U-Pb geochronology of zircon and rutile from the Kokchetav metamorphic belt, northern Kazakhstan, and its tectonic implications

The Kokchetav ultrahigh-pressure (UHP) metamorphic belt, located in the western part of the Central Asia Orogenic Belt (CAOB), consists of a well-studied diamond-bearing western domain and a less well-known coesite-bearing domain. To better understand the time of subduction and exhumation of the eastern domain and the tectonic evolution of the entire Kokchetav microcontinent, we conducted SIMS U-Pb dating of zircon from eclogite and rutile from whiteschist of the eastern domain. We identified two types of zircon from the Kulet eclogite: type-I zircon grains possess a large, low-cathodoluminescence (CL) magmatic core with high Th/U ratios of 1.0-1.5, yielding a Proterozoic protolith age (1437 +/- 29 Ma), and a thin rim of intermediate or high-CL. Type-II zircon crystals are rounded to ovoid, and have very low Th/U ratios (<0.1-0.2), yielding two population ages (532 +/- 58 and 497 +/- 5 Ma), which we interpret as the ages of peak metamorphism and decompression metamorphism, respectively. Rutile separates from three samples of whiteschist have low Th/U ratios (<<0.1). U-Pb isotopic analyses yielded two ages (526 +/- 9 and 498 +/- 11 Ma), interpreted to represent two stages of retrograde metamorphism. Combined with geochronological data from the literature and our results indicate that UHP rocks from both the eastern and western domains represent Proterozoic protoliths, and that they were subjected to nearly coeval UHP metamorphism at 527-537 Ma, even though the two domains formed at different peak P-T conditions. Exhumation took place during 526-498 Ma for the eastern domain, comparable to the western domain (526-507 Ma). The UHP slabs were exhumed from depths of 100-200 km to a mid -crustal level over 20-25 My. The final suturing of the uplifted Kokchetav microcontinent against the Stepnyak paleo-island arc occurred at <= 500 Ma. Our new SIMS U-Pb age data and previous petrological and structural analyses provide new constraints on the metamorphism and tectonic evolution of the Kokchetav microcontinent, and on the continental history in the western part of the CAOB.