The geology and petrology of the pre-camhrian basement "between Sirdal and Åseral, Vest Agder, Norway

Abstract

The field relations and petrography of the rocks of the PreCambrian basement complex between Sirdal and Aseral, comprising two series of high-grade metamorphic gneisses separated by a structural discontinuity, syntectonic granites, intrusive quartz monzonites with thermal metamorphic aureoles and basic dykes, are described. During orogeny the gneisses were subjected to intense poly-phase deformation, three regional and two localised phases of which have been recognised. Minor fold relics within augen gneiss in the lower gneiss sequence suggest that this rock was involved in earlier deformation. the climax of metamorphic crystallisation occurred at the low-pressure granulite facies-amphibolite facies boundary with mineral parageneses corresponding closely with the sillimanite-cordierite-orthoclase subfacies of the Abukuma-type cordierite amphibolite facies except for the additional occurrence of orthopyroxene. Major and trace elements X.R.F. analyses of gneissic and some igneous rocks are presented. These data reveal significant differences between basic rocks of the two gneiss series, basic gneisses with different mineral assemblages and to a lesser extent different lithostatigraphical units in the upper gneiss series. Electron microprobe analyses of alkali feldspar, plagioclase, biotite, hornblende, clinopyroxenes, orthopyroxene, sphene, magnetite, ilmenite, chlorite, and garnet from several rock types are presented. With the exceptions of alkali feldspar, magnetite and ilmenite all minerals are chemically homogeneous and represent original equilibrium compositions. The chemical inhomogeneity of alkali feldspar resulted from post-crystallisation leaching and redistribution of alkalies, resistance to which is related to grain size. Equilibrium during original feldspar crystallisation is indicated by the restricted composition of plagioclase coexisting with alkali feldspar. The distribution of titanium and magnesium between coexisting silicates indicates equilibrium compositions, influenced by oxygen fugacity, the nature of the coexisting iron oxides and the tetrahedral aluminium content of the hydrous phases in addition to the rock composition. The application of several means of multicomponent paragenesis analysis reveals that the various mineral assemblages can be interpreted in terms of variations in major element rock composition and oxygen fugacity. The widespread molybdenite mineralisation is considered to have been transported from depth in siliceous hydrothermal solutions into the gneisses, especially where the strike of the gneissic layering coincided with deep fractures. Fixing of the metal as sulphide occurred particularly in the vicinity of pre-existing fahlband sulphide due to release of sulphur in the local environment of increased oxygen fugacity.