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- Burchardt, Steffi, et al.
(författare)
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Progressive growth of the Cerro Bayo cryptodome, Chachahuén volcano, Argentina – implications for viscous magma emplacement
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 5. |
- Mattsson, Tobias, et al.
(författare)
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Magma Deformation During Laccolith Emplacement: Examples From Iceland And Argentina
- 2018
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Ingår i: Geological Society of America Abstracts with Programs.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- 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 show extensive internal, magmatic 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. By combining field mapping, anisotropy of magnetic susceptibility and microstructural analysis, we studied the emplacement of two laccoliths emplaced within the upper kilometer of the crust in a single intrusive event. The rhyolitic Sandfell laccolith, Iceland, which intruded at a depth of 500 m and the dacitic Cerro Bayo laccolith, Argentina. The initial growth stage of the two laccoliths is characterized by contact parallel flow indicators that formed during inflation of the laccolith. The second growth stage encompass brecciation and fracturing in the rim of the intrusion. Fractures in the Sandfell laccolith occur in layers and individual fractures are parallel to strain-localization bands in coherent rhyolite. A dominantly oblate magmatic fabric in the fractured areas, S-C fabrics in flow bands and conjugate geometry of strain-localization bands and the fractures demonstrate that the magma was sheared and compacted by the continuous intrusion of magma into the bodies. This further implies that the rims of the magma bodies essentially solidified during the intrusive event. In the third stage of growth, the stalled rim of the laccolith is breached, which promotes vertical growth of the magma body facilitated by larger steeply-dipping faults and shear zones. Our observations indicate that syn-emplacement changes in magma rheology play a major role in the emplacement of viscous magma intrusions in the upper kilometer of the crust.
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| 6. |
- Mattsson, Tobias, 1990-, et al.
(författare)
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Quantifying the crystal cargo of the Cerro Bayo cryptodome, Argentina; A window into pre-emplacement magma processes and storage conditions
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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