REE, Th, Hf, Ta in Bamble gabbros (southern Norway) and their amphibolitized equivalents: implications for gabbro tectonic setting☆

Abstract The Bamble gabbros (“hyperites”) are a group of partly metamorphosed Proterozoic basic intrusions in the Bamble Sector of southern Norway. They are discordant to structures formed during the Kongsbergian (1.54 Ga) high-grade metamorphism, yet are themselves partially amphibolitized. In some localities, transitions from gabbro to amphibolite occur over only a few metres. A previous study of closely spaced gabbro-amphibolite sample pairs revealed that amphibolitization was accompanied by increases in K, Fe 3+ , P, H 2 O, Rb, Cl±Zr and decreases in Ca, Fe 2+ , S, Zn ± Si ± Co. New data for REE, Th, Hf, Ta in the same samples indicate that the only possible metamorphic modification of these elements is a barely significant increase in La in one locality. Eu/Eu ∗ ratios are preserved. This adds confidence to the use of REE as petrogenetic indicators in amphibolitized basic rocks. The resistance of Sm/Nd and Lu/Hf ratios to the amphibolitization may indicate that these isotopic systems could be used to gain geochronological and petrogenetic information from the gabbros and their amphibolitized equivalents. The combined chemical data fingerprint the gabbros as similar to present-day destructive plate-margin-related basic rocks. This is surprising as it has previously been widely supposed that they are an expression of the tensional alkaline magmatism that was widespread in the North Atlantic province at ∼ 1.25 Ga . It thus seems more likely that the Bamble gabbros are related to renewed subduction after the ∼ 1.25 Ga anorogenic period. The geochemistry of these widespread gabbros must be accounted for in future Proterozoic Scandinavian tectonic reconstructions.