可以推动具有重要经济意义的矿床的形成。然而，在这些系统中，堤坝通常被认为是连接岩浆储存区域和喷发地点的相对简单的通道。然而，人们越来越意识到，岩脉内的物理和化学岩浆过程在时间和空间上可能非常小，从而产生岩脉内岩浆行为不同的区域（例如，Magee et al., 2016; Holness et al., 2017）。例如，岩脉可能会对流，在某些地区集中岩浆上升（Holness et al., 2017），而岩脉的其他部分可能会停滞并允许分层发生（Upton & Thomas, 1980）。因此，岩脉内的岩浆过程比许多先前的研究和模型所考虑的更为复杂。关键的是，我们不知道岩脉内部的这种复杂性如何影响喷发地点的位置、矿物（包括矿床）的分布，或者岩浆在运输过程中的演化和流变。解决这一知识缺口将为火山危害评估和矿产勘探提供至关重要的基本见解。该项目将研究格陵兰岛东南部约11亿年的Tugtutôq巨型岩脉，它代表了元古代裂谷岩浆活动的一个庞大阶段（Upton & Blundell, 1978）。巨型岩脉，如Tugtutôq中的岩脉，非常宽（10 -100米），通常包含矿物层状带（例如，Upton & Thomas, 1980），已知形成重要的矿物和金属矿床（例如，Chaumba & Musa, 2020）。这些巨大的岩脉是研究岩脉内部岩浆过程的绝佳天然实验室，因为它们包含离散的区域（例如分层豆荚和假定流动区域），岩浆在这些区域的表现不同，可以很容易地取样。先前的矿物学和地球化学研究提供了对Tugtutôq Giant Dykes岩石形成历史和相关Ti-Fe矿床的广泛了解（见Steenfelt et al., 2016）。然而，岩脉注入的机制、原始矿物层状的形成以及这些侵入体内部域之间的相互作用仍未得到研究。本博士将专门研究Tugtutôq巨型岩脉内岩浆运动和结晶的物理和化学记录，以确定对以下因素的控制：(1)岩浆流动的局部化，这可能对喷发地点的位置和分布产生关键影响；(2)冻结岩浆中矿物的聚集和演化，揭示岩浆化学在成岩作用和伴生矿床形成过程中是如何被改变的。得到的数据将帮助研究人员确定岩浆如何穿过岩脉并在岩脉内停滞，以及这些过程如何影响岩浆演化、矿化和喷发方式。

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.