CHARACTERIZATION OF THE PALEOPROTEROZOIC MANTLE BENEATH THE FENNOSCANDIAN SHIELD: GEOCHEMISTRY AND ISOTOPE GEOLOGY (Nd, Sr) OF ~1.8 GA MAFIC PLUTONIC ROCKS FROM THE TRANSSCANDINAVIAM IGNEOUS BELT IN SOUTHEAST SWEDEN

Abstract

Geochemistry and Sr-Nd isotope geochemistry of ca. 1.8 Ga, mafic intrusions of the Transscandinavian Igneous Belt (TIB-1) in the Fennoscandian shield were studied in southeastern Sweden. These rocks show LILE-LREE-enriched, HFSE-depleted, calc-alkaline, continental arc signatures in the north, grading to slightly less enriched, oceanic affinities southward.εNd(1.80) values range from +2.0 to +0.7 and Sr/Sr(1.80) from 0.7022 to 0.7029 (with one outlier at 0 and 0.7033), without correlation to fractionation (e.g., Mg#) or crustal contamination, indicating sources that are mildly depleted. The most depleted ratios occur in the south, trending with the geochemistry toward more enriched compositions northward. The sources represent depleted mantle wedge material that was subjected to enrichment not long before (TDM ca. 2.0 Ga), i.e., during the preceding arc subduction (2.1-1.82 Ga), and/or during the TIB-1 magmatism itself, by hydrous fluids with a sediment and/or melt input increasing northward. The TIB-1 magmatism occurred above a south(west)ward-retreating subduction zone along the continental margin of the juvenile Svecofennian continent at 1.81-1.76 Ga.