The timing of partial melting and UHP metamorphism in the Kumdy-Kol region (Kokchetav Massif, Northern Kazakhstan)

The Kokchetav Massif of northern Kazakhstan is the best-known metamorphic diamond locality among numerous ultrahigh-pressure (UHP) terranes in the world. At the Kumdy-Kol deposit, diamondiferous rocks are interbedded with granitic gneisses, and biotite gneisses; some have been migmatized. Some granite gneisses and migmatites were formed by partial melting of diamondiferous rocks. To verify such suggestion, sensitive high resolution ion microprobe (SHRIMP) U-Pb dating of zoned zircons from migmatites at the Kumdy-Kol region was performed to constrain the age of partial melting of the Kokchetav UHP metamorphic rocks. Most age data from core and rim domains of zircon separates are concordant. The apparent 206/Pb238U ages for core (524 ± 6 Ma) and rim domains of zircons (522 ± 7 Ma) are nearly identical within analytical error. All SHRIMP analyses of zircons from three samples fall in the range 507 - 538 Ma with the weighted mean age for all zircon domains at 523 ± 4 Ma (MSWD = 4.6). Taking the mean ages of UHP metamorphism (530 - 337 Ma) and late amphibolite-facies overprinting (517 ± 5 and 515 ± 5 Ma) we conclude that exhumation from diamond-grade depths (> 150 km) to mid-crustal depths (~ 30 km) must have been completed within about 15 - 20 Ma. Our data show that migmatization of UHP pelites occurred slightly later than the peak metamorphism (~ 530-537 Ma), and the decompression, partial melting took place during exhumation of diamondiferous rocks from mantle depths to amphibolite-facies conditions at mid-crustal levels.