SHALKAR OPHIOLITE COMPLEX, NORTHERN KAZAKHSTAN: STRUCTURAL SETTING, AGE, GEOCHEMISTRY, AND GENESIS
N.L. Dobretsov, M.M. Buslov, D. Rubatto*, and I.Yu. Safonova
Institute of Geology and Mineralogy, Siberian Branch of the RAS,
3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia
* Research School of Earth Sciences, Australian National University, Canberra, ACT0200, Australia
Keywords: Ophiolites, complex of parallel dikes, gabbro-pyroxenites, basalts, geochemistry, geochronology, Kokchetav microcontinent
Pages: 471-481
Abstract
The Shalkar ophiolite complex consists of a series of tectonic sheets, including lavas, a complex of parallel dikes, layered complex of gabbro and gabbro-pyroxenites intruded by dikes of plagiogranites. The tectonic sheets are subsided northward and are associated with the base of a large tectonic nappe made up of rocks of the Kokchetav microcontinent. The microcontinent and ophiolites are jointly thrust upon the Kokchetav megamelange complex with HP-UHP rocks. It is inferred from the chemical composition and structural position that the Shalkar ophiolites were formed under the conditions of suprasubduction zone of extension and belong to 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 can be due to a jump of 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.
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