Shalkar ophiolite complex (northern Kazakhstan): Structural setting, age, geochemistry and genesis

The Shalkar ophiolite complex consists of a series of tectonic slabs including lavas, a complex of parallel dikes, and a layered complex of gabbros and gabbro-pyroxenites intruded by dikes of plagiogranites. The tectonic slabs are subsided northward and are confined to the base of a large nappe made up of rocks of the Kokchetav microcontinent. The ophiolitcs and fragment of the microcontinent are jointly thrust over the Kokchetav megamelange complex with HP-UHP rocks. It is inferred from the chemical composition and structural position that the Shalkar ophiolites formed under the conditions of suprasubduction zone of extension and occur in the setting of the initial stage of formation of the Stepnyak island arc. The upper age of the ophiolites is 485 ± 6 Ma, as determined for zircons from plagiogranites using the U-Th-Pb SHRIMP method. The age of the Shalkar ophiolites is within 495-485 Ma. This interval is marked by a considerable rearrangement in the subduction zone, which might have been due to the jump of the subduction zone to the hinterland of the Kokchetav microcontinent and to the origin of a zone of extension within its limits with the formation of ophiolites.