Metasomalism associated with the North Gorog Centre, South Greenland

Abstract

This thesis describes the alkali metasomatism and fenitisation associated with the North Qoroq nepheline syenite centre in the Gardar alkaline igneous province of South Greenland. Alkali metasomatism occurs both within the syenite centre and in the country rocks of granite gneiss, quartzitic and tones and basalt lavas. Within the syenite centre the metasomatism produces distinctive petrographic textures. The mafic minerals develop a characteristic poikilitic texture, a feature seen less commonly in the felsic phases which tend to recrystallise to a fine grained feldspathoidal matrix. The metasomatic activity produces significant changes in whole-rock chemistry and a detailed discussion is given of computational methods in the representation of whole-rock chemical change. One approach is adopted for the examination of the metasomatic changes and it is shown that the metasomatism took place at near constant volume and was dominated by desilicification coupled with addition of sodium. Examination of the felsic phases in the syenite centre demonstrates the usefulness of cathodoluminescence microscopy in studying textural changes during the metasomatism of the feldspars. The causes of luminescence are examined and it is shown that the presence of red luminescent albite is indicative of fluid activity in the rock. Where reaction with the metasomatising fluid is most intense, nepheline and sodalite are formed at the expense of K-feldspar while albite remains in the rock and occurs in a granular mosaic with the feldspathoids. Modification of mafic silicate compositions indicates that the metasomatism in the syenites was initially under the influence of Na- and volatile-rich fluids at relatively high temperature and low oxygen fugacity. Differences in magmatic mineral assemblages and localised reactions in micro-environments gave rise to a patchy and variable metasomatism. Decreasing temperature and increasing oxygen fugacity led to the later formation of acmitic pyroxene. The mafic mineralogy developed in the country rocks shows that higher oxygen fugacities prevailed, and with decreasing temperatureproduced riebeckite, acmite and haematite. . . . Fluid inclusion studies indicate that more than one fluid phase was active in and around the North Qoroq centre and mixing between magmatic and meteoric fluids is suggested. Apatite chemistry demonstrates that the rare earth elements are mobile during the metasomatism and were probably transported in fluorine and/or carbonate complexes. It is proposed that maximum temperatures of metasomatism were no higher th 650°c in the syenites and quartzite and less than 500°c in the granite-gneiss and fluid activity persisted to at least 100°c. The fluids emanating from the syenites were Initlally very sodium-rich, silica-poor and had low oxygen fugaclty. Fluid evolution from the syenites was by continuous fractionation to a fluorine-nch magmatic residue. Comparisons are drawn with other Gardar centres and and alkaline Igneous centres around the world and suggestions are made for future research topics.