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Boron sources of tourmaline-rich Nb-Y-F-pegmatites in south Norway: Implications for pegmatite melt origin

Urheber*innen

De La Cruz,  Erika
External Organizations;
GFZ SIMS Publications, Deutsches GeoForschungsZentrum;

Müller,  Axel
External Organizations;
GFZ SIMS Publications, Deutsches GeoForschungsZentrum;

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Trumbull,  R.
3.1 Inorganic and Isotope Geochemistry, 3.0 Geochemistry, Departments, GFZ Publication Database, Deutsches GeoForschungsZentrum;
GFZ SIMS Publications, Deutsches GeoForschungsZentrum;

Faria,  Pedro
External Organizations;
GFZ SIMS Publications, Deutsches GeoForschungsZentrum;

Andersen,  Tom
External Organizations;
GFZ SIMS Publications, Deutsches GeoForschungsZentrum;

Erambert,  Muriel
External Organizations;
GFZ SIMS Publications, Deutsches GeoForschungsZentrum;

Kristoffersen,  Magnus
External Organizations;
GFZ SIMS Publications, Deutsches GeoForschungsZentrum;

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Volltexte (frei zugänglich)

5026461.pdf
(Verlagsversion), 25MB

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Zitation

De La Cruz, E., Müller, A., Trumbull, R., Faria, P., Andersen, T., Erambert, M., Kristoffersen, M. (2024): Boron sources of tourmaline-rich Nb-Y-F-pegmatites in south Norway: Implications for pegmatite melt origin. - Precambrian Research, 410, 107474.
https://doi.org/10.1016/j.precamres.2024.107474


Zitierlink: https://gfzpublic.gfz-potsdam.de/pubman/item/item_5026461
Zusammenfassung
Tourmaline is common in rare element pegmatites of the Nb-Y-F (NYF) type in the south-central part of the Proterozoic Sveconorwegian orogen in southern Norway. In the global context, however, tourmaline appears rare in this type of pegmatite. This study aims to explain the unusual tourmaline abundance in these pegmatites and the origin of boron (B) in the respective melts, and to raise awareness of tourmaline in NYF pegmatites generally. Tourmalines from six pegmatites in three Sveconorwegian lithotectonic units: Bamble, Kongsberg and Idefjorden, were investigated in terms of their mineral chemistry and δ11B values, in addition to bulk rock analyses of pegmatites and host rocks. Tourmalines in pegmatites from Bamble and Kongsberg record B isotopic compositions (δ11B = -1.0 to + 9.9 ‰) that are heavy relative to continental crust and mantle sources. In contrast, tourmaline in pegmatites and host rocks from Idefjorden have light B isotopic ratios (δ11B = -14.8 to −12.5 ‰) that are typical crustal values. We suggest that the latter melts were sourced from orthogneisses at depth. We relate the heavy B isotopic composition of Bamble and Kongsberg pegmatites to regional Na-metasomatism by fluids sourced from Mesoproterozoic shallow marine sediments. This is supported by previously published δ11B ratios from metasomatized Bamble host rocks. The spatial association of pegmatites with Na-metasomatism in the basement rocks suggests that metasomatism enhanced the fertility and B-concentration in the affected lithologies, favouring partial melting and the formation of tourmaline-bearing pegmatites. These findings contribute to understanding the petrogenesis of Sveconorwegian pegmatites but they also imply that B can play a greater role in the formation of NYF pegmatites than previously thought and that tourmaline has value as a petrogenetic tool in this type of pegmatites as well as in the Li-Cs-Ta (LCT) type to which is it is more commonly applied.