Viscous-brittle deformation of shallowly emplaced silicic magma : Implications for outgassing and volcanic hazards

• Main Author: Witcher, Taylor
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: Uppsala universitet, Mineralogi, petrologi och tektonik 2024
• Subjects: magma deformation; magmatic outgassing; magmatic intrusions; rhyolite; Sandfell laccolith; crystal mush; volcanic hazards; Geology; Geologi; Iceland
• Online Access: http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-524861

Silicic magma in the shallow crust has the potential to violently erupt, depending on its ability to release overpressures caused by magmatic volatiles (outgassing). Deformation-induced outgassing is prevalent along volcanic conduit margins, where ascending magma is sheared at high rates. However, this mechanism limits outgassing to the contact with the host rock, leaving the bulk of the magma untouched and full of volatiles. This thesis presents a different mechanism of silicic outgassing that affects the interior volume of a magma body as well as the margins. Here, we present a case study of deformation features within the Miocene Sandfell laccolith, Eastern Iceland: a 0.57 km3 dome-shaped rhyolitic magma body with ~5 vol% phenocrysts and a microcrystalline groundmass. Similar textures have been reported in lava domes and intrusions with various compositions and crystallinities. The range of deformation features are 1. porous flow bands, 2. elongated pores within flow bands, 3. 1–5 cm long tensile fractures aligned in bands, 4. 5–20 cm fractures within bands (often multiple fracture sets), and 5. breccia (densely spaced bands that are no longer distinguishable). The bands in each category range in length from ‘lenses’ (~15 cm) to laterally expansive (several meters), and usually taper at the tips. The bands are interlayered with coherent, undeformed rhyolite, and their morphology varies between planar, undulating, and anastomosing. The chapters within this thesis characterize the spatial distribution of each stage of ‘fracture banding’ and interpret their role in magma emplacement (Paper I); analyze the textures of each deformation stage on a micro-scale to interpret the rheology of the magma during formation (Paper II); investigate the mineral assemblage of fracture fillings and apply results to metal separation from parent magma in early ore systems (Paper III); and attempt to experimentally recreate fracture bands in a laboratory setting (Paper IV). The results of these chapters suggest the ...