Discordant layering relations in the Fongen–Hyllingen basic intrusion

Abstract

Modal stratification or layering on the scale of centimetres to metres is a common macroscopic feature of many basic intrusions. In some cases this rhythmic layering is accompanied by gradual changes in the composition of mineral solid solutions (cryptic layering) and by the entry and exit of certain minerals (phase layering). These compositional variations, which evidently reflect progressive fractional crystallization, are generally assumed to be orthogonal to rhythmic layering, as predicted by crystal settling models for the origin of layered basic intrusions, and they have been used to establish ‘cumulate stratigraphy’ of many layered intrusions¹. In the southern part of the synorogenic Fongen–Hyllingen complex², Norway, well developed rhythmic layering is markedly discordant to phase and cryptic layering. Regressions in the cryptic and phase layering that are believed to reflect magma addition are also discordant to the rhythmic layering. These features raise questions as to the stratigraphical significance of phase layering and add to the mounting evidence in favour of in situ crystallization for the origin of layered igneous rocks^3,4.