To obtain geochronological data from polymetamorphic rocks is a complex but in many geological projects required task. Especially for polymetamorphic S-type granitoids the use of zircons is often difficult, time consuming, accompanied with doubts about a correct and convincing data interpretation.
Zircons can be influenced by metamorphic overprints and anatectic processes. For example resorption, magmatic and metamorphic overgrowth, high temperature recrystallization and Pb-loss can change the age informations of a zircon partially or completely. These processes are generally accompanied by crystallographical and chemical changes which are indicated by distinct internal structures. These characteristics can be objectively documented, even in small scale of several micrometers, by using a Scanning-Electron-Microscope - Cathodoluminescence (SEM-CL) facility.
Sometimes a core in a zircon is already visible with
a standard microscope. But often zircons are free of
inclusions, fully transparent, overgrown without any optical indication but all these features are exclusively visible using SEM-CL techniques. Depending on primary ages, smallest amounts of inherited components can lead to discordant data points. Additionally a polyphase and different evolution of these cores during their pre-detrital and pre-anatectic history gives reason for characteristic discordia fans and not well defined lower intercepts.
SEM-CL pre-investigations improve the selection strategy for all zircon dating methods and the petrogenetic significance of different habitus can be checked. All typological habitus interpretations are limited by influence of inherited, sometimes dominating but optical invisible components.
For the presented case study we analysed 4 different polymetamorphic orthogneisses from the Upper Austroalpine Silvretta Nappe (Central Alps).
The main structural features occurring inside zircon populations of the same habitus 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 features (magmatic zoning, resorbed areas, inherited component) are present simultaneously.
In former projects the SEM-CL studied zircon populations are analysed:
1. Grain size and habitus depending multi-grain analyses, obtained in collaboration with R. Frei (Laboratory for Isotope Geology, Univ. Bern).
2. Conventional single grain measurements, obtained from S. Sergeev (ETH-Zürich), to verify the multi-grain interpretations with potentially concordant single zircons of gemstone quality. This approach of precise age determination was dominated by the problem to find inheritance free zircons without using SEM-CL documented grains.
3. Evaporation technique was used by B. Hellermann (Univ. Fribourg) in collaboration with U. Klötzli (Vienna). The aim was to obtain age informations for the anatexis and the protolith by stepwise evaporation. But without direct SEM-CL control, the problem of try and error was obvious. Subsequently, the approach to determine the youngest magmatic event was dominated by the characteristic problem of mixed age informations during the first evaporation steps. But convincing data about early evolution stages of the zircons are available.
The observation on these 3 methods lead to the development of a dating technique for SEM-CL pre-investigated bisected zircons in cooperation of the Univ. Fribourg with the MPI-Mainz. This approach enables to find fast and successful the rare candidates for a concordant point of the youngest magmatic stage, recover this individual grain and analyse it.
The results of our bisected and SEM-CL documented zircons are compared with the only binocular controlled zircon data using standard dating techniques.