ABAI SYENITE MASSIF, EASTERN KAZAKHSTAN: AGE, FORMATION MODEL AND GEODYNAMIC SIGNIFICANCE

Studies have been made of the Abai alkaline rocks massif, located in the northeastern part of the ChingizTarbagatai zone of Eastern Kazakhstan. The Chingiz-Tarbagatai zone of Eastern Kazakhstan is located in the western part of the Central Asian Orogenic Belt (CAOB), the lithospheric evolution of which continued during the Paleozoic and was associated with the basin closure in the system of the Paleoasian Ocean. The 9 km² massif is characterized by an isometric shape with a clearly defined concentric zonal structure and represents a multiphase intrusion composed of several varieties of syenite. Potassium feldspar is the most prevalent mineral in syenite, while plagioclase, pyroxene (5 to 15 vol. %), and amphibole (5 to 20 vol. %) are less common. Most of the massif is composed of nepheline syenite rocks containing 5–10 vol. % of nepheline. Accessory minerals include apatite, zircon, and ilmenite. Studies of mineral composition and geochemical characteristics of the rocks revealed that the syenites formed during the evolution of a single magma melt composed of alkali-syenite or monzonite. This magma was formed probably as a result of primary alkaline-mafic magma differentiation. The U-Pb LA-ICP-MS dating of magmatic zircon grains for the first time yielded a relibale rock age estimate of 401–398 Ma, which corresponds to the Emsian stage of the Early Devonian. This refutes the previously accepted ideas about the Early Permian age of the massif and its intraplate geodynamic nature. When the data on the composition and age of the massif are compared with the data on the geological evolution of the region, it is apparent that the Abai syenite massif formation is related to the extensional processes in response to subduction of the Junggar-Balkhash oceanic lithosphere underneath the Chingiz-Tarbagatai zone.

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