Genesis of dykes and mantle-crust interaction in the Keregetas swarm, Eastern Kazakhstan

Abstract

Post-orogenic dyke swarms have occurred in many orogens within the CAOB. The dyke swarms that contains various rocks can provide information about the process of mantle magmas differentiation during uplift and interaction with crustal material at different levels. The Keregetas dyke swarm (Eastern Kazakhstan) contain more than 200 dykes of different compositions, intrusions of biotite granites and amphibole leucogranites in a comparatively small area. We present geochronology, petrography, mineralogy, major and trace element chemistry and Nd isotopic data. U-Pb (zircon) and Ar-Ar (amphibole) geochronological data indicate that all rocks were emplaced synchronously in the Early Permian (292–282 Ma). Detailed study of the mineralogical and geochemical data suggests that the two processes determined the magmatic rock diversity within Keregetas swarm: differentiation of the parental mafic magma and melting of crustal sources at different levels. All rocks show positive ℇNd(T) values which confirms the role of upper mantle in their formation. A two-stage model for the formation of the Keregetas dyke swarm was proposed: (1) differentiation and assimilation of mafic magma and the formation of leucogranitic magma during partial melting in the lower crust, and (2) the formation of granodiorite and granite magmas as a result of partial melting in the middle crust. The geodynamic setting responsible for the formation of the Keregetas dyke swarm was the process of post-collisional lithosphere extension in the Early Permian, occurring against the backdrop of a thermal anomaly in the upper mantle caused by the activity of the Tarim mantle plume.