The role of fluids during retrograde metamorphism, associated with thrust zones in the Caledonides of north Norway

Bennett, David George (1990) The role of fluids during retrograde metamorphism, associated with thrust zones in the Caledonides of north Norway. University of Southampton, Doctoral Thesis.

Abstract

The Caldeonian nappes of North Central Norway record similar D₁ to D₄ deformation histories. They experienced Barrovian style metamorphism ranging from lower greenschist to upper amphibolite (Kyanite-Sillimanite grade) conditions. Peak metamorphism occurred during D₂. Retrogression of the peak metamorphic assemblages is concentrated along thrust zones between the nappes, related to fluid flow after peak metamorphism. The most commonly observed retrograde reactions are the replacement of garnet by chlorite and of kyanite by sericite. These reactions are exothermic rehydration reactions and lead to small volume increases. Microstructural evidence indicates that retrogression occurred at a post tectonic stage. Four stages of vein formation have been identified. Pre-D₂ veins have been extensively recrystallised and contain a range of H₂O-CO₂-(N₂-CH₄)-NaCl fluid inclusions which predominantly occur along grain boundaries. These are CO₂ rich and of low salinity. Inclusions in less recrystallised areas are similar in composition but more H₂O rich. Textural and microthermometric evidence suggests that many inclusions suffered preferential water loss during recrystallisation and became CO₂ enriched. The most water rich inclusions best represent fluids derived from prograde metamorphic devolatilisation reactions. These fluids have salinities less than 6 weight % NaCl equivalents and (CH₄₊ N₂)/CO₂ ratios of 0.15 ± 0.1. Veins which developed after D_2 but prior to D_3 contain fluid inclusions which provide evidence of phase separation between a H_2O rich H_2O-CO_2-(CH_4-N_2)-NaCl liquid and a CO_2-(CH_4-N_2) rich vapour. Phase separation occurred in response to reducing pressures during uplift, at conditions of ≈380^oC and 2.5 to 3.1 kbar. Syn- to post-D₃ veins contain inclusions of similar composition to the H₂O rich liquid. Syn- to post-D₄ veins which formed at the same time as the retrogression of the thrust zones, contain a population of hypersaline aqueous fluid inclusions. These saline fluids range in salinity between 27 and 50 weight % NaCl equivalent. They are NaCl dominated but also contain appreciable concentrations of K, Ca and Fe. They have Na/K and Na/K ratios similar to those of high salinity fluids from the Salton Sea geothermal system. This similarity, coupled with results of alkali geothermometry suggest that they have undergone almost complete equilibration with their host rocks at temperatures of 300 ± 50^oC. Fluid/rock weight ratios calculated for the phase of retrogression are low (F/R = 0.008 to 0.08). Biotites in the thrust zones are relatively chlorine enriched and record the passage of the late stage brines along the thrust zones. The high salinities of the fluids related to the retrograde reactions, and the low fluid/rock ratios imply that fluid infiltration of the thrust zones was limited, and that the infiltrating fluids became more saline as rehydration reactions proceeded. Comparison of the brine compositions with those of the retrograde mineral assemblages suggests that they are consistent with the assemblage Ab-Ms-Pg-Qtz in pelites at a pH of 4.7. With the exception of Cl, the major components in the fluids were internally buffered during retrogression by mineral-fluid exchange equilibria. The predominant controls on the retrogression were the supply of water to the reaction sites, the PT conditions of infiltration and the rate and degree of completion of the rehydration reactions. The late stage brines have relatively consistent oxygen isotopic compositions (δ18O = + 8.0 to + 12.7). In contrast they record a wide range of δD from -147.6 to -64.7. The most likely source of the brines was either a modified meteoric water or a formation water containing a component of meteoric water. The structurally higher thrust zones may have been infiltrated by fluids from the surface after their uplift to relatively shallow levels in the crust. The Hogtind Thrust zone may also have received fluids from the surface, however formation/metamorphic waters may also have migrated into the zone from the sedimentary material of the Fossbakken Nappe as it was overridden by the nappe pile and emplaced onto the Baltic Shield.

This record has no associated files available for download.

Contributors

Download statistics