U-PB SINGLE-ZIRCON DATING UNDER CATHODOLUMINESCENCE CONTROL (CLC-METHOD): APPLICATION TO POLYMETAMORPHIC ORTHOGNEISSES

Abstract

The dating of polymetamorphic rocks with the U-Pb zircon method often results in discordant data. Thus a new technique combining cathodoluminescence imaging (CL) and thermal ion mass spectrometry (TIMS) measurements was developed. Cathodoluminescence-controlled U-Pb dating (CLC-method) works with bisected zircons and was successfully applied to the multistage S-type granitoids of the Swiss Silvretta nappe. CLC-method enables the resolution of the different anatectic events and later major overprints (Cambrian, Ordovician, Silurian) and gives information about the protolith (Archaean) of the investigated gneisses. This is achieved by conventional U-Pb dating of selected grains, recovered from the SEM (scanning electron microscope) mounts after cathodoluminescence (CL) imaging.Applying this new combination of two established techniques yields geologically meaningful U-Pb zircon ages even for complex polymetamorphic rocks.