Genetic Relationship Between Carbonatities-lamprophyres In Maoniuping,western Sichuan: Li-Ca Isotopic Evidence

The Maoniuping deposit is the third largest carbonatite-type rare earth deposit in the world.the deposit genesis is closely related to the carbonatite-alkaline complex.Previous studies on the deposit have mainly focused on carbonatites-syenites and have made certain research results,but there are still controversies on the metasomatic material composition and rock genesis in the source area.Previous research shows that lamprophyres are closely related to gold mineralization.As a rare earth elements deposits,the genetic relationship between carbonatites and lamprophyres,lamprophyres and rare earth mineralization in Maoniuping is rarely reported.In this work,we take carbonatites-lamprophyres as an entry point,analyzing the isotopic composition of Li and Ca,combined with the analysis of major and trace elements and Pb-Sr-Nd isotopes to investigate the source area characteristics,petrogenesis and the relationship between rock genesis and rare earth mineralization.The carbonatites,syenites and lamprophyres of the Maoniuping deposit are enriched in large ionic lithophile elements,light rare earth elements and lack high field strength elements,with geochemical characteristics of subduction environment.In addition,The Pb-Sr-Nd isotopes of the three types of rocks we collected are located between the EMⅠand EMⅡ,with high(⁸⁷Sr/⁸⁶Sr)_iand lowεNd_（t）,indicating that the rock’s originate from the metasomatic enriched mantle.Relevant studies show that Li and Ca isotopes can be a good tracer of the source region characteristics of rocks,while Ca isotopes in mantle derived rocks have the potential to trace recyclic carbonate.The carbonatites,syenites and lamprophyres of the Maoniuping have similar Li isotopic compositions as the fresh mid-ocean ridge basalts,the averageδ⁷Li values of the three types of rocks collected are3.0‰,2.3‰,and 2.1‰,respectively.Of which,carbonatites and lamprophyres have significantly lower Ca isotopic compositions,The averageδ^44/42Ca values of 0.30±0.01‰and 0.31±0.01‰,respectively.The Pb-Sr-Li isotopic simulation of the three types of rocks shows that the metasomatism of their source areas may be closely related to marine sediments and altered oceanic crust.However,Sr-Nd-Ca isotopes of carbonatites and lamprophyres further show that the source area has the addition of about 10%carbonate-rich marine sediments.The rare-earth elements distribution form,Pb-Sr-Nd isotopic composition and diagenetic age of the three types of rocks are not obvious different,indicating that they originated from the same parent magma.The higher La/Yb ratio,lower La content,the right dipping REE distribution model show that the three types of rocks are produced by partial melting of the mantle.Compared with carbonatites and syenites,lamprophyres has significantly higher contents of Mg O,Ni and Cr,which is closer to the characteristics of mantle primitive magma,suggesting that the magma has undergone fractional crystallization.The characteristics of rare earth and trace elements consistent with the liquid immiscibility model of carbonate-silicate indicate that carbonatites and syenites are controlled by liquid immiscibility in the later stage.In the process of rare earth mineralization,the magmatism of the three types of rocks plays an important role in providing ore-forming fluids and materials.The mantle source area,fractional crystallization and magmatic immiscibility may all have an important impact on the enrichment of rare earth in the deposit.