Constraints on the fluid enhanced eclogitisation of granulite domains in the Bergen Arcs, Norway

Abstract

High-pressure (HP) and ultra-high-pressure (UHP) terranes are often associated with the presence of fluids (e.g. hydrous mineral assemblages, pegmatite veins or melts). However, examples of progressive conversion of anhydrous mineral assemblages, to hydrous ones, are rare. Holsnøy Island is host to a unique setting whereby hydrous eclogite domains are perfectly preserved juxtaposing anhydrous granulite domains. The structural evolution of the system on Holsnøy Island can be observed via different stages, each indicated by an increase in fluid availability and degree of ductile deformation. This thesis identified 5 stages of deformation, based on structural cross-cutting relationships, and P–T condition estimations for four of the stages (1, 3, 4 and 5), using forward phase equilibria modelling. Stage 1 is the formation of pseudotachalytes and fractures in the granulite domains, which provide the initial pathway for the infiltration of fluid. The metamorphic overprint of the pseudotychylytes limit the occurrence of stage 1 of deformation to maximum P–T conditions of 15.2–15.7 kbar and 675–685 °C. Stage 2 of deformation is the formation of small-scale discrete shear zones along the partially hydrated domains. Mineralogically, this stage of deformation is indicated by the breakdown of garnet grains to form omphacite– kyanite–zoisite–plagioclase symplectites. Stage 3 of deformation is the formation of peak, sheared eclogite domains at P–T conditions of 21–22 kbar and 670–690 °C. Stage 4 of deformation is the formation of a retrogressed-eclogite (R-eclogite), a phengite-rich domain at P–T conditions 16–17 kbar and 680–700 °C. Lastly, is stage 5 of deformation, which is the retrogression of the system to amphibolite-facies at P–T conditions 11.8–12.8 kbar and 720–732 °C. Combined, these different stages of deformation illustrate the first calculated P–T path experienced by the Holsnøy domains (presented in Chapters 2 and 5). Systematic sampling of altered domains and their closest granulite protolith provided a comparative study, thus allowing the geochemical characterisation of the fluid infiltrating the Holsnøy granulites. Stage 3 of deformation has been interpreted to be rock-buffered based on whole-rock δ18O analyses, trace element chemistry, and Sm–Nd chemistry. In contrast, a fluid-buffered system was interpreted for stage 4 of deformation. Calculated δ18O and δD values for the fluid based on measured mineral isotopic values are consistent with a metamorphic fluid reservoir, interpreted in Chapter 2. Trace element analyses and Sm–Nd systematics, presented in Chapter 3, suggest the possibility of a sedimentary unit, with mafic components as a likely source of fluids for the eclogitisation of granulite domains on Holsnøy Island. Lastly, this thesis investigated the exhumation rates of the peak eclogite-facies altered domain (stage 3 of deformation). Calculated minimum exhumation rates of 41– 6122 mm/yr were obtained based on diffusion durations in zoned garnet grains, and constraints on pressure decrease from eclogite-facies to amphibolite-facies (stage 5). These extreme exhumation rates experienced by the HP terranes on Holsnøy Island cannot be explained by known exhumation mechanisms. Therefore, this highlights the need to reassess the general scientific consensus that pressure can be used as a proxy for depth in the Earth’s crust, and propose new mechanisms for the emplacement of HP and UHP terranes. Combined, the chapters provide a series of new datasets for the eclogitisation process on Hoslnøy Island. This ulstimately contributes to the better understanding of deep crustal metamsomatism and metamorphic processes at HP and UHP conditions.