The geochemistry of orthoamphiboles and coexisting cordierites and phlogopites from South Norway

A number of orthoamphiboles (18), cordierites (9) and phlogopites (5) in Mg-rich rocks from the Bamble Sector of the Fennoscandian Shield have been separated and analysed for major and 20 trace elements.The ratio of Na to Al^IV in the Bamble orthoamphiboles is approximately 15, which is a smaller ratio than calculated for many other areas. This may result from the low Na content of the majority of parent rocks.The AFM diagram shows that orthoamphiboles termed anthophyllites by one author overlap with gedrites of other authors and vice-versa. In addition there is a wide variation of the AFM components for all orthoamphiboles. This variation can be accounted for by the presence or absence of coexisting cordierite or clinoamphibole. Finally the AFM positions of orthoamphiboles from 3 separate investigations are split into Al-poor and Al-rich fields. This can be explained, at least in part, by the relative abundance of Si and Na in the rock.The partition of Fe²⁺ and Mg between the coexisting minerals is comparable with that established for other metamorphic terrains in spite of the Mg enrichment of the South Norway rocks, indicating probable stability between the minerals.The three coexisting phases show a different preferential partition of trace elements. Orthoamphiboles are enriched in Sc, V, Y, Zr and lanthanides, cordierites in Cu, Ga and Sr, and phlogopites in Cl, Cr, Co, Ni, Rb and Ba. The trace elements concentrations and partitions, for orthoamphibole-phlogopite pairs display strong similarities to those of hornblende-biotite pairs from igneous granites.