Abstract
Carbonatites play a key role in mantle metasomatism and host industrially critical mineral deposits of rare-earth-elements (REE). However, the evolution of carbonatite magmas and their relation with REE enrichment in the crust remain unclear. Here, we report a multi-modal evolutionary sequence of Zhuozi carbonatites in Paleoproterozoic orogenic belt, North China Craton. These rocks are closely symbiotic in field and exhibit similar Sr-Nd isotopes (87Sr/86Srt = 0.70340 – 0.70558, εNdt = -2.8 – 2.0), indicating a common mantle source. Carbonatite-I is represented by calcio-carbonatite whose calcite shows negligible Pb anomalies, suggesting minor crustal contamination. It underwent fractional crystallization to form dolomite-rich carbonatite-II with amounts of crustal quartz contamination. This rock has high whole-rock REE abundances and high abundance of REE minerals, but its calcite has low REE contents. Carbonatite-II has sediment-like C-O isotopes (−1.7 to −0.3 ‰, 15.1 – 19.5 ‰, respectively) similar to carbonatite-I. Both rocks originated from metasomatic mantle sources by Paleoproterozoic subducted slab. Carbonatite-III and IV show mantle-like δ13CPDB (−5.2 – −5.0 ‰ and −5.2 – −4.9 ‰) and high δ18OV-SMOW (13.2 – 15.1 ‰ and 14.1 – 15.8 ‰) values. They are characterized by silicate assemblages of diopside and tremolite, suggesting that the carbonatites are residual melts following interaction with wall rocks. This resulted in CO2 degassing and decreasing δ13C. REE in carbonatite-III are primarily hosted by calcite rather than by REE minerals. Carbonatite-IV is characterized by occurrence of quartz and REE minerals. It lacks diopside and has calcite with low REE contents, indicating that carbonatite-IV formed at relatively lower temperature than carbonatite-III. These rocks record successive fractional crystallization and interaction with crust. Carbonatite-I and carbonatite-II represent the continuous evolution of initial magma, with REE enrichment by fractionation of calcite in later carbonatite-II. Relatively low temperature suppressed the partitioning of REE into calcite, and the addition of silicon promoted REE mineralization in carbonatite-II. REE mineralization in carbonatite-IV resulted from silica assimilation, which enhanced the mobility of REE in the residual melts. Therefore, fractional crystallization and drop in temperature of primary carbonatite magmas and their interaction with wall-rocks promoted REE enrichment and deposition in the crust.