Geochronological data are often difficult to obtain from polymetamorphic rocks. The complicated history of gneisses, anatectic or metamorphic rocks is reflected in a complex mineral assemblage. Also the minerals themselves can show different growing zones and phenomena due to metamorphic influences.
Zircons are good candidates for dating because they
have a very high closure temperature. But they can also be influenced by metamorphic overprints or anatexis. These processes are visible in the interior structure of the crystals, sometimes also in the outer shape.
Cathodoluminescence (CL) is a very good technique for imaging such internal structures especially in comparism with backscattered photos.
As shown by intensive CL investigations from S-type granitoids of the Swiss Silvretta Nappe, the structural features occurring inside zircon populations of the same outer shape and grain size are quite different. Perfectly zoned zircons as a result of the last magmatic event (i.e. anatexis) are present beside crystals bearing old inherited components from the protolith. Both types can be in common with grains showing large resorbed areas with very intense luminescence and thin outer magmatic rims. In some zircons all three structural types (magmatic zoning, resorbed areas, inherited component) are present simultaneously.
Dating such zircons by conventional multi and single grain techniques normally ends in discordant data points. The parallel use of CL and conventional U-Pb dating was also not sufficient because large differences in the internal structures lead to a kind of gamble when doing the CL photo on grain A and dating grain B.
In order to improve the U-Pb dating on zircons from polymetamorphic rocks (especially S-type granitoids) a new procedure was developed, combining CL studies and U-Pb single zircon dating by vapour digestion or conventional techniques on the same grain.
After mounting suitable zircons in a low luminescent resin, these grains are polished with Pb free materials until the core zones of the zircons are exposed. After coating with carbon, a photo database of the zircon population is collected using a scanning electron microscope (SEM)-CL. The chemical composition of inclusions occuring inside the zircons can be identified qualitatively by an energy dispersive system (EDX) .
On the basis of these CL photos, suitable zircons are selected: for dating the last magmatic event, the perfectly zoned zircons are preferred, whereas large inherited cores can be used to get information about the age of the protolith.
The next step is to recover these bisect crystals from the resin by mechanical techniques in a clean laboratory. These recovered zircons are then dissolved by vapour digestion and spiked with a U-Th-Pb mixspike, sometimes followed by micro coloumn chemistry. In the case of perfectly zoned magmatic zircons also the evaporation technique can be used.
These procedures were first applied to the orthogneisses of the "Flüelagranitic Association" and to the "Mönchalpgneisses", all belonging to the upper Austroalpine Silvretta crystalline complex. These rocks were already dated by multi grain analyses (Bern) and single grain analyses (ETH Zürich) resulting in discordant data points, with the exception of two single zircons out of nine gaving concordant ages.
With this new technique for selecting zircons, concordant ages were obtained for all four gneiss types, constraining the Ordovician anatexis of the investigated S-type granitoids.
In a next step the dating of the protolith and a possible high pressure event will be tried, using crystals with large cores or resorbed grains. This study shows, that the described combination of CL-investigation and U-Pb single zircon dating on the same grain is a useful opportunity to obtain concordant U-Pb data on metamorphic S-type granitoids.