Formation and accretionary evolution of the Kokchetav complex, northwestern Kazakhstan: Insights from structural styles, and detrital zircon geochronology and Lu-Hf isotopes

Abstract

Understanding how subduction processes incorporate continental fragments into accretionary complexes is important for constraining the formation of accretionary orogenic belts. The Kokchetav Complex is a distinctive geological unit in the northwestern Altaids of northwestern Kazakhstan that is ideal for investigating such processes. It hosts microcontinental fragments, and accretionary and (ultra)high-pressure ([U]HP) metamorphic rocks. We undertook U-Pb dating and Lu-Hf isotopic analyses of detrital zircons from quartzites, schists, gneisses, and sandstones in the metasedimentary belt, gneissic basement rocks, (U)HP metamorphic belt, and ophiolitic mélange in the Kokchetav Complex. The quartzite–schist rocks yield three primary zircon age clusters: middle–late Paleoproterozoic (1.8–1.6 Ga), middle Mesoproterozoic to early Neoproterozoic (1.4–0.8 Ga), and latest Neoproterozoic to Early Devonian (544–406 Ma), with a few isolated zircons dated between 3.6 and 2.8 Ga. These rocks have diverse detrital zircon U-Pb age spectra (unimodal to multi-modal) with variable Hf isotope ratios (−10.9 to +22.0). The zircons have a mixed origin from the early Paleozoic Stepnyak Arc and Precambrian continental fragments, and were not derived from the gneissic basement, indicating an affinity with the Proterozoic Musgrave–Albany Orogen in Australia. Consequently, these zircons reveal that a continent fragment from the northwestern Rodinia supercontinent exists within the Kokchetav Complex, which retained its original nature from the Neoproterozoic to early Paleozoic and was later incorporated into the accretionary complex as tectonic sheets and slices during convergence between the Kokchetav microcontinent and Stepnyak Arc. Our results further indicate that, rather than being part of a unified Precambrian block, the Kokchetav Complex incorporated continental fragments and younger accreted materials that formed in an early Paleozoic supra-subduction zone setting.