Late Early Carboniferous Bimodal Magmatism Along the Northern Margin of the North China Craton and Its Tectonic Significance

ABSTRACT

During the Late Palaeozoic, southward subduction of the Paleo‐Asian Ocean (PAO) generated widespread magmatism along the northern margin of the North China Craton (NCC). In the Chahar Right Rear Banner–Shangdu area, located in the central segment of this margin, a late Early Carboniferous bimodal assemblage comprising hornblende gabbro and granite porphyry has been identified. Integrated field observations, petrography, whole‐rock geochemistry, zircon LA‐ICP‐MS U–Pb dating, and zircon Hf isotopes were used to constrain the emplacement age, magma sources and tectonic significance of this assemblage. Hornblende gabbro yields zircon U–Pb ages of 320 ± 3.5 and 319 ± 1.9 Ma. It is characterised by low SiO ₂ , low‐K tholeiitic affinity, high Mg ^# , enrichment in LREEs and LILEs, depletion in HFSEs and strongly negative zircon ε _Hf ( t ) values. These features indicate derivation from a lithospheric mantle source metasomatised by subduction‐related fluids. Granite porphyry yields a zircon U–Pb age of 326 ± 5.9 Ma. It shows high SiO ₂ , strong peraluminosity, enrichment in LILEs, depletion in HFSEs, pronounced negative Eu anomalies, tetrad REE patterns and negative zircon ε _Hf ( t ) values. These features suggest that it is an aluminous A‐type granite derived mainly from partial melting of the ancient lower crust, probably triggered by mafic underplating. Combined with regional Carboniferous magmatic data, this bimodal magmatism is interpreted to be consistent with a subduction‐related extensional setting along the northern NCC margin, and may have involved localised back‐arc extension associated with slab rollback.