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The dynamic emplacement of felsic magma in the upper crust
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Mineralogy Petrology and Tectonics. Uppsala University.ORCID iD: 0000-0003-0717-4014
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 [en]
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: urn:nbn:se:uu:diva-363445ISBN: 978-91-513-0483-0 (print)OAI: oai:DiVA.org:uu-363445DiVA, id: diva2:1257124
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
List of papers
1. Syn-Emplacement Fracturing in the Sandfell Laccolith, Eastern Iceland: Implications for Rhyolite Intrusion Growth and Volcanic Hazards
Open this publication in new window or tab >>Syn-Emplacement Fracturing in the Sandfell Laccolith, Eastern Iceland: Implications for Rhyolite Intrusion Growth and Volcanic Hazards
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.

Keywords
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:nbn:se:uu:diva-340868 (URN)10.3389/feart.2018.00005 (DOI)000429857800001 ()
Funder
The Royal Swedish Academy of SciencesSwedish Research Council, 2015-03931_VR
Available from: 2018-02-05 Created: 2018-02-05 Last updated: 2018-10-19Bibliographically approved
2. Floor subsidence and roof and wall-rock deformation during the emplacement of the Mourne Mountains granite pluton; Insights from the regional fracture pattern
Open this publication in new window or tab >>Floor subsidence and roof and wall-rock deformation during the emplacement of the Mourne Mountains granite pluton; Insights from the regional fracture pattern
(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
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:nbn:se:uu:diva-363441 (URN)
Available from: 2018-10-18 Created: 2018-10-18 Last updated: 2018-10-19
3. Progressive growth of the Cerro Bayo cryptodome, Chachahuén volcano, Argentina – implications for viscous magma emplacement
Open this publication in new window or tab >>Progressive growth of the Cerro Bayo cryptodome, Chachahuén volcano, Argentina – implications for viscous magma emplacement
Show others...
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Cryptodome and dome collapse is associated with volcanic hazards, such as, explosive eruptions, pyroclastic flows, and volcanic edifice collapse. Study of the growth and evolution of volcanic domes provides vital information on the link between dome growth and the development of weakness zones that may cause collapse. The Cerro Bayo cryptodome is superbly exposed in the eroded Miocene Chachahuén volcano in the Neuquén basin, Argentina. Cerro Bayo is a >0.3 km3 trachyandesitic cryptodome that intruded within the uppermost kilometer of the Chachahuén volcano. Here we investigate the emplacement of the Cerro Bayo cryptodome using structural mapping, photogrammetry, 3D structural modelling and measurement of magma flow indicators, brittle deformation features and magnetic fabrics with anisotropy of magnetic susceptibility (AMS). Magma flow fabrics near the margin are concentric and indicate contact-parallel flow and internal inflation of the body. Magmatic and magnetic fabrics and fracture patters in the interior of the cryptodome are more complex and outline several structural domains. These domains are separated by magmatic shear zones that accommodated intrusion growth. The shear zones locally overprint the earlier formed concentric fabric. The nature of the structural domains shows that emplacement of Cerro Bayo occurred in three stages that resemble the endogenous to exogenous growth of volcanic domes. The formation of magmatic shear zones during cryptodome formation may have a profound effect on cryptodome stability by creating weakness zones that increase the risk of collapse.

Keywords
Cryptodome, Intrusion emplacement, AMS, magmatic breccia, magma flow, volcanic hazards, laccolith, dome collapse, viscous magma
National Category
Geology
Research subject
Earth Science with specialization in Mineral Chemistry, Petrology and Tectonics
Identifiers
urn:nbn:se:uu:diva-363407 (URN)
Available from: 2018-10-18 Created: 2018-10-18 Last updated: 2018-10-19
4. Quantifying the crystal cargo of the Cerro Bayo cryptodome, Argentina; A window into pre-emplacement magma processes and storage conditions
Open this publication in new window or tab >>Quantifying the crystal cargo of the Cerro Bayo cryptodome, Argentina; A window into pre-emplacement magma processes and storage conditions
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

Shallow plumbing systems with intermediate to evolved volcanic systems can show complex magma recycling and recharge systematics. Such evidence is often stored in the information looked within the crystal cargo found in shallow systems. The Cerro Bayo cryptodome provides a classic example of such processes and was emplaced in the upper kilometre of the Miocene, back-arc Chachahuén volcano. Cerro Bayo formed during an extensive dome-building (and collapse) stage of the Chachahuén volcano, and is composed of a porphyritic trachyandesite which contains numerous mafic enclaves. This contribution investigates the deeper magma plumbing system and magmatic processes preceding the emplacement of Cerro Bayo with petrography, mineral chemistry, crystal size distributions and thermobarometry. The main crystals found in Cerro Bayo are plagioclase and amphiboles, found as mixed phenocryst/antecryst and glomerocryts, constituting about 30 vol. % of the rock. Minor mineral phases include pyroxene, apatite and magnetite. Several crystal populations are identified within the Cerro Bayo, which indicate differences within the growth histories of the populations. Some plagioclase crystal display resorbed textures and show zones with sharp increases in An mol% (up to 25 %), indicative of recharge events with hot (more mafic) magma. Whereas other crystals record only small temperature shifts, likely induced by latent heat of crystallisation. Amphibole and clinopyroxene barometers yield average crystallisation pressure estimates that range from 740 to 1036 MPa, however, they overlap within error of the barometers, revealing a storage region prior to shallow emplacement at about 30 to 40 km depth in the lower crust. The voluminous crystal cargo, diverse zoning patterns, sieve-textured plagioclase and the abundant enclaves in Cerro Bayo suggest that the magma was derived from a crystal mush storage region that was mobilised and partly mixed by the recharge of mafic melt.

Keywords
Laccolith, Cryptodome, Magma Plumbing System, Thermobarometry, Crystal size distribution, Chachahuén volcano
National Category
Geology
Research subject
Earth Science with specialization in Mineral Chemistry, Petrology and Tectonics
Identifiers
urn:nbn:se:uu:diva-363408 (URN)
Available from: 2018-10-18 Created: 2018-10-18 Last updated: 2018-10-19
5. Magma plumbing for the 2014–2015 Holuhraun eruption, Iceland
Open this publication in new window or tab >>Magma plumbing for the 2014–2015 Holuhraun eruption, Iceland
Show others...
2016 (English)In: Geochemistry Geophysics Geosystems, ISSN 1525-2027, E-ISSN 1525-2027, Vol. 17, no 8, p. 2953-2968Article in journal (Refereed) Published
Abstract [en]

The 2014–2015 Holuhraun eruption on Iceland was located within the Askja fissure swarm butwas accompanied by caldera subsidence in the Barðarbunga central volcano 45 km to the southwest. Geophysicalmonitoring of the eruption identified a seismic swarm that migrated from Barðarbunga to theHoluhraun eruption site over the course of two weeks. In order to better understand this lateral connectionbetween Barðarbunga and Holuhraun, we present mineral textures and compositions, mineral-meltequilibriumcalculations, whole rock and trace element data, and oxygen isotope ratios for selected Holuhraunsamples. The Holuhraun lavas are compositionally similar to recorded historical eruptions from theBarðarbunga volcanic system but are distinct from the historical eruption products of the nearby Askja system.Thermobarometry calculations indicate a polybaric magma plumbing system for the Holuhraun eruption,wherein clinopyroxene and plagioclase crystallized at average depths of 17 km and 5 km,respectively. Crystal resorption textures and oxygen isotope variations imply that this multilevel plumbingsystem facilitated magma mixing and assimilation of low-d18O Icelandic crust prior to eruption. In conjunctionwith the existing geophysical evidence for lateral migration, our results support a model of initial verticalmagma ascent within the Barðarbunga plumbing system followed by lateral transport of aggregatedmagma batches within the upper crust to the Holuhraun eruption site.

National Category
Geology
Research subject
Earth Science with specialization in Mineral Chemistry, Petrology and Tectonics
Identifiers
urn:nbn:se:uu:diva-304630 (URN)10.1002/2016GC006317 (DOI)000384808200001 ()
Funder
Swedish Research Council
Available from: 2016-10-06 Created: 2016-10-06 Last updated: 2018-10-19Bibliographically approved

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