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.

The entire population of magmatic oscillatory zoned zircons in a migmatitic granite from the Tjärnesjö intrusion, southwest Sweden, reveal fi ne-scale brittle fracturing. The oscillatory zoned fragments are rotated but not dispersed. Fractures between individual fragments are sealed by newly formed CL-bright zircon. Hydraulic fracturing is the most probable mechanism. The internal structure of fractured zircons and the LREE-enriched, low Th character of CL-bright zircon both suggest that cracks between oscillatory zoned zircon fragments were rapidly sealed after fracturing by CL-bright zircon, precipitated from hydrothermal fl uids. Zircon fracturing and crack-sealing has been dated by SIMS ion-probe and U-Th-Pb isotopes to 920 ± 51 Ma (lower intercept age, 2σ, MSWD = 1.09) with a limit for the youngest possible age of 960 ± 16 Ma ( 207 Pb/ 206 Pb, 2σ, MSWD = 0.23) dated by sector-zoned rims forming overgrowths on the fractured cores.