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

Abstract

U–Th–total Pb dating of zircons from the Vånga granite, south Sweden, by electron microprobe (EMP) revealed three different zircon generations. U–Th–Pb analyses of finely zoned zircons, existing both as rounded cores and overgrowth rims, yielded a well defined U–Th–total Pb model age of 1448±25 Ma (1[sgrave]). This age is interpreted to date the intrusion of the Vånga granite, which is notably older than previously suggested. Hf- and F-rich overgrowths yielded a U–Th–total Pb model age of 1128±51 Ma (1[sgrave]). Based on the Hf- and F-enrichment and morphological features, it is suggested that the overgrowths formed by a dissolution–precipitation process in a hydrothermal environment. In few cases magmatic zircon overgrows clear, unzoned cores, 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 model age of c. 1700 Ma, which fits with a previously determined age of refractory zircons in a c. 1445–1465 Ma old Karlshamn-type granite. This agreement supports the previous interpretation that the Vånga granite crystallized from a residual liquid separated from the Karlshamn granite magma. The new intrusion age of c. 1450 Ma indicates that the Vånga granite intruded contemporaneously with an extensive thermomagmatic activity further west in the Eastern Segment of the Sveconorwegian orogen. The c. 1130 Ma age of zircon overgrowths has tectonic implications on the N–S trending Protogine Zone; a major shear zone that marks the eastern boundary of Sveconorwegian reworking. Since equally old syenites and dolerites occur within the Protogine Zone, the age of these hydrothermal overgrowths indicates that the crust affected 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.

Key Words:

• Vånga granite
• zircon
• geochronology
• chemical age dating
• electron microprobe
• hydrothermal zircon