Ion micro-probe U-Pb zircon geochronology of peak and retrograde stages ofultrahigh-pressure metamorphic rocks from the Kokchetav massif, northern Kazakhstan

Sensitive high resolution ion micro-probe (SHRIMP) U-Pb analyses of zoned z ircons were performed to constrain the age and geodynamic significance of t he Kokchetav ultrahigh-pressure (UHP) metamorphic rocks. which recrystalliz ed at P > 60 kbar and T> 1000 degreesC. Carhodoluminescence images were cor related with micro-assemblages of mineral inclusions in zircons from diamon d-bearing gneisses. These revealed UHP mineral-bearing cores and low-pressu re mineral-bearing rims, some zircons also retain inherited cores. Our SHRI MP data identified four discrete stages: Middle Proterozoic protolith age, 537 +/- 9 Ma for UHP metamorphic conditions, 507 +/- 8 Ma for the late-stag e amphibolite facies overprint, and 456-461 Ma for post-orogenic thermal ev ents. Thus, we conclude that Middle Proterozoic supracrustal protoliths of the Kokchetav UHP-HP rocks were subducted to mantle depths in the Middle Ca mbrian, and exhumed to mid-crustal levels in the Late Cambrian. The exhumat ion rate calculated from the SHRIMP data and relative P-T conditions is app roximately 5 km/Ma. The rapid exhumation rate for the massif is in the same range as other UHP metamorphic terranes, and suggests that slab decoupling may be a driving force for exhumation of UHP metamorphic rocks from mantle depths. (C) 2001 Elsevier Science B.V. All rights reserved.