Revisiting the “Irtish tectonic belt”: Implications for the Paleozoic tectonic evolution of the Altai orogen

Abstract

The Irtish tectonic belt is one of the most important suture zones in the Central Asian Orogenic Belt (CAOB). The various models invoked to explain the tectonic evolution of this belt in relation to Paleozoic continental growth include: (1) syn-subduction strike-slip duplication of a single long-lived arc, and (2) collision of many arcs and micro-continents. Here we report new results from detailed field studies, geochemistry and zircon U–Pb geochronology on the major rock units along the Irtish belt both in Kazakhstan and in the Chinese Altay. Our new results, together with information from published data, suggest that the Altai-Mongolia terrain witnessed several episodes of subduction. The Junggar terrain, which formed by multiple collisions of intra-oceanic arcs, seamounts and ophiolite suites prior to 330Ma, subsequently amalgamated with the Altai-Mongolia terrain at around 320Ma along the Irtish suture zone. Following the clockwise rotation of the Siberian craton and the N-directed movement of the Junggar terrain, the sinistral strike-slip motion along the Irtish suture zone resulted in ductile shearing, partial melting of the crustal material, and the formation of mylonites, syn-tectonic migmatite and gneissic granites. The sinistral strike-slip deformation of the Irtish suture zone culminated at ca. 275Ma. The diverse 275–250Ma magmatism (mafic–ultramafic complexes, bimodal intrusives, mafic and silicic dykes, A-type and I-A-type granites) is related to magma pulses generated by the Permian Tarim mantle plume. Our model of multiple episodes of subduction below the Altai arc is different from previous models and proposes that the subduction continued until Triassic with the Permian subduction under the Altai arc and a late Permian large-scale south-directed thrusting.