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Floor subsidence and roof and wall-rock deformation during the emplacement of the Mourne Mountains granite pluton; Insights from the regional fracture pattern
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Mineralogy Petrology and Tectonics.ORCID iD: 0000-0003-0717-4014
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Mineralogy Petrology and Tectonics.
Physics of Geological Processes, The NJORD Centre, Department of Geo-sciences, University of Oslo, Box 1047, Blindern, 0316 Oslo, Norway.
Formerly at the Department of Earth Sciences, Uppsala University, Villavägen 16, 752 36, Uppsala, Sweden.
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The Mourne Mountains magmatic centre in Northern Ireland consist of five successively emplaced granites and has traditionally been viewed as a type locality of a magma body emplaced by cauldron subsidence, primarily because the granites truncate the host-rock bedding. Cauldron subsidence makes space for magma through ring dyking and floor subsidence. However, the Mourne granites were more recently re-interpreted as laccoliths. Laccolith intrusions form by inflation and dome their hosts. Here we perform a detailed study of the deformation in the host rock to the Mourne granite pluton in order to test its emplacement mechanism. We use the regional fracture pattern as a passive marker and microstructures in the contact-metamorphic aureole to constrain large-scale magma emplacement-related deformation. In addition, we use virtual scan lines to investigate the fracture sets. The dip and azimuth of the regional fractures are very consistent on the roof of the intrusion and can be separated into four steeply-dipping sets dominantly striking SE, S, NE, and E, which rules out pluton-wide doming. In contrast, fracture orientations in the wall to the granites in the NE show contact-strike parallel shear. In several samples from the contact-metamorphic aureole, segregations are displaced by fractures and shear zones, and indicate that the initial granite intrusion did not cause significant deformation of the host, while later granite pulses deformed the aureole rocks. Based on the north-eastward inclined granite-granite contacts, sub-vertical joints in the granites inclined to the west, and the westward younging succession of the granites and the displaced metamorphic segregations, we propose that multiple mechanisms involving asymmetric ‘trap-door’ floor subsidence and deflection of the north-eastern wall of the intrusion parallel to a propagating ring-dyke accommodated the emplacement of the granites.

Keywords [en]
Mourne Mountains granite pluton, magma emplacement, Hawick Group, cauldron subsidence, laccolith, trap-door floor subsidence
National Category
Geology
Research subject
Earth Science with specialization in Mineral Chemistry, Petrology and Tectonics
Identifiers
URN: urn:nbn:se:uu:diva-363441OAI: oai:DiVA.org:uu-363441DiVA, id: diva2:1256783
Available from: 2018-10-18 Created: 2018-10-18 Last updated: 2018-10-19
In thesis
1. The dynamic emplacement of felsic magma in the upper crust
Open this publication in new window or tab >>The dynamic emplacement of felsic magma in the upper crust
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Felsic magma intrudes earth’s upper crust through a variety of mechanisms. Magma intrusion growth and shape have mainly been explained in terms of host rock properties and intrusion depth, while considering the magma as an overpressurised fluid. However, volcanologists view a magma as a rheologically evolving fluid, which affects the magma flow in volcanic conduits. This thesis seeks to explore intrusion dynamics during magma emplacement by taking both the magma and the host rock into account. The first part of the thesis investigates the emplacement of the Sandfell laccolith/cryptodome, the Cerro Bayo cryptodome and the Mourne granite pluton. Both cryptodomes grew initially by inflation, which resulted in contact-parallel magma flow. Later during the emplacement, the rim of the intrusions viscously stalled as indicated by brecciation and fracturing in the intrusion rims, which then forced them to grow vertically. Our observations suggest that rheological changes in the magma during intrusion growth may control the shape of the cryptodomes/laccoliths. Previously proposed emplacement mechanisms of the Mourne Mountains granite pluton were tested by investigating host-rock deformation and the surrounding contact-metamorphic aureole. The aureole displays contact-metamorphic segregations that were later deformed by brecciation and shearing. The consistent regional fracture patterns in the pluton roof indicate that it was not widely domed, while the north-eastern wall of the pluton was deflected parallel to the strike of the contact. These observations suggest that multiple mechanisms emplaced the pluton, involving both floor subsidence and deflection of the roof and wall.

The last part of the thesis studies the magma plumbing system to the Holuhraun 2014-15 eruption with mineral and whole-rock geochemistry and thermobarometry. The Holuhraun eruption was accompanied by subsidence in the Bárðarbunga caldera but occurred in the Askja volcanic system. Our results show that the Holuhraun eruption was fed from a vertically extensive magma plumbing system in the Bárðarbunga volcanic system.

The works of this thesis highlight that felsic magma emplacement in the upper crust involves multiple and dynamic mechanisms that control the growth and shape of the intrusion and that the interplay between magma and host-rock properties needs to be considered.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2018. p. 69
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1736
Keywords
laccolith, cryptodome, magma emplacement, magma flow, magma rheology, granite, rhyolite, pluton, AMS, XCT, thermobarometry
National Category
Geology
Research subject
Earth Science with specialization in Mineral Chemistry, Petrology and Tectonics
Identifiers
urn:nbn:se:uu:diva-363445 (URN)978-91-513-0483-0 (ISBN)
Public defence
2018-12-07, Hambergsalen, Geocentrum, Villavägen 16, Uppsala, 09:00 (English)
Opponent
Supervisors
Available from: 2018-11-13 Created: 2018-10-19 Last updated: 2018-11-30Bibliographically approved

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