A petrological and structural study of the leaving Gneiss-Dome, Southern Norway

Abstract

The Levang granite forms an elliptical body occupying the core of a broad antiformal structure, and covering approximately 50 sq. km. Surrounding this mass of granitoid gneisses is a series of banded gneisses of very variable composition, including amphibolites, quartzltes, and nodular sillimanite gneisses. Against these rocks the granitic gneisses make a sharp, though in places 'interfingered' contact. Agmatitl relationships are never seen. Further south, on the PortOr sub-peninsula, migmatitic granitoid gneisses are exposed which have mineral assemblages identical to those of rock types from within the main Levang granite. The composition of the Levang granite gneisses is variable, ranging from quartz monzonltic to tonalitic types lacking potash feldspar, but on the whole corresponds fairly closely with the minimum melting composition of the granite system. Over much the greater area the Levang granite is strongly foliated, and everywhere shows some degree of preferred planar or linear orientation of minerals. The foliation always parallels the contacts, and it defines a complex fold system, within the granite which is in complete structural conformity with that of the surrounding banded gneisses and migmatites. The overall symmetry of the structure is triclinic, the plunge of the linear elements varying from horizontal to vertical. The β axis of the domal structure in the eastern part of the granite mass is completely overturned. Traced eastwards from the core of the dome, P trends initially due east north east, steepens to vertical, and then plunges due west at angles of 60 to 70 . The very complex tectonic evolution has probably involved disharmonic folding and two phases of deformation. Within the Levang granite amphibolitic sheets and lenses are invariably conformable to the fold pattern defined by the granite gneiss foliation. In the Helligesvann brachyanticline, a regular sequence of granitoid gneisses of variable composition, interlayered with a thick amphibolitic sheet which forms an unbroken horizon, indicates the presence of a 'ghost' stratigraphy. Investigation of the potash feldspar obliquities shows that all the potash feldspars, from granitic gneisses, migmatites, and pegmatites, are more or less fully ordered maximum microclines. The fact that some of the granitic gneisses approximate to the minimum melting composition is not conclusive evidence in favour of an origin through anatexis or fractional crystallization, in view of the spatial distribution of these rocks, their metamorphic textures, and the nature of the basic inclusions. The Levang granite is a metamorphic rock, the result of the transformation in an open system of a pre-existing series of supra- crustal rocks by processes of granitization and metasomatism.