The geochemistry of the grØnnedal- ĭka alkaline complex, South Greenland

Abstract

Analyses are given of 137 foyaites and related syenites from the grØnnedal- ĭka alkaline complex, South Greenland. The distribution of the felsic normative components in the Lower Laminated Series (Emeleus 1964) is interpreted partly in terms of the early settling of nepheline followed by nepheline and feldspar together. The laminated feldspathic syenites identified at the top of the Lower Series are described and their significance is discussed. The Upper Series shows no pronounced differentiation of felsic components. The problem of describing systematically the complicated chemical variation among the cumulus rocks of the complex is to seme extent overcome by the application of R-mode factor analysis, the principles of which are explained. The chenical characteristics of the various minor syenite units distinguished by Emeleus (1964) are described. Chemical comparisons between all of the syenite units leads to the postulation of three broad 'taagma associations" contributing to the complex. Analyses are also given of 50 alkaline dykes belonging to later magmatic episodes. From relationships in the system Na(_2)O-K(_2)O-Al(_2)O(_3)- SiO(_2), it is argued that the members of the peralkaline phonolite suite are related principally by the fractionation of feldspar approximating to Ab(_55)Or(_40)An(_5) in composition. The bearing of these rocks on phase equilibria in the analogous natural system is discussed, and consideration is given to the possible origins of the initial peralkaline phonolite magma. The chemistry of a number of severely altered dykes of the same type is considered, and an account is given of the chemical and mineralogical changes occurring during alteration. The geochemistry of suites of lamprophyrlc and trachyte dykes is examined and possible relationships between them are discussed. Finally, the magma types thought to be represented in the complex are reviewed, and the significance of their distribution in time is considered together with possible modes of origin.