The norite-mangerite relationships in the Bjerkreim-Sokndal layered lopolith (southwest Norway)

Abstract

The Transition Zone between the anorthositic-noritic rhythmic units and the mangeritic unit of the Bjerkreim-Sokndal layered lopolith (southwest Norway) is characterized by thick layers of ultramafic rock (UML), interleaved with norites in the lower part and mangerites in the upper part. In the latter rocks large mesoperthite crystals occur in a fine-grained groundmass (porphyritic texture). Rapid variations in chemical composition are shown by the mafic minerals, especially by olivine (Fo 50 to Fo 19) and by Ti-magnetite (10–19% TiO 2). The whole-rock composition of the UML does not change in the upper part of the Transition Zone, and the Fe (Fe + Mg) ratio of the mangerites remains constant (Bowen trend). Formation of the UML through liquid immiscibility is rejected because: (1) the liquidus minerals are not always identical in the Fe-rich and associated silica-rich rocks; (2) the distribution of P 2O 5, REE and Ba between associated rocks are not in agreement with experimental data; (3) the major elements vary along parallel trends in the Creigh diagram. Cumulative processes and adcumulus growth can best explain the UML, oscillatory nucleation can account for the norites, and flotation of mesoperthite is plausible in the mangerites, where accumulation of feldspar is evidenced by a strong positive Eu anomaly and porphyritic textures. The mangeritic unit of the lopolith is interpreted as a flotation cumulate at the roof of the magma chamber. The residual liquids of the Bjerkreim-Sokndal differentiation were either trapped in the mangerites, or expelled in the form of dykes, or were mixed with quartz mangeritic liquids above the intrusion.