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Title: Geochronological and geochemical constraints on the tectonothermal history of Shetland
Author: Walker, Stephanie
Awarding Body: Royal Holloway, University of London
Current Institution: Royal Holloway, University of London
Date of Award: 2017
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Abstract:
This thesis aims to provide understanding of the pre-Devonian geology of Shetland, an archipelago northeast of Scotland, in three main ways. Firstly, assessment of the mode of formation for the Archaean gneisses on the North Roe peninsula using geochemistry and Nd isotopes to characterise their petrogenesis. Secondly, the timing of garnet growth within the metamorphic rocks of Shetland is determined using Lu-Hf and Sm-Nd geochronology, to provide direct constraints on the timing of metamorphism. Thirdly, deformation fabrics within the metamorphic rocks of Shetland are assessed using high-precision Rb-Sr mica geochronology. The felsic gneisses that crop out on the North Roe peninsula in northwest Mainland Shetland have chemical characteristics of a typical TTG (tonalite- trondhjemite- granodiorite) suite, and Nd isotopes that indicate extraction from a depleted mantle source during the Neoarchaean. Although this is similar to the age and source to the Lewisian Gneisses in northern mainland Scotland, evidence of metamorphism and deformation during the Neoproterozoic, Silurian, and Devonian preclude its direct correlation. The dating of metamorphic garnet indicates that the pre-Devonian rocks of Shetland are polyorogenic. Garnet growth related to the early Neoproterozoic Renlandian accretionary orogen is conspicuous by its absence, given the widespread evidence of this event in previous geochronological studies. However, there is evidence of garnet growth related to the mid Neo- proterozoic Knoydartian orogeny on the island of Yell. A cryptic metamorphic event between c. 630-585 Ma is found on the Walls peninsula and the island of Unst, and may relate to similar ages for metamorphic lithologies in Svalbard. Caledonian orogenesis is widespread throughout Shetland and appears to have occurred in an episodic manner. Episodes of Caledonian garnet growth are broadly synchronous with those in mainland Scotland, and major tectonothermal events occurred between 480-465 Ma; 455-445 Ma; and c. 430 Ma relating to the Grampian I and II orogens and the Scandian continental collision of Laurentia and Baltica. Dating of mica fabrics by high-precision Rb-Sr geochronology shows that deformation in Shetland was polyphase. Ages of 723-702 Ma from the Wester Keolka Shear Zone indicate a Knoydartian age for this structure, and preclude the correlation of this structure with the Silurian Moine Thrust. Mica Rb-Sr ages are comparable with the garnet ages determined in this study, and provides supporting evidence that Shetland has a similar metamorphic and deformational history to the Caledonides in mainland Scotland.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.792684  DOI: Not available
Keywords: Geochronology ; Geochemistry ; Isotope ; Garnet ; Mica ; Lu-Hf ; Sm-Nd ; Rb-Sr ; Shetland ; Scotland ; Caledonides ; TTG ; Archaean
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