Journal of Petrology | Volume 40 | Number 6 | Pages 909-933 | 1999
© Oxford University Press 1999
Petrology and Geochemistry of the Tromøy Gneiss Complex, South Norway, an Alleged Example of Proterozoic Depleted Lower Continental Crust
Mineralogical–Geological Museum, SARS Gate 1, N-0562 Oslo, Norway
Received May 1, 1998; Revised typescript accepted December 8, 1998
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Abstract |
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A granulite-facies Precambrian meta-igneous gneiss complex at Tromøy, South Norway, which was previously assumed to represent a 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 Tromøy gneiss complex consists of mafic and tonalitic gneisses (SiO2 = 60–70 wt %) that are intruded 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 (2
); (2) metamorphic overgrowths at 1125 ± 23 Ma (2); (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 Tromøy gneiss complex is not a typical example of ‘depleted lower continental crust’, nor has it been highly metasomatized or severely depleted by metamorphic fluids.
KEY WORDS: crustal differentiation; Proterozoic juvenile magmatism; Sveconorwegian high-grade metamorphism; U–Pb zircon SIMS data
* Corresponding author. Telephone: +47 22 85 17 89. Fax: +47 22 85 18 00. e-mail: t.l.knudsen{at}toyen.uio.no