Early Carboniferous mafic dike–syenitic granite association in the Atengtao Mountain, Yili Block (NW China): Geochronology, petrogenesis, and tectonic significance

Abstract

The Carboniferous is an important period for understanding tectonic evolutionary history and subduction processes of Chinese western Tianshan orogen. Here, we present results of petrologic, geochemical, and in situ LA-ICP-MS zircon U-Pb and Hf isotope data from an Early Carboniferous mafic dike-syenitic granite association in Atengtao Mountain, Yili Block, aiming to constrain their ages and petrogenesis and tectonic significance. Zircon U-Pb dating results reveal that the mafic dikes and syenitic granites were coevally emplaced at ca. 345-349 Ma. Geochemically, the mafic dikes contain high MgO, Nb, TiO 2 , and Ta contents, low SiO 2 contents, and high Na 2 O/K 2 O ratios, showing Nb-enriched affinities. Additionally, they are enriched in LILEs, relatively depleted in HFSEs, and positive εHf (t) values (+1.2 to +8.7), likely representing melts derived from partial melting of a depleted mantle wedge metasomatized by subducted slab-derived fluids, with minor involvement of asthenospheric components. Compared with mafic dikes, the coeval syenitic granites display calc-alkaline I-type features and have high SiO 2 , Al 2 O 3 , and low MgO, Mg # (<40), Ni, and Cr contents, considered to have been derived from the reworking of the Precambrian basementwith an input of mantle-derived components. These mafic dike-syenitic granite associations likely formed in an arc-related setting rather than postcollision or mantle plume setting. In addition, pronouncedly higher zircon saturation temperature of the syenitic granites, compared with the coeval I-type granitoids from western Tianshan, imply the occurrence of a thermal anomaly in this period (ca. 345 Ma). With these studied results, together with previously published data, we argue that a slab breakoff model was likely responsible for the mafic dike-syenitic granite association, and related rocks, in the western Yili Block during Early Carboniferous.