Bovay, Thomas (2021). Origin and pressure-temperature-time-fluid evolution of a subducted volcanoclastic sequence: the Theodul Glacier Unit (Western Alps, Switzerland). (Thesis). Universität Bern, Bern
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Abstract
Ophiolites found within mountain belts testify the closure of oceanic basins at convergent margins and consists of variably hydrated exhumed fragments of oceanic lithosphere that may have undergone high-pressure metamorphism and deformation. This study investigates the Theodul Glacier Unit (TGU), a tectonic slice of volcanoclastic material within the Zermatt Saas meta-ophiolite (Western Alps) that was subducted during the closure of the Piemonte-Ligurian ocean. This well-exposed and preserved rock association offers the opportunity to gain insights into three crucial aspects of subduction, namely (1) the composition and variability of the input crustal material, (2) the variation of the geothermal gradient and (3) the generation and transfer of fluids within the slab. The location of the TGU within the meta-ophiolites of the Zermatt-Saas Zone (ZSZ) and its unusual lithostratigraphy raises question about its origin. In Chapter 1 it is shown that the preserved heterogeneous layering of the TGU is composed of lithologies with mafic (OIB) to felsic (UCC) composition that all contain a variable detrital input of Permian age. The internal structures and lithologies of the TGU are not directly comparable to the sedimentary cover of the ZSZ, or to the continental outliers embedded within the ZSZ, nor to crystalline basements. The TGU is thus interpreted as a volcanoclastic sequence and as a new type of allochthonous sedimentary cover of the Piemont-Ligurian oceanic crust in the Western Alps. Collisional belts such as the Alps, necessarily include units with diverse origin and age and complex metamorphic evolutions. In the Western Alps, mono-metamorphic Alpine rocks record clockwise, high-pressure and low-temperature paths, where relatively small domains of UHP relicts are preserved. In contrast, pre-Alpine basement rocks evolved along hotter geotherms typical of collision and extension. In Chapter 2, garnet Lu-Hf ages from different lithologies of the TGU yield a restricted garnet crystallization time window between 50.3 and 48.8 Ma (± 0.5%, 2SD). The Alpine ages were measured even in garnet with complex 2-stages zoning observed in the schists, which further indicates an Alpine mono-metamorphic evolution for the TGU. Multiphase equilibrium thermodynamic modelling of garnet, phengite and rutile constrains a tight ß-shape pressure-temperature (P–T) path with significant variation in pressure (26.5 ± 1.0 to 15.0 ± 1.0 kbar) in a very limited temperature range of ~30 °C. The older 50.3 ± 0.3 Ma Lu-Hf age is related to the high-P stage, whereas initial exhumation was rapid and led to isothermal decompression within 1 Myr prior to a reheating stage. Distinct stages of the ß-shape metamorphic path correspond to different geotherms, where the reheating is best explained by upwelling of hot asthenospheric mantle material and transient storage of the unit at the crust–mantle boundary. Enigmatic garnet porphyroblasts from one type of schists have many similarities with those from other samples (Chapter 4). However, this garnet type yields a Lu-Hf isochron date of 168.7 ± 1.8 Ma, which awaits confirmation before a scenario including a Jurassic stage can be fully evaluated. In subduction zones, the aqueous fluids generated through hydrous phase breakdown have individual signatures, which equilibrated oxygen isotope composition is function of the bulk δ18O, rock chemistry and P–T conditions. Garnet from TGU lithologies show important intracrystalline drop of ~8 ‰ in δ18O, corresponding to sharp chemical zoning between a xenomorphic core and a euhedral rim (Chapter 3). Such variation in oxygen isotope is inconsistent with a closed system evolution and demand influx of external fluids in isotopic disequilibrium. Thermodynamic and δ18O models constrain the interaction with external fluids at high–P and imply that the large amount of low δ18O H2O required for the isotopic shift was in isotopic equilibrium with the surrounding serpentinites. The calculated time-integrated fluid flux across the TGU rocks is above the open-system behaviour threshold and argues for pervasive flow across the unit. The transient rock volume variations caused by lawsonite breakdown in the TGU schist is identified as the possible trigger for the pervasive fluid influx. This process can eventually form a transient water-filled porosity network with sufficient connection to allow fluid mobilization and promote pervasive infiltration of external fluids. Thus, a sustained fluid income can prevent the newly formed porous network to collapse and efficiently modify bulk δ18O. The work conducted in this thesis showed the importance and challenges of applying a multidisciplinary approach that takes into account field geology, petrology, modelling, geochronology and geochemistry in order to gather accurate information for reconstructing the geological evolutions of metamorphic units. This detailed record can then inform our understanding of processes such as crustal formation and recycling, as well as dynamics of subduction.
| Item Type: | Thesis |
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| Dissertation Type: | Cumulative |
| Date of Defense: | 31 March 2021 |
| Subjects: | 500 Science > 550 Earth sciences & geology |
| Institute / Center: | 08 Faculty of Science > Institute of Geological Sciences |
| Depositing User: | Sarah Stalder |
| Date Deposited: | 08 Dec 2025 15:45 |
| Last Modified: | 08 Dec 2025 23:25 |
| URI: | https://boristheses.unibe.ch/id/eprint/6947 |
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