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Syn-Emplacement Fracturing in the Sandfell Laccolith, Eastern Iceland: Implications for Rhyolite Intrusion Growth and Volcanic Hazards
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Mineralogy Petrology and Tectonics.
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Mineralogy Petrology and Tectonics. Center for Natural Hazards and Disaster Science, Uppsala University, Uppsala, Sweden.ORCID iD: 0000-0002-3316-658X
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.ORCID iD: 0000-0002-9385-7614
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Mineralogy Petrology and Tectonics. Center for Natural Hazards and Disaster Science, Uppsala University, Uppsala, Sweden.
2018 (English)In: Frontiers in earth science, E-ISSN 2296-6463, Vol. 6, article id 5Article in journal (Refereed) Published
Abstract [en]

Felsic magma commonly pools within shallow mushroom-shaped magmatic intrusions, so-called laccoliths or cryptodomes, which can cause both explosive eruptions and collapse of the volcanic edifice. Deformation during laccolith emplacement is primarily considered to occur in the host rock. However, shallowly emplaced laccoliths (cryptodomes) show extensive internal deformation. While deformation of magma in volcanic conduits is an important process for regulating eruptive behavior, the effects of magma deformation on intrusion emplacement remain largely unexplored. In this study, we investigate the emplacement of the 0.57 km3 rhyolitic Sandfell laccolith, Iceland, which formed at a depth of 500 m in a single intrusive event. By combining field measurements, 3D modeling, anisotropy of magnetic susceptibility (AMS), microstructural analysis, and FEM modeling we examine deformation in the magma to constrain its influence on intrusion emplacement. Concentric flow bands and S-C fabrics reveal contact-parallel magma flow during the initial stages of laccolith inflation. The magma flow fabric is overprinted by strain-localization bands (SLBs) and more than one third of the volume of the Sandfell laccolith displays concentric intensely fractured layers. A dominantly oblate magmatic fabric in the fractured areas and conjugate geometry of SLBs, and fractures in the fracture layers demonstrate that the magma was deformed by intrusive stresses. This implies that a large volume of magma became viscously stalled and was unable to flow during intrusion. Fine-grained groundmass and vesicle-poor rock adjacent to the fracture layers point to that the interaction between the SLBs and the flow bands at sub-solidus state caused the brittle-failure and triggered decompression degassing and crystallization, which led to rapid viscosity increase in the magma. The extent of syn-emplacement fracturing in the Sandfell laccolith further shows that strain-induced degassing limited the amount of eruptible magma by essentially solidifying the rim of the magma body. Our observations indicate that syn-emplacement changes in rheology, and the associated fracturing of intruding magma not only occur in volcanic conduits, but also play a major role in the emplacement of viscous magma intrusions in the upper kilometer of the crust.

Place, publisher, year, edition, pages
2018. Vol. 6, article id 5
Keywords [en]
laccolith, cryptodome, magma flow, intrusion emplacement, strain localization, magma degassing, volcanic hazards
National Category
Geology
Research subject
Earth Science with specialization in Mineral Chemistry, Petrology and Tectonics
Identifiers
URN: urn:nbn:se:uu:diva-340868DOI: 10.3389/feart.2018.00005ISI: 000429857800001OAI: oai:DiVA.org:uu-340868DiVA, id: diva2:1180110
Funder
The Royal Swedish Academy of SciencesSwedish Research Council, 2015-03931_VRAvailable from: 2018-02-05 Created: 2018-02-05 Last updated: 2018-10-19Bibliographically approved
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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Publisher's full texthttp://journal.frontiersin.org/article/10.3389/feart.2018.00005/full

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Mattsson, TobiasBurchardt, SteffiAlmqvist, Bjarne S. G.Ronchin, Erika

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