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The origins of accreted units in the Monian Composite Terrane, North Wales UK

Groome, Niall 2023. The origins of accreted units in the Monian Composite Terrane, North Wales UK. PhD Thesis, Cardiff University.
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The subduction complex of the Monian Composite Terrane (MCT) records two distinct episodes of subduction – late Neoproterozoic to early Cambrian, and early Ordovician – separated by a transient transcurrent regime. The first episode is characterised by the high P-T Penmynydd Terrane and the accretionary Gwna Complex, which formed at the continental margin of Gondwana, backed by the Avalonian arc. Transcurrent faulting along the arc front juxtaposed terranes of various origins against one another to assemble the MCT. The lower Ordovician marked a short-lived renewal of subduction before back-arc rifting led to detachment of the Avalonian microcontinent. This project combines field relations with geochemical petrographic and mineralogical information to discuss the genesis subduction-related MCT units to decipher deformed remnants of past tectonic environments, and to interpret the convergent regime that led to their destruction, and ultimate preservation. The Gwna Complex preserves a range of deformation states, largely dependent on volume of accreted trench-fill sediments. Block-in-matrix mélange, thick disrupted turbidites and, and olistostromes are common in areas dominated by clastic sediment. Sediment starved areas exhibit predominantly imbricated units of ocean floor material with partially preserved ridge-trench ocean plate stratigraphy (OPS) – semi-coherent fragments of sea floor sedimentary successions and upper parts of igneous substrate from a subducting slab – while block-in-matrix mélange is confined to thin, isolated pelitic beds in highly sheared zones between low shear lenticular units. Basalt substrates exhibit MORB-OIB geochemical signatures with significant variability of enrichment over short distances that may represent off-axis seamount development. Dual OPS sequences are proposed, representing seamount stratigraphy and adjacent ocean floor stratigraphy. Intraplate volcanism, likely in the form of a petit-spot seamount, was also recognised through a combination of field evidence, petrography, and whole rock geochemistry. Metabasites in the Penmynydd Terrane contain sodic amphiboles including glaucophane and magnesio-riebeckite paired with epidote, indicating peak blueschist facies metamorphism. Relict pillow textures and consistent geochemical compositions with the Gwna Complex indicate formation at the same subduction zone. The Cemaes Group is recognised as a separate episode of subduction recorded in sedimentary mélange. A continuous volcaniclastic base of continental arc affinity indicates resumption of subduction and arc magmatism, followed by a gradual period of uplift and resultant olistostrome deposition.

Item Type: Thesis (PhD)
Date Type: Completion
Status: Unpublished
Schools: Earth and Environmental Sciences
Funders: KESS2
Date of First Compliant Deposit: 26 May 2023
Last Modified: 31 May 2023 11:13

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