No 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.ε Nd (1.80) values range from +2.0 to +0.7 and 87 Sr/ 86 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 (T DM 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.ANDERSSON ET AL.

Mafic intrusive rocks (1.79-1.78 Ga) of the Transscandinavian Igneous Belt (TIB) and the c. 1.87 Ga Hedesunda Igneous Complex in the Fennoscandian Shield of south-central Sweden were studied using whole-rock and isotope geochemistry.Rock types vary from gabbros/norites (and leucogabbros) to quartz diorites, with Mg# between 76 and 49, and wt% SiO 2 between 43.6 and 59.7, indicating some variation in evolutionary levels and variable cumulus components. Geochemical signatures are calc-alkaline to shoshonitic, large ion lithophile elements and light rare earth elements enriched and high-field strength elements depleted of continental-arc type.e Nd (t) ranges between þ1.0 and þ2.7, and 87 Sr/ 86 Sr(t) between 0.7020 and 0.7038. There is no systematic correlation between chemical parameters and isotope ratios. These isotopic data overlap with other mafic plutonic TIB rocks; samples from the Dala Province (DP) tend to overlap with the c. 1.7 Ga basic Dala lavas of TIB at slightly elevated relative Sr/Nd ratios.With two exceptions, the e Nd (t) of þ1 to þ2 conform to an isotopically 'mildly depleted' source, typical for mafic TIB rocks and many Svecofennian rocks in the region. Reported values above e Nd (t) þ2.0 are scarce in the TIB. Mantle sources represent depleted mantle wedge material that was enriched by fluids/melts not long before (T DM c. 2.0 Ga), that is during subduction in the preceding Svecofennian (2.0-1.87 Ga) and/or during the TIB-0&1 event (1.85-1.78 Ga). The palaeotectonic settings inferred are active continental margins; N-S-directed convergence at 1.87 Ga and E-W-directed at 1.79-1.78 Ga.

No abstract