Conventional U-Pb dating versus single-grain Pb evaporation dating of complex zircons from a pegmatite in the high-grade gneisses of southwestern Sweden

U-Pb multi-grain dating of zircons from a pegmatite dyke in southwestern Sweden yield analyses which define a discordia with upper and lower intercept axes of 1510 +/- 32 Ma and 631 +/- 64 Ma respectively. Dating by the single-zircon evaporation method on prismatic zircons from the same dyke indicates a primary crystallization age of 1409 +/- 20 Ma. In addition, four oval zircon grains yield a Pb-207/Pb-206 age of 969 +/- 27 Ma. The older single-zircon evaporation age is interpreted as representative of pegmatite emplacement, whereas the younger age is suggested to correspond to isotopic disturbance and new zircon growth during a late Sveconorwegian metamorphic event. To explain the false intercept ages in the U-Pb concordia diagram, this study suggests a three-stage model including (1) crystallization of zircons during pegmatite intrusion, (2) episodic lead-loss and new zircon growth during high-grade metamorphism and (3) a recent lead-loss event. As the dykes are strongly deformed, the protolith age of the pegmatite provides an upper age limit for the gneiss-forming event. The single-zircon age of 969 +/- 27 Ma is interpreted to be associated with this event, which took place during high-grade metamorphic conditions.