APPLICATION OF IN SITU Rb-Sr DATING AND GEOCHEMICAL INVESTIGATIONS ON A NEW, 1.52-1.56 GA. OLD PEGMATITE FIELD IN THE ARCHIPELAGO OFF GOTHENBURG, IDEFJORDEN TERRANE, WESTERN SWEDEN

Sammanfattning: In this study, Rb-Sr geochronology was used to determine the crystallization/cooling ages of muscovites and biotites from mapped primitive barren and rare element pegmatites in the Stora-Le-Marstrand Formation in the southwestern archipelago of Gothenburg. The results indicate that primitive barrens can be linked to the DG2 deformation event that occurred 1.56-1.54 Ga and suggest an anatectic origin and limits the migmatisation event to 1547 Ma. The study also found that the Leucosome sample from the SLM Fm. and the barrens have similar geochemical fractionation indicators, which supports the theory that these barrens originate from the partial melt of the SLM Fm. A correlation was identified between the degree of fractionation and the cooling age of the barrens, with younger barrens showing greater fractionation as seen in parameters such as Ba/Rb, Rb/Sr and trace element concentrations of Li, Ta and Cs in muscovites, biotites, garnets and feldspars. Rb-Sr ages of biotites were used as an exhumation age indicating that the area of the SLM Fm. west of Gothenburg reached a temperature of 300-350°C at around 938 to 973 Ma, which is consistent with the end of the Sveconorwegian orogeny. The study also found that the beryl-bearing rare element pegmatites on the island of Donsö are of cogenetic origin yielding calculated cooling ages of 1520 ± 6.4 2s Ma, the study suggest the source for these pegmatites may be another deformation event or the last intrusive stage of the Hisingen suite. The study also found that fast data acquisition with the handheld XRF is suitable for a rough estimate of the degree of fractionation and that the Rb-Sr dating process can be made faster and easier by using the initial Sr isotope ratios of 0.715 ± 0.015 (for enriched magmatic rocks) by Rösel & Zack (2022). Overall, this study provides new insights into the formation and evolution of the pegmatites in the Idefjorden terrane and highlights the potential of using Rb-Sr geochronology for dating pegmatites. The correlation between fractionation degree and cooling age of the barrens and rare element pegmatites, as well as the determination of exhumation ages from biotites samples, add to our understanding of the geodynamic processes occurred in this part of the Idefjorden terrane.