The chemistry and origins of Proterozoic low-potash, high-iron, charnockitic gneisses from Tromøy, south Norway

Abstract

New major and trace element data for 79 acid-intermediate charnockitic gneisses (the Tromøy gneisses) and 16 associated metabasites from the island of Tromøy show that this part of the 1200–900-m.y. Sveconorwegian zone is occupied by rocks of unusual composition. Overall values for K and Rb are the lowest yet reported for any granulites, and K/Rb ratios are very high. Cs and Th are also low and, abnormally for granulites, so are Ba, Sr and Zr. Ba/Sr ratios are similar to those in other suites, but K/Ba and K/Sr are higher. These features may partially be reflecting unusual pre-metamorphic lithologies, but it is considered more likely that they are largely the product of metamorphically induced depletion processes involving metasomatism. There is some indication that the Na 2O/CaO and normative Ab/An ratios may also have been modified during metamorphism. Data for the presumed relatively immobile elements Cr, Co, Ni and V support an igneous origin for the Tromøy gneisses, but the presence of a paragneiss component cannot be ruled out. A characteristic of the gneisses is their high iron content, and spatial and temporal considerations point towards a genetic link with the iron-rich, intrusive rapakivi suites of Finland, Sweden and south Greenland. If the Tromøy gneisses do represent material of this type, it would seem to follow that potash fractionation has been extreme.