Abstract
The entire population of magmatic oscillatory zoned zircons in a migmatitic granite from the Tjärnesjö intrusion, southwest Sweden, reveal fine-scale brittle fracturing. The oscillatory zoned fragments are rotated but not dispersed. Fractures between individual fragments are sealed by newly formed CL-bright zircon. Hydraulic fracturing is the most probable mechanism. The internal structure of fractured zircons and the LREE-enriched, low Th character of CL-bright zircon both suggest that cracks between oscillatory zoned zircon fragments were rapidly sealed after fracturing by CL-bright zircon, precipitated from hydrothermal fluids. Zircon fracturing and crack-sealing has been dated by SIMS ion-probe and U-Th-Pb isotopes to 920 ± 51 Ma (lower intercept age, 2σ, MSWD = 1.09) with a limit for the youngest possible age of 960 ± 16 Ma (207Pb/206Pb, 2σ, MSWD = 0.23) dated by sector-zoned rims forming overgrowths on the fractured cores.
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