Sill emplacement in wet sediments: fluid inclusion and cathodoluminescence studies at Grunehogna, western Dronning Maud Land, Antarctica

Abstract

The Grunehogna and Kullen sills of western Dronning Maud Land were emplaced into water-saturated, partially lithified sediments of the mid-Proterozoic Ritscherflya Supergroup. Fluid inclusion and optical and scanning electron microscope cathodoluminescence studies of quartz grains within and adjacent to the sills (the ‘sill-sediment interactive zone’, referred to as the ‘interactive zone’) showed that only a single generation of later fluid inclusions is present. The fluids are Na-K-Ca-Cl aqueous solutions with temperatures of homogenization between 95 and 105 °C. Individual quartz particles in the sedimentary rocks above the interactive zone have unique fluid inclusion populations. Petrographic and cathodoluminescence evidence indicated that there are three types of quartz grains in the interactive zone. These consist of (i) a xenocrystic type, (ii) a high-temperature quartz that crystallized either from the magma or as a result of anatectic melting of xenolithic (and/or xenocrystic) material, and (iii) euhedral quartz crystals that have grown in vugs in the peripheral zone. Cathodoluminescence of xenocrystic grains indicated the existence of healed microfractures along which fluid inclusions were concentrated and secondary and authigenic overgrowths on older generation quartz grains had developed. The presence of zonal luminescence patterns of detrital particles, coupled with evidence for equilibrium polygonization in the sedimentary rocks adjacent to the peripheral zone up to about 8 m above the upper contact of the Grunehogna sill suggested local thermal tempering or sintering and recrystallization. Luminescence of detrital quartz particles in the overlying Hogfonna Formation reflected variation in the characteristics of their source terrains, and showed limited evidence of heating by the magma. The interactive zone fluid inclusions are formation waters which penetrated microfractures during diagenesis. We show that no large-scale hydrothermal circulation system is likely to develop following intrusion into a water-saturated environment.