Composition and U-Th-total Pb model ages of polygenetic zircons from the Vanga granite, south Sweden: An electron microprobe study

U-Th-total Pb dating of zircons from the Vanga granite, south Sweden, by el ectron microprobe (EMP) revealed three different zircon generations. U-Th-P b analyses of finely zoned zircons, existing both as rounded cores and over growth rims, yielded a well defined U-Th-total Pb model age of 1448+/-25 Ma (1 sigma). This age is interpreted to date the intrusion of the Vanga gran ite, which is notably older than previously suggested. Hf- and F-rich overg rowths yielded a U-Th-total Pb model age of 1128+/-51 Ma (1 sigma). Based o n the Hf- and F-enrichment and morphological features, it is suggested that the overgrowths formed by a dissolution-precipitation process in a hydroth ermal environment. In few cases magmatic zircon overgrows clear, unzoned co res, which could hardly be dated by EMP due to low U, Th, and Pb contents. A single analysis of one of these cores, however, yielded a U-Th-total Pb m odel age of c. 1700 Ma, which fits with a previously determined age of refr actory zircons in a c. 1445-1465 Ma old Karlshamn-type granite. This agreem ent supports the previous interpretation that the Vanga granite crystallize d from a residual liquid separated from the Karlshamn granite magma. The ne w intrusion age of c. 1450 Ma indicates that the Vanga granite intruded con temporaneously with an extensive thermo-magmatic activity further west in t he Eastern Segment of the Sveconorwegian orogen. The c. 1130 Ma age of zirc on overgrowths has tectonic implications on the N-S trending Protogine Zone ; a major shear zone that marks the eastern boundary of Sveconorwegian rewo rking. Since equally old syenites and dolerites occur within the Protogine Zone, the age of these hydrothermal overgrowths indicates that the crust af fected by Protogine Zone deformation is not exotic relative to the crust of Blekinge, nor is it conceivable to relate the metamorphic contrast across the Protogine Zone to deformation east thereof.