ION MICRO-PROBE U-PB ZIRCON GEOCHRONOLOGY OF PEAK AND RETROGRADE STAGES OF ULTRAHIGH-PRESSURE METAMORPHIC ROCKS FROM THE KOKCHETAV MASSIF, NORTHERN KAZAKHSTAN

Abstract

Sensitive high resolution ion micro-probe (SHRIMP) U–Pb analyses of zoned zircons were performed to constrain the age and geodynamic significance of the Kokchetav ultrahigh-pressure (UHP) metamorphic rocks, which recrystallized at P>60 kbar and T>1000°C. Cathodoluminescence images were correlated with micro-assemblages of mineral inclusions in zircons from diamond-bearing gneisses. These revealed UHP mineral-bearing cores and low-pressure mineral-bearing rims, some zircons also retain inherited cores. Our SHRIMP data identified four discrete stages: Middle Proterozoic protolith age, 537±9 Ma for UHP metamorphic conditions, 507±8 Ma for the late-stage amphibolite facies overprint, and 456–461 Ma for post-orogenic thermal events. Thus, we conclude that Middle Proterozoic supracrustal protoliths of the Kokchetav UHP–HP rocks were subducted to mantle depths in the Middle Cambrian, and exhumed to mid-crustal levels in the Late Cambrian. The exhumation rate calculated from the SHRIMP data and relative P–T conditions is approximately 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.