A tentative study of the U-Pb isotope system in zircons from the Mylonite Zone, south-east Norway

Abstract

About 80% of the zircons separated from a protomylonitized gneissic granite in the Mylonite Zone in SW Scandinavia consist of fragments. The crystals have sharp edges and are dominated by (100) prisms and (101) pyramids. U-Pb isotope analyses of four unabraded size-fractions result in highly discordant ²⁰⁶Pb/²³⁸U, ²⁰⁷Pb/²³⁵U and ²⁰⁷Pb/²⁰⁶Pb ages. The fractions lie close together on a discordia with an upper intercept at 1333±187 Ma. The inclusion of two air-abraded fractions in the calculations gives an upper intercept at 1418±65 Ma. A discordia through only the two abraded fractions gives an upper intercept at 1503±97 Ma. This latter intercept is interpreted as a crude estimate of the minimum age of the intrusion. The morphology of the zircons, typical for high temperature alkalic granites, and the lack of growth rims and visible cores, in connection with the high percentage of broken crystals with pitted faces, suggest leaching of primarily Pb as the main cause for the discordancy. We thus conclude that mylonitization events lead to profound disturbances of the U/Pb isotope systems and suggest that zircons from strongly mylonitized rocks should be tested as a method to date mylonitization processes directly.

Key Words:

• Mylonite
• zircon dating
• zircon morphology
• upper intercept age
• air abraded fraction
• Southwest Scandinavian Gneiss Complex