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Title: Integrated crustal processes : micro-scale to macro-scale
Author: Dempster, Tim
ISNI:       0000 0001 2418 3399
Awarding Body: University of Glasgow
Current Institution: University of Glasgow
Date of Award: 2014
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This thesis contains a selection of forty-three research papers [1-43], published by the author that investigate key processes in the formation and stabilization of continental crust. The processes are addressed at a micro-scale and linked to crustal processes at a global scale. Many of the studies included in this thesis take an integrated but novel approach, typically combining disciplines in ways different to "traditional" research on crustal rocks and/or studying mineral groups in ways not typically used to investigate crustal processes. Metamorphic equilibrium in rocks is driven by the thermodynamic forces controlling the stability of mineral assemblages, and is inhibited by kinetic inertia of mineral reactions. It is the balance between these two factors that controls metamorphic reactions. Much existing literature on metamorphic processes is concerned with assessing thermodynamically constrained equilibrium conditions. This thesis includes many studies that emphasize the importance of chemical disequilibrium preserved by individual minerals and the kinetic consequences for metamorphic processes [1,2,3,4]. Deformation and volume diffusion are recognized as key factors in allowing thermodynamic equilibrium to be established between minerals and the significance of many metamorphic changes is re-evaluated. It is within polymetamorphic rocks that the kinetic "problems" associated with metamorphic reactions are most apparent, such that only rocks that have experienced multiple events at similar temperatures reliably retain the evidence [5]. The importance of kinetic controls on equilibrium is emphasized both in their effect on major rock-forming minerals and in ground breaking petrological studies of accessory phases. Solid-state diffusion is a key to allowing chemical and isotopic transport within grains, however, communication between grains is typically controlled by the characteristics of grain boundaries such as the presence, absence or geometry of fluids. As such grain edges are probably the most important, but perhaps the least studied, "phase" present in any rock. Innovative approaches have been used to investigate the characteristics of mineral boundaries [8,9], the extent of isotopic exchange within rocks [6] and the role that highly localized fluid infiltration may have on the metamorphic responses of the crust [7,9]. Many of the studies address key factors controlling metamorphic processes and use a variety of different minerals, such as garnet [1], muscovite [2,3,7], apatite [8,9] and zircon [10,11] to assess crustal behaviour. The use of accessory minerals to directly understand a range of metamorphic processes is a unique aspect to the research. Studies included here describe zircon and apatite textures in-situ within rocks. Zircon was previously thought of as an exceptionally stable unreactive mineral, but in a series of studies is shown to be one of the most reactive and hence informative of silicate minerals capable of recording reaction histories and fluid movements through a wide range of crustal conditions [12,13,14,15,16]. In studies of crustal behaviour, time is a key component and investigations of the fundamental controls on metamorphic processes have been integrated with data from thermochronometers to provide insights into the low temperature cooling history of the crust. Rb-Sr and K-Ar geochronology on biotite and muscovite, lower temperature thermochronometers including apatite fission track, U-Th-He on apatite and zircon, together with cosmogenic isotopes are applied in a range of investigations to assess unroofing histories. The impact of spatial and temporal variability of exhumation rates is linked back to metamorphic and structural processes within the deeper crust [17,18,24]. The variation acts as a trigger to structural collapse [19], causes major diachroneity in metamorphic events [18] and facilitates significant lateral heat transfer that impacts on isograd patterns [22]. Surprising general conclusions are reached suggesting that some thermochronometers can not record erosional unroofing but may reveal the thermal influence of fluid movements [20,21]. Factors such as localized uplift, and fluid and magma movements in the crust, are explored further as effective agents for modifying lateral and vertical geothermal gradients in a range of tectonic settings and point to considerable complexity in the geothermal gradients of orogenic belts [22,23,24,25]. The exotic nature of crustal terranes in the British Caledonides is addressed in a range of collaborative studies, through a variety of isotopic determinations, constraining movements, amalgamation histories and events within crustal blocks [26,27,28]. Such studies are then integrated with petrological and stratigraphic evidence to present models for crustal evolution in the Caledonides [29,31,32] and in addition develop general models for the formation of metamorphic terranes in strike-slip environments [30]. The approach of using detailed characterization of minerals to understand metamorphic rock-forming processes is similarly applied to deciphering magmatic processes in the crust. A wealth of published research on the petrogenesis of igneous rocks focusses on bulk rock geochemical and isotopic approaches to study the origin of the melts and examples of such investigations are included here [35,36,37]. However many of the studies included in this thesis emphasise small-scale chemical disequilibrium, question this approach, and open new avenues to investigate magmatic processes. The evolution of slowly cooling granite magma chambers is studied at a range of different scales using zoned feldspars [37,38,39] and accessory minerals [42,43]. Crustal contamination [40], magma mixing [39,42,43], sub-solvus crystallization [37,38], and late stage permeability [42] are all processes that are investigated through detailed textural and geochemical characterization of magmatic minerals. The importance of inefficient diffusion is again emphasized and the controls on melt permeability during crystallization are highlighted. Such techniques may yield unprecedented details of the magmatic processes that complement traditional whole rock geochemical and isotopic investigations. Small-scale processes that operate in magma chambers are also linked to models of large-scale crustal generation processes, including the formation of the enigmatic Late Proterozoic massif anorthosites [41]. The theme throughout the thesis is integration of geological disciplines that are not commonly combined. Metamorphic processes traditionally linked to thermodynamics are investigated via kinetic controls, such as deformation and fluid access [3,4,13,16]. Denudation histories traditionally linked to surface processes are integrated with metamorphic histories and structural evolution [e.g. 18,19,25]. Magmatic systems traditionally investigated using bulk rock geochemical and isotopic approaches are instead studied using disequilibrium crystallization histories of minerals [e.g. 38,39,42,43].
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (D.Sc.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QE Geology