Metamorphism in the roots of mountain belts and its effect on rock technical properties : A case study of the Eastern Segment, Sveconorwegian orogen

Abstract: Deciphering the metamorphic evolution of the roots of mountain belts is essential for understanding deep tectonic processes involved in mountain building. The Sveconorwegian orogen in southwestern Scandinavia, formed during the Rodinia assembly in the transition between the Mesoproterozoic and Neoproterozoic Eras, has been exhumed and provides access to interior deep structural levels of the ancient mountain belt. The Eastern Segment of the Sveconorwegian Province corresponds to the underthrusting continental crustal block that was metamorphosed under variable temperature conditions and differences in the availability of hydrous fluids. As a result, the bedrock across the Eastern Segment has different mineral assemblages and textural characteristics.This thesis investigates the metamorphic behaviour of the deep-seated bedrock in the Eastern Segment from two approaches. Bedrock is a widely exploited raw material in Sweden for use in crushed aggregate production. The performance of aggregates is steered by the textural and mineralogical properties, which, in turn, are determined by the geological processes. The first part of the study aims to assess the influence of metamorphic conditions on the technical properties of crushed bedrock aggregates. The second part focuses on the metamorphic evolution resulting from the limited introduction of hydrous fluids in a ~25 km wide discrete deformation zone with steeply dipping and anastomosing structures in the easternmost part of the Eastern Segment. At the present erosion level, this deformation zone delimits the Sveconorwegian ductile deformation to the east.The results of the first two studies indicate that technical properties of bedrock vary systematically with the macro-fabric and microtextures across the Eastern Segment. Rocks of granitic and gabbroic compositions show a close relationship between the mode of recrystallization and the resistance to fragmentation and wear. Changes in the state of recrystallization reflect the differences in the metamorphic temperature and availability of hydrous fluids. As a result, rocks show variations in the mineral assemblage, size, and shape of the crystals. High performance of aggregates is linked to high textural complexity, which is characterized by non-uniform grain-size distributions and irregular grain boundaries. In granitic and gabbroic rocks, these textural parameters stemmed from metamorphism under low availability of hydrous fluids and at variable temperatures, below 600 °C and above 750 °C. In granites, the recrystallization of quartz and feldspar at these conditions yielded complex rock textures. In gabbroic rocks, however, the textural complexity is determined by the preservation of relict igneous textures due to limited hydration. High-grade metamorphism associated with hydration and partial melting yielded low resistance of granitic and gabbroic aggregates to fragmentation and wear. This investigation illustrates how knowledge of metamorphic processes effectively supports the prediction of functional properties of bedrock aggregates.Two following studies focus on the metamorphic evolution within the easternmost deformation zone of the Sveconorwegian Province. Textural analysis linked to petrological and geochronological data was performed on samples from a key locality of Mesoproterozoic syenodiorite, in which the syenodioritic rock preserves different states of deformation and metamorphic recrystallization. The textural relationships show partial to complete replacements of the primary igneous minerals, including the formation of metamorphic zircon at the expense of igneous baddeleyite and zirconolite. The findings demonstrate that metamorphic reactions were induced by the influx of hydrous fluid in deformed zones. The deformation assisted the infiltration of hydrous fluid, resulting in different states of recrystallization and deformation: while fully recrystallized rocks record equilibrium at 540–600 °C and 9–12 kbar, rocks in undeformed domains remained in a metastable near-pristine igneous state. This study contributes to the understanding of the metamorphic processes that operated at depths of 35–40 km within a ductile deformation zone in the easternmost part of the Eastern Segment and the frontal parts of the Sveconorwegian orogen.