Disturbed radiometric ages and their bearing on interregional correlations in the SW Baltic Shield

Abstract

RbSr whole-rock and zircon UPb isotopic work is reported from the northern part of the region between the Mylonite and Protogine Zones in the southern Baltic Shield. Three age determinations on polymetamorphic gneissic granites were carried out. Two of these yielded UPb upper-intercept ages of ∼1650 and ∼1675 Ma, respectively, which is approximately one hundred Ma less than expected from the combined field evidence and earlier isotopic age determinations. Although the new discordias appear well defined, various criteria suggest that the UPb isotope system was disturbed. Thus, the RbSr system had been opened, but nevertheless the RbSr errorchrones are consistent with the UPb upper-intercept ages, and an abraded zircon fraction suggests an older age than the unabraded fractions. The third UPb age determination resulted in a poorly constrained discordia. Its upper intercept ranges between 1575 and 1605 Ma, depending on the number of fractions considered. This paper tests a three-stage model, where the intrusion age is set identical to the 1780 Ma intrusion age of the undeformed assumed protolith. The first two analysed rocks are consistent with a model suggesting intrusion at 1780 Ma followed by partial opening of the isotopic systems early (∼1200 Ma) and late (∼950 Ma) during the Sveconorwegian orogeny. An abraded fraction together with the time of the suggested last opening of the system 950 Ma ago defines a two-point discordia with an upper-intercept age of ∼1745 Ma. This age is in close agreement with the assumed intrusion age. The isotope data from the third gneissic granite cannot be fitted to this simple model. It is concluded that the analysed rocks could belong to the granitoids of the Transscandinavian Igneous Belt, but if they do, they were reworked during the Sveconorwegian orogeny. As a corollary, it follows that the regions separated by the northern part of the Protogine Zone largely had a common pre-Sveconorwegain geological history. Another important implication is that the intrusion ages in the area between the Mylonite and Protogine Zones are older than those of the Trans-Labrador Batholith in Canada.