A new tectonic model for the Itmurundy Zone, central Kazakhstan: linking ocean plate stratigraphy, timing of accretion and subduction polarity

The Itmurundy Zone of Central Kazakhstan is a key structure in the core of the Kazakh Orocline representing a typical Pacific-type orogenic belt hosting accretionary complex, ophiolite massifs and serpentinite mélange. The main controversies in the existing tectonic models of the Itmurundy Zone are about the timing of subduction and accretion, the direction and kinematics of subduction and the number of oceanic plates. A new model for the early Paleozoic tectonic story of the Itmurundy Zone is postulated in this paper, based on new detailed geological and U–Pb detrital zircon age data, combined with previously documented geological, U–Pb age, microfossil, geochemical and isotope data from igneous rocks, deep-sea sediments and greywacke sandstones. The present study employs the Ocean Plate Stratigraphy (OPS) model to explain the tectonic processes involved in the evolution of the Itmurundy Zone and to present a holistic story of Ordovician oceanic plate(s), which accretion formed an accretionary complex. The detailed mapping allows distinguishing three types of OPS assemblages: (1) Chert-dominated, (2) OIB-hosting, and (3) MORB-hosting. The U–Pb ages of detrital zircons from sandstones of OIB and Chert types show unimodal distributions with similar main peaks of magmatism at 460–455 Ma in the provenance, and their maximum depositional ages (MDA) span 455–433 Ma. Two samples from OPS Type 3 show the peaks of magmatism both at ca. 460 Ma and the MDA of 452 Ma and 459 Ma, respectively. The MDA of sandstones and microfossils data from chert show the younging of strata to the south and SE in Types 1 and 2 and to NEE for Type 3 (in present coordinates) suggesting double-sided subduction to the NNW and SEE and, accordingly, the co-existence of pieces of two oceanic plates in Ordovician time. The U–Pb zircon data from both igneous and clastic rocks indicate a period of subduction erosion in early Ordovician time. As a whole, the accreted OPS units of the Itmurundy Zone record the timing of subduction and accretion from the early Ordovician to the early Silurian, i.e., 60 Ma at shortest.