The influence of carbonated components on the petrogenesis of the Abaga basalts, Xilin Gol League (Inner Mongolia, China)

The Abaga volcanic field of Inner Mongolia, northern China, is situated west of the Daxinganling-Taihangshan gravity lineament and within the eastern part of the Central Asian orogenic belt (the Xing-Meng orogenic belt). This volcanic field consists of alkali basalts and tholeiites. Previous studies reported eruption ages (14.57−2.55 Ma; Luo and Chen, 1990; Ho et al., 2008; Zhang et al., 2016; Wu et al., 2023) based solely on exposed samples. The range of eruption ages and the chronostratigraphic stages of the study area, based on drill core and outcrop samples, have not been deciphered until now. In addition, the petrogenesis of the tholeiites remains poorly understood. In this study, we present Ar-Ar ages, major and trace element data, and Sr-Nd-Pb-Hf-Mg isotopic values for drill core and outcrop samples from the Abaga basalts. These data are used to identify the eruption stages of this volcanic field, determine whether sedimentary carbonates were present in the magma source, and discuss the genesis of both the alkali basalts and the tholeiites. Based on Ar-Ar dating results and the geological features of the basalts, four eruption stages have been identified. The earliest effusive eruption dates back to 21.38 Ma, which is ∼6.81 m.y. older than previously reported ages. In addition, the Abaga alkali basalts display concentrations of TiO2 (2.57−3.58 wt%) and Na2O + K2O (4.05%−6.36%), along with CaO/Al2O3 (0.58−0.95), Hf/Hf* (0.46−0.83), and Zr/Hf (41.24−47.23) ratios. Their δ26Mg (−0.43‰ to −0.31‰) and δ66Zn (0.47‰−0.50‰; Liu et al., 2022) values are similar to those of carbonated basalts, demonstrating that their magma source was metasomatized by recycled carbonates. The modeling results for δ26Mg and (Zn/Fe) × 104 values illustrate that <10% carbonate was added to the mantle source. Compared to the alkali basalts, the tholeiites display higher Hf/Hf* values (0.78−0.89) and δ26Mg ratios (−0.35‰ to −0.24‰) and lower TiO2 abundances (2.50%−2.91%), CaO/Al2O3 values (0.68−0.85), and Zr/Hf ratios (38.57−42.08). These characteristics suggest that the tholeiites may have been generated by interactions between alkali basalts and the lithosphere. The two-end-member mixing calculation using 143Nd/144Nd and δ26Mg values demonstrates that the addition of 20%−60% lithospheric melt to the alkali basalt can account for the petrogenesis of the tholeiites. Subduction of the Pacific plate likely contributed to the addition of carbonate to the mantle source. In conclusion, these newly reported data provide new insights into the petrogenesis of the basalts.