Secondary elemental mobility within the Støren greenstones, Norway

A treatment of the geochemistry of both pillowed and massive Javas from the Benna area, Trondheim, is presented. Normally 'immobile' elements (TiO2, Hf, Se, P2O5 and REE) are shown to have been susceptible to very significant degrees of post-crystallization mobility. The pillow Javas are relatively
depleted in these elements compared to the less altered massive Javas, while the three components of individual pillows have systematic differences in chemistry and elemental ratios. In particular, rare earth element (REE) rich components (pillow selvedges) have higher light rare earth element (LREE)/heavy rare earth element (HREE) ratios. During these secondary geochemical changes, trace elements and REE behaved coherently as indicated by the presence of strong interelement correlations. As the degree of carbonatization can be clearly related to the concentration of these elements, carbonate rich domains being relatively depleted in comparison to carbonate poor domains, the redistribution of chemistry is attributed to metasomatism involving a CO2 rich fluid phase. This condition would enhance the mobility
of these high valency cations by the formation of carbonate complexes. As this metasomatic process can be modelled using the Rayleigh distillation law, and because fractional crystallization is subject to the same law, caution is advised in the use of trace elements and REE in constructing petrological models for
metamorphosed basic sequences. In addition, it is shown that the probable primary igneous composition of these greenstones is not unique to any specific geotectonic environment.