Liquid immiscibility acting on the formation of the Miaoya carbonatite-syenite complex in the South Qinling Belt, Central China

Abstract

As one of the largest carbonatite-syenite complexes enriched in rare earth elements (REE) in China, the magma-hydrothermal processes of the Miaoya carbonatite-syenite complex (MCSC) are of great significance with respect to the enrichment of REE, and have therefore attracted much attention. Although the hydrothermal mineralization process of the Miaoya carbonatite-associated REE deposits has been well addressed, studies concerning its detailed magmatic process remain relatively limited. Here, we present new zircon U-Pb age, whole-rock major and trace element, and Sr-Nd-Pb isotope composition data of the MCSC, to help provide a comprehensive understanding of its source characteristics and related liquid immiscibility process. The Miaoya syenites studied here formed at 410.8 ± 1.9 Ma, consistent with the formation ages of alkaline rocks (377–455 Ma) in the South Qinling Belt. Both samples in the MCSC share homogeneous Sr-Nd-Pb isotopes (i.e., carbonatites: 87Sr/86Sr(i) = 0.70361–0.70366, εNd(t) = +2.6–+2.8, 206Pb/204Pb = 17.81–18.13, 207Pb/204Pb = 15.51–15.53, and 208Pb/204Pb = 37.67–38.27; syenites: 87Sr/86Sr(i) = 0.70286–0.70524, εNd(t) = +2.0–+2.6, 206Pb/204Pb = 18.33–18.83, 207Pb/204Pb = 15.55–15.57, and 208Pb/204Pb = 37.88–38.38), suggesting a common enriched mantle source. The plagioclase identified in the carbonatites and the calcite observed in the syenites fully support the interaction between carbonate and silicate melts during the liquid immiscibility process. Similar to the liquid immiscibility experimental results, the Miaoya carbonatites are more enriched in Ca, Mn, Sr, and REE than the syenites, but are more depleted in Na, K, Al, Si, and high field strength elements, such as Nb, Ta, Zr, and Hf, further demonstrating that liquid immiscibility provided a crucial contribution to the formation of the MCSC. In combination with typical REE-rich carbonatite-alkaline complexes (e.g., the Mianning-Dechang carbonatite-associated REE deposit belt of the western Yangtze Block, South China, and the Mountain Pass carbonatite-associated REE deposits of California, USA), we propose that the enriched mantle and the liquid immiscibility are significant for the REE enrichment of carbonatite-alkaline complexes.