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

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

May 1977

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1977e%26psl..35..105c&link_type=abstract

Earth and Planetary Science Letters, Volume 35, Issue 1, p. 105-115.

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Scientific paper

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 Na2O/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.

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