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Disturbed radiometric ages and their bearing on interregional correlations in the SW Baltic Shield |
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| Authors: | A Lidh H Schberg K Annertz |
| Institution: | a Department of Geology, Lund University, Sölvegatan 13, S-223 62, Lund, Sweden b Swedish Museum of Natural History, Box 50007, S-104 05, Stockholm, Sweden |
| 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 ( 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. |
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