Petrology and geochemistry of the Tromoy gneiss complex, South Norway, an alleged example of Proterozoic depleted lower continental crust

A granulite-facies Precambrian meta-igneous gneiss complex at Tromoy, South Norway, which was previously assumed to represent of fragment of strongly large ion lithophile element (LILE)-depleted lower continental crust, has been reinvestigated using major and trace element data, radiogenic isotopes and secondary ion mass spectrometry (SIMS) U-Pb geochronology. The Tromoy gneiss complex consists of mafic and tonalitic gneisses (SiO₂ = 60-70 wt %) that are itruded by trondhjemitic dykes (SiO₂ > 70 wt %). The mafic and tonalitic members are metaluminous, low-K rocks that have characteristic negative spikes for niobium and positive spikes in lead, are moderately enriched in middle rare earth elements-light rare earth elements (MREE-LREE) and have relatively flat MREE-heavy REE (HREE) patterns. Their compositions resemble evolved magmas in modern oceanic island arcs. The trondhjemites have major element compositions close to minimum melts in either mafic or tonalitic systems. They display low LILE and LREE contents, with high K/Rb (up to > 13 000), and their REE patterns are concave in the MREE to HREE and have a positive Eu anomaly. SIMS U-Pb analyses of zircons from the mafic gneiss, tonalite and one trondhjemite suggest three different episodes of zircon growth: (1) oscillatory zoned magmatic cores at 1198 ± 13 Ma (2σ); (2) metamorphic overgrowths at 1125 ± 23 Ma (2σ); (3) later fluid-controlled embayments and paths of zircon reworking. The mafic gneisses and tonalites have indistinguishable magmatic ages. The trondhjemites originated as anatectic melts in the mafic-tonalitic rock complex during high-grade metamorphism at 1100 Ma; their most likely source was a leucogabbroic or dioritic facies within the igneous complex. Nd, Sr and Pb isotope data suggest involvement of mantle- and crustal-derived source components in the petrogenesis of the gneiss protolith, probably in a subduction-zone setting. The present data show that the Tromoy gneiss complex is not a typical example of 'depleted lower continental crust', nor has it been highly metasomatized or severely depleted by metamorphic fluids.