Kokchetau-Issykkul terrain of the Kazakhstan Paleozoic continent and the Ordovician latitude of the continent

摘要

Research subject. The Kazakhstan (Kazakh-Kyrgyz) paleocon-tinent attracts much research attention. In this article, we present data on the tectonic structure of the Early Paleozoic Kokchetau-Issykkul sialic terrain. Materials and methods. The geodynamic position of the Kokchetau-Issykkul terrain was determined by analysing samples of Ordovician rocks collected across 11 sites using the paleomagnetism method. Thus, we studied the paleo-magnetism of red sandstones, siltstones, siltstones and basalts, as well as that of andesite and tuffaceous rocks in marine sediments, which affiliation to different Ordovician formations was established on the basis of trilobite, brachiopod, conodont and graptolite fauna. Results. Early Paleozoic rocks are exposed in the northern part of the studied terrain (Kokchetau district) and in the Issyk- Kul region, which covers southern Kazakhstan, northern Kyrgyzstan and western Xinjiang. The sutures of Chistopol-Terskei and Iradir-Ili ocean basins serving as the boundaries of the Kokchetau-Issykkul terrain are located in the allochthoni us position. A multi-storey system of napes composed of pre-Pa-leozoic and Early Paleozoic rocks of the oceanic crust, volcanic oceanic island arcs and continental slope were studied near the Terskei margin of the terrin. This system of napes is covered by sediments containing graptolites of the Floian-Darriwilian age. Subduction-related volcanism near this margin of the terrain is of the Darriwilian and Sandbian age, while Ⅰ-type granites are of the Darriwilian age. The over-thrusting of the Iladyr-Ili oceanic crust on the Ili border of the terrain also occurred in the middle Ordovician. Subduction-related magmatism occurred here in the Late Darriwilian and Sandbian periods. Conclusion. The conducted studies testify to the geodynamics of the region. In the Early and Middle Ordovician, the Kokchetau-Issykkul terrain was a sialic volcanic arc separated from other sialic terrains by ocean basins. In the Middle Ordovician, the counter subduction of the oceanic crust under the Kokchetau-Issykkul island arc occurred. This process led to the closure of the Chistopol-Terskei and Iradir-Ili ocean basins in Middle Ordovician and to the formation the Kazakhstan continent by the collision of the Early Paleozoic sialic terrains and oceanic island arcs. According to the study of the paleomagnetism of Ordovician rocks in the Kokchetau-Issykkul island, the position of the middle part of this arc is determined to be at the paleogeographic latitude of 9.1 ±5.4°.