Evidence of the Collison of the Precambrian Kokchetav Microcontinent With Future Asia During Paleozoic

Formation of the Kokchetav subduction-collision zone (northern Kazakhstan): Insights from zircon U-Pb and Lu-Hf isotope systematics

Authors:

Glorie et al

Abstract:

The Kokchetav subduction-collision zone is located in the western part of the ancestral Central Asian Orogenic Belt. This zone is built up by the Precambrian Kokchetav microcontinent which includes a HP-UHP metamorphic belt, and the North Kokchetav tectonic zone (NKTZ) which represents an accretionary complex between the Kokchetav microcontinent and the adjacent Stepnyak island-arc. The entire region is widely intruded by Palaeozoic granitoids which were emplaced after the collision of the Stepnyak island-arc with the Kokchetav microcontinent. We present zircon U-Pb ages and Lu-Hf systematics in zircon to better characterize the tectonic evolution of the Kokchetav subduction-collision zone.

The Lu-Hf results indicate that the Kokchetav basement rocks are derived from late Neoarchaean – early Palaeoproterozoic (~ 2.5 Ga) crust. For the granite-gneiss basement of the Kokchetav microcontinent, early Mesoproterozoic (Grenville-age, ~ 1.17-1.14 Ga) zircon U-Pb crystallization ages were obtained. For the NKTZ, two main age-components were recognized: (1) an oldest Mesoproterozoic age-component (~ 1.20-1.05 Ga) similar as for the Kokchetav microcontinental zircons, and (2) a younger Early Cambrian (540-520 Ma) or Late Cambrian - Early Ordovician (~ 490-480 Ma) age-component. Th/U ratios (< 0.1) are indicative of a metamorphic origin for both Early Cambrian and Late Cambrian - Early Ordovician zircon populations. The oscillatory zoned Mesoproterozoic zircons have higher Th/U ratios (> 0.1) and are typical of a magmatic protolith. The distinction between both zircon types is supported by characteristic cathodoluminescence images. These results confirm previous observations, indicating early Palaeozoic high-grade metamorphism (~ 540-520 Ma) and collisional deformation (~ 490-480 Ma) of the Mesoproterozoic Kokchetav zone as a result of subduction-accretion and collision of the Stepnyak island-arc to the Kokchetav microcontinent. For two additional samples of the Balkashin granitic complex Early Devonian (~ 415-395 Ma) zircon crystallization ages (magmatic Th/U ratios) were obtained. The occurrence of a Mesozoic xenocryst within these leucogranites may indicate that they were emplaced in a continental-arc setting.