Episodic bimodal magmatism in Central Inner Mongolia, China: Insights from geochronological, geochemical, and Sr-Nd isotopic evidence

This study combines zircon U-Pb ages, whole-rock major and trace elements, and Sr-Nd isotope data to investigate the petrogenesis and tectonic implications of late Paleozoic magmatic rocks in central Inner Mongolia of the southeastern Central Asian Orogenic Belt. The results indicate the presence of two stages of bimodal magmatism from the late Carboniferous to early Permian. The late Carboniferous (ca. 323 Ma) bimodal associations consist mainly of gabbroic and granitic intrusive rocks, whereas the early Permian (ca. 273 Ma) bimodal suites are dominated by dolerite and granite porphyry dikes. The gabbroic intrusive rocks and dolerite dikes are calcalkaline, slightly enriched in light rare earth elements and depleted in Nb, Ta, Ti, and have low initial 87Sr/86Sr ratios (0.70317-0.70510) and high epsilon Nd(t) values (+3.1 to +7.4). They are both interpreted to result from partial melting of a depleted spinel-lherzolite mantle source that was metasomatized by subduction-related components. The granitic intrusive rocks show I-type granite affinity and similar geochemical features to coeval gabbroic rocks, suggesting they were derived from a melt-induced anatexis of basaltic lower crust. The early Permian granite porphyry dikes show characteristics of LREE enrichment, negative Nb-Ta anomalies and positive Zr-Hf anomalies. They have higher La/Yb and Th/Zr ratios and epsilon Nd(t) values, and lower initial 87Sr/86Sr ratios than the coeval dolerite dikes. These features suggest that the early Permian felsic dikes were most likely generated by partial melting of a juvenile lower crust mixing with minor mantle-derived components. An integrated study with regional geological data suggests that the episodic bimodal suites in central Inner Mongolia formed in a postcollisional to within-plate rift setting after the closure of the Paleo-Asian Ocean.