The chemistry of barium anomalies in the Berisal Complex, Simplon Region, Switzerland

The petrographic and geochemical features of two zoisite–celsian gneiss outcrops from the Berisal Complex, characterised by a syn-kinematic mineral assemblage that contains celsian and barium white micas and a maximum whole-rock BaO content of 8.36 wt%, are described. The outcrops are enclosed in a larger body of garnet-bearing two-mica augen-gneiss, which has intrusive contacts with the surrounding garnet–biotite–muscovite paragneiss, and also contains small outcrops of two-mica clinozoisite gneiss. The zoisite–celsian gneisses are strongly enriched in all alkaline-earth elements, are depleted in the alkali elements, and have high Zr/Hf and Nb/Ta ratios compared with the surrounding gneisses. The zoisite–celsian, two-mica clinozoisite, and garnet-bearing two-mica augen-gneisses have Al/CNK molar ratios >1, and the zoisite–celsian gneisses are also enriched in Zr, Y, and Nb. Chondrite-normalised patterns for the rare earth elements (REE) show light REE enrichment, with a negative Eu anomaly. On the basis of field and geochemical data it is argued that the barium anomalies in the Berisal complex are the result of igneous fractionation of barium into anorthosite-dominated cognate inclusions within a larger volume of calc-alkaline peraluminous melt. On the basis of U–Pb zircon ages, a conservative estimate for the age of magma crystallisation is placed at 460 ± 10 Ma, and thus was related to significant late Ordovician granitoid magmatism in Gondwana-derived microcontinents during collision between Avalonia-Cadomia and Gondwana.