| dc.description.abstract | The field area is situated in the marble deposit in Akselberg, Velfjorden in the southern part of the Helgeland Nappe Complex, and is mined by Brønnøy Kalk AS. The marble is intruded by several granitic dikes. Fieldwork and microscopic analysis show that these dikes consists of three texturally different rock types; 1) Pegmatites, 2) Homogranular aplite and 3) Heterogranular aplite. The Pegmatites occurs as both lenses and dikes, with sizes around 2 meters high and 3 meters wide. Almost all pegmatites found appear in the southern part of the area. The aplites occur as dikes with 5 cm to 1 meter thickness, they appear all over the mapped area, and there doesn’t seem to be any favored distribution with regards to the aplite type. It seems, however, that the amount of dikes is decreasing from south to north. The microscopic analysis showed that the pegmatites had homogranular grain distribution, with a mean grain size of 1 mm to 2.5 cm. The homogranular aplites showed grain sizes around 0.2 - 2 mm, while the heterogranular aplites had porphyries around 1-2 mm (some grains up to 4 mm), in a matrix from 0.1 - 0.4 mm. For the aplites there is a slight increase in grain size from south to north.
The geochemical composition of the three rock types shows that they are peraluminous and calc-alkaline, this indicates that they are derived from S-type melts, in a continental arc environment. S-type melts also points to LCT-pegmatites. Several geochemical trends indicate that the sourcemelt was in melted condition long before crystallization commenced, and therefore is more differentiated, for instance they have a high SiO2-content. The pegmatites, and homogranular- and heterogranular aplites show little differences in geochemical composition, thus they are most likely formed during the same magmatic event. The rocks of the Velfjorden area can be genetically linked to two magmatic events, one involving regional migmatization linked to the intrusion of the earliest plutons in the Bindal Batholith from 481-468Ma (biotite granite near Brønnøysund, anatectic granite on the Vega islands and tourmaline granite in the Velfjorden area). The second event is local migmatization linked to the intrusions of the Velfjorden plutons 447-448Ma (Hilstadfjellet, Akset-Drevli, Sausfjellet, and the two smaller ones Svarthopen and Aunet). Barnes et al. (2002) interpreted a zone of ~1 km with local migmatization around the plutons. The geochemical compositions of the pegmatites and homogranular- and heterogranular aplites, corresponds with the local migmatites. The field area is situated around 500-1000 meters of the western border of the Akset-Drevli pluton. This, and the geochemical evidence indicates that the aplitic and pegmatitic intrusions where formed by local migmatization during emplacement of the Akset-Drevli pluton 448Ma.
Comparison of marble situated <1 meter from aplite dikes, with marble which was not near dikes, showed very small differences in macroscopic, microscopic and geochemical analysis. This indicates that the dikes have not influenced the marble during intrusion. This could be due to the fact that these intrsusions are small, and would cool rapidly, and that the local migmatization also affected the marble in terms of the temperature, so that the differences in temperature where small. | nb_NO |
