Petrogenesis and tectonic implications of the Early Carboniferous high‐K granites with enriched isotope signatures in the northern North China Craton

Abstract

Although an extensive continental arc magmatism was recorded in the northern North China Craton (NCC) during the Carboniferous, a general geodynamic model has not been constructed, and associated crust–mantle processes are not fully understood. Here, we carried out systematic geochronological and geochemical investigations on the alkali‐feldspar granite pluton in the Zhaojinggou area, northern NCC. U–Pb dating of high‐U–Th zircons failed to obtain concordant ages due to severe metamictization, while apatite U–Pb dating yielded a perfect age of 327.6 ± 4.1 Ma, indicating the emplacement of granites in the Early Carboniferous. The granites have high SiO2 (74.99–76.88 wt%) and K2O (5.73–6.43 wt%) contents and exhibit enriched LREEs, K, Rb, Th, U, and Pb, and depleted Nb, Ta, P, and Ti contents, which are typical of highly fractionated, I‐type high‐K granites formed under a continental arc setting. Moreover, the high‐K granites show highly enriched Nd isotopic compositions (with whole‐rock and apatite εNd(t) of −33.6 to −16.3) that are distinctly lower than other Carboniferous continental arc rocks in the northern NCC (εNd(t) from −18.5 to −9.9). These geochemical characteristics indicate that the high‐K granites possibly originated by partial melting of the Archean TTG gneisses and mafic rocks in the lower crust. Involvement of mantle‐derived fluids is crucial to induce the low‐temperature melting, and sufficient fractional crystallization was reached during subsequent magmatic evolution. Previous studies suggest that the Carboniferous continental arc rocks in the northern NCC have mantle‐derived or crust–mantle mixed sources. Our study, however, indicates that there also exist coeval crust‐derived magmatic rocks in this area. In combination with previous work and this study, we support the hypothesis that the widespread Carboniferous continental arc magmatism in the northern NCC is related to the southward subduction of the Palaeo‐Asian Ocean (PAO), and the different magma sources and extensive crust–mantle interactions are responsible for the formation of diverse types of magmatic rocks with contrasting isotopic compositions. We also draw attention to the fact that phosphate minerals such as apatite may be a preferable chronometer relative to high‐U–Th zircon in a highly fractionated igneous system.