Geochronology and Sr–Nd–Hf isotopic composition of the granites, enclaves, and dikes in the Karamay area, NW China: Insights into late Carboniferous crustal growth of West Junggar

• The Karamay granites and dikes were emplaced at ∼300 Ma.
• Geochemical and isotopic studies of granites and their enclaves indicate a crust- and mantle-derived magma mixing process.
• The dikes were derived from partial melting of an enriched lithospheric mantle.
• Ridge subduction in an island arc environment is supported.

New whole-rock major and trace elements, and zircon U–Pb and Hf–Nd isotope compositions are reported for the Karamay dikes, enclaves, and host granites in the West Junggar, NW China. Zircon U–Pb dating of the Karamay pluton yields an age of 300.7 ± 2.3 Ma for the enclave and 300.0 ± 2.6 Ma for the host granite, which was intruded by dike with an age of 298 Ma. The host granites exhibit relatively low SiO2 contents and A/CNK and Ga/Al ratios, low initial 87Sr/86Sr ratios (0.703421–0.703526) and positive εHf(t) (5.5–14.1) and εNd(t) (7.3–8.1) values with a young model age, suggesting that they are I-type granites and were mainly derived from a juvenile lower crustal source. The enclaves and dikes belong to an andesitic calc-alkaline series and have high MgO concentrations at low silica content and positive εHf(t) (7.6–13.2, 14.2–14.9) and εNd(t) (6.8–8.3, ∼6.9) values. They are enriched in LILEs (Rb, Ba and U) and LREE and depleted in HFSEs (Nb and Ta) with insignificant negative Eu anomalies, indicating that the melts were derived from an enriched lithospheric mantle modified by subducted oceanic crust-derived melts and minor fluids, followed by fractional crystallization. The Karamay host granites and enclaves are of mixed origin and are most probably formed by the interaction between the lower crust- and lithospheric mantle-derived magmas, and were intruded by the unmixed dikes subsequently. The upwelling mantle through a slab window in an island arc environment might have triggered partial melting of the lithospheric mantle and its subsequent interaction with the granitic magma, further suggesting that the ridge subduction played an important role in the crustal growth of West Junggar.

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