Ultrahigh-pressure metamorphism of the diamond-bearing gneisses in the Kokchetav massif, USSR, has been dated at 530 ±7 Ma (2&sgr,) by SHRIMP ion-microprobe analysis of metamorphic zircons. Zircon xenocrysts as old as 2000 Ma have retained original isotopic compositions through the extreme metamorphic conditions and indicate that the protolith that was buried to diamond-forming pressures included Early Proterozoic components. Highly variable reset-ting of U-Pb isotopic systems in zircon xenocrysts at the time of metamorphism suggests that diffusive lead loss had commenced but was incomplete, a disequilibrium observation consistent with relatively transient exposure of the rocks to extreme crustal depths.

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