The Itmurundy Pacific-type orogenic belt in northern Balkhash, central Kazakhstan: Revisited plus first U-Pb age, geochemical and Nd isotope data from igneous rocks

摘要

The Itmurundy zone/belt is located in the northern Balkhash area of central Kazakhstan. Geologically it belongs to the Kazakh orocline located in the western Central Asian Orogenic Belt (CAOB), north of the Tarim craton and west of the Junggar block. The Itmurundy belt, which surprisingly has remained unstudied in terms of up-to-date geochronological, geochemical and isotope methods compared to other regions of the CAOB, was revisited and reinvestigated. The belt possesses a very complicated geological structure and hosts rocks of mantle, orogenic and post-orogenic associations. This paper focuses on the orogenic association and presents original geological data, first U-Pb age and first up-to-date geochemical and Nd isotope data from igneous rocks. The orogenic association of the Itmurundy belt includes volcanic and sedimentary rocks of three formations, Itmurundy (O1-2), Kazyk (O2-3) and Tyuretai (O-3-S-1), and represents an accretionary complex. The most lithologically diverse Itmurundy Fm. (O1-2) consists of oceanic basalt, pelagic chert, hemipelagic siliceous mudstone and siltstone, and greywacke sandstones. Both sedimentary and igneous rocks were strongly deformed by syn- and post-accretion processes, which, in places, formed duplex structures. The igneous rocks are basalt/dolerite/gabbro, andesibasalt, trachybasalt and diorite. The diorite yielded a U-Pb age of ca. 500 Ma. The subalkaline volcanic and subvolcanic rocks belong to the tholeiitic series. Based on major oxides three groups of rocks can be distinguished: high-Ti, mid-Ti and low-Ti. The rocks of these three groups are variably enriched in LREE (La-N = 122, 23 and 2 in average, respectively) showing LREE enriched (high-Ti), LREE depleted (mid-Ti) and flat (low-Ti) REE patterns. The high-Ti group shows enrichment in Nb, Th, and Zr compared to the mid-Ti and low-Ti groups. The low-Ti group is special for the Nb troughs in primitive mantle normalized multi-element diagrams, which are typical of supra-subduction settings. The values of epsilon Nd are mostly positive for the mid-Ti and low-Ti groups, but negative for the high-Ti group. The geochemical features of the igneous rocks suggest their formation in oceanic (oceanic floor and oceanic island/seamount) and supra-subduction (intra-oceanic arc) settings. In general, the structural position, lithology and deformation styles of Itmurundy sedimentary and igneous rocks and the geochemical features of the igneous rocks all accord well with the models of Ocean Plate Stratigraphy (OPS) and Pacific-type orogeny. Thus, the Itmurundy belt at northern Balkhash represents an Ordovician-Silurian Pacific-type orogenic belt formed at a convergent active margin of the Paleo-Asian Ocean. (c) 2019 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.