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The formation and evolution of the Central Arran Igneous Complex

Gooday, Robert 2018. The formation and evolution of the Central Arran Igneous Complex. PhD Thesis, Cardiff University.
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Abstract

The Central Arran Igneous Complex – a system of intrusive and pyroclastic volcanic rocks – is one of the least understood onshore ‘central complexes’ of the British Palaeogene Igneous Province (BPIP). The BPIP is part of the wider North Atlantic Igneous Province, formed during opening of the North Atlantic Ocean and the arrival of the Iceland plume at the base of the lithosphere. Despite being known about for over a century, the magmatic and volcanic processes that formed the Central Arran Igneous Complex remain poorly understood. The Central Arran Igneous Complex comprises a number of granitic and dioritic intrusions, a caldera-fill succession of pyroclastic and sedimentary rocks intruded by a dolerite sill, and a series of dykes ranging in composition from picrite to pitchstone. This study uses a combination of field mapping, whole-rock elemental geochemistry, radiogenic isotope geochemistry, and U-Pb zircon geochronology to determine how these units and their magmas formed, how they relate to one another, and their relationship with other igneous rocks throughout the BPIP. Detailed study of the intra-caldera stratigraphy allows a prolonged volcanic history to be pieced together, with pyroclastic units separated by erosional surfaces indicating periods of volcanic quiescence and sedimentary processes. Following the initial period of caldera collapse, which was accompanied by highly explosive eruptions, volcanism was generally less explosive and formed abundant high-grade and lava-like ignimbrites. The majority of magmas were derived from a mantle source compositionally transitional between N-MORB and the Iceland plume. Geochemical differences between the mafic units are derived from different degrees of melting, from different source regions. All magmas show some degree of crustal contamination by various units, and their isotope geochemistry can be used to constrain the poorly understood crustal architecture in the region. U-Pb zircon geochronology shows that magmatism in central Arran took place over a very short space of time. The difference in interpreted 206Pb/238U ages between the oldest (Allt Ruadh Member ignimbrites; 58.92 ± 0.19 Ma) and the youngest (granitic Glenloig Hybrids; 58.71 ± 0.07 Ma) dated units is

Item Type: Thesis (PhD)
Date Type: Submission
Status: Unpublished
Schools: Earth and Ocean Sciences
Subjects: Q Science > QE Geology
Funders: Natural Environment Research Council
Date of First Compliant Deposit: 4 February 2019
Last Modified: 05 Feb 2019 13:54
URI: http://orca-mwe.cf.ac.uk/id/eprint/119175

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