The Early Paleozoic Alkaline-carbonatite Magmatism From The South Qinling And The Relevant Nb-REE Mineralization Mechanism

Niobium and rare earth elements（REE）play a critical role in modern industries,which mainly come from carbonatite related deposit.Several processes have been proposed to be responsible for rare-element mineralization in carbonatites,including liquid immiscibility,fractional crystallization,and hydrothermal processes,but the mechanism that leads to ore-grade enrichments of Nb and REE in carbonatite is still in a matter of debate.Alkaline rocks,including alkaline volcanic rocks and carbonatite-silicate intrusive complexes,are widely distributed within the South Qinling Belt,Central China,and some of them contain low grade REE-Nb mineralization.However,there are still some debates on the main Nb and REE mineralization stage and characteristics of the mantle source for these alkaline rocks.In this study,Miaoya and Shaxiongdong alkaline-carbonatite complexes and Tianbao alkaline volcanic rocks are selected to evaluate above problems.Here,we present a comprehensive study of geology,geochronology,geochemistry,and mineralogy of the three alkaline rocks.Our new dataset allows us to propose a new genetic model for the origin of carbonatite and alkaline and the enrichment of REE and Nb in these rocks.Miaoya complex consists of syenite,calcite carbonatite,and ankerite carbonatite.Shaxiongdong complex consists of meta-pyroxenite,syenite,and carbonatite,and fenitization occurs within the meta-pyroxenite and syenite result from reaction with alkali fluids exsolved from carbonatite magma.The Tianbao alkali volcanics are divided into basaltic tuff,alkali basalt,and trachytic rocks,and basaltic rocks always occur as basal units.U-Th-Pb dating of zircon and titanite in different rocks suggest they formed at～430-440 Ma.Thus,these magmatic rocks were likely related to a continental rifting event that occurred in the northern part of the Yangtze Block during the opening of Mianlue ocean in the Paleozoic.Tianbao rocks exhibit enrichment of LREE,Th,U,Nb,Ta,Zr,and Hf,albeit with negative anomalies of K,Pb,and Sr on the primitive mantle-normalized trace element spidergram.Tianbao alkaline basalt and trachyte have similar Sr-Nd-Pb isotope compositions,but SiO₂ content shows a slightly positive correlation with(⁸⁷Sr/⁸⁶Sr)_i and negative correlation withεNd（t）,indicating that crustal contamination was involved in the evolution of at least the more siliceous magmas.MELTS and AFC modeling reveals that trachyte can evolve from the alkali basalt with<5%contamination of local metasedimentary or felsic rocks.The batch melting modelling indicates that the alkali basalt could be formed via partial melting of amphibole-bearing garnet peridotite.The initial Sr-Nd-Pb isotopes of these volcanics as well as Miaoya and Shaxiongdong complexes are consistent with those of global ocean island basalts,with mixed components of depleted MORB,HIMU and EMI type mantle sources,which require recycled crustal materials into their mantle source.Therefore,alkaline rocks in South Qinling were generated by partial melting of amphibole-bearing mantle reservoir,which likely formed by previous subduction event.In Miaoya and Shaxiongdong,carbonatite and associated silicate rocks have nearly identical Sr–Nd isotopes,implying that they were derived from a common source of carbonated silicate magmas.Whole-rock geochemistry and fractionation modeling suggest the syenites evolve from a parental alkali basalt magma by separation of cumulate pyroxenite in the complex.Trace elements,such as Nb,Zr,and REE will be concentrated in the residual melts during above fractional crystallization processes.Carbonatites and associated silicate rocks are distinct in their major element compositions and mineral assemblages,which is difficult to explain by fractional crystallization.Thus,we propose that the carbonatite melts separated from carbonated silicate melts when the latter reached saturation in carbonate phases after extensive crystallization of silicate minerals at the late stage of syenite.Carbonatites always have the highest REE content in both Miaoya and Shaxiongdong complexes,suggesting REE are obviously compatible in the unmixing carbonatite.In conclusion,fractionation of the parental carbonated silicate magmas will lead to Nb and REE enrichment in the residual melts and finally lead to the unmixing of REE enriched carbonatites.Then,we evaluate how carbonatite evolution effect Nb and REE mineralization through mineralogy studies of carbonate and Nb-REE minerals.Firstly,it should be noted that syenite and trachyte also host some Nb ores.For example,Miaoya syenite contain columbite,Nb-rutile,and betafite and Tianbao trachyte contain columbite,Nb-rutile,and aeschynite.Generally,calcite crystallized at the early stage in carbonatites,while ankerite formed at the late stage.Sr-rich calcite also crystallized in Shaxiongdong complex after crystallization of earlier calcite.It suggests that carbonatite magmas become more enriched in Fe-Mg-Mn±Sr during the fractionation.Nb minerals are mainly found in early calcite carbonatite,suggesting their magmatic origin.Nb minerals in Miaoya includd columbite,Nb-rutile,and betafite,while Nb is mainly hosted in pyrochlore in Shaxiongdong.At this stage,trace burbankite also crystallized as inclusions within calcite in Shaxiongdong.However,REE minerals crystallized mainly at the Fe-Mg-Mn±Sr stage,which should be equivalent to the salt melt stage distinguished by previous studies.REE minerals in Miaoya complex mainly include allanite-（Ce）,monazite-（Ce）,parisite-（Ce）and bastn（?）site-（Ce）.In Shaxiongdong complex,REE minerals include ancylite-（Ce）,monazite-（Ce）,carbocernaite,allanite-（Ce）,bastn（?）site-（Ce）as well as trace parisite-（Ce）.In conclusion,in the South Qinling Belt,Nb minerals are mainly of magmatic origin,while REE mineralization mainly occurs at the salt-melt stage of carbonatite evolution.In this study,detailed textural observations together with in situ elemental and U-Th-Pb and Nd isotopic analyses on phosphates unravel the regional Triassic metamorphic overprint on the Silurian carbonatite-alkaline complexes.Apatite grains from Miaoya and Shaxiongdong complexes commonly show typical replacement textures related to fluid metasomatism,due to coupled dissolution-reprecipitation.The altered apatite domains,which contain abundant monazite-（Ce）inclusions or are locally surrounded by fine-grained monazite-（Ce）,have average REE concentrations lower than primary apatite.A second type of monazite-（Ce）,not spatially associated with apatite,shows porous textures and zoning under BSE imaging.Spot analyses of these monazite-（Ce）grains have variable U-Th-Pb ages of 210-410 Ma and show a peak age of 230 Ma,which is significantly younger than the emplacement age（440-430 Ma）but are roughly synchronous with a regionally metamorphic event related to the collision between the North China Block and Yangtze Block along the Mianlue suture.We suggest these monazite-（Ce）grains crystallized from the early Silurian carbonatites and have been partially or fully modified during a Triassic metamorphic event,partially resetting U-Pb ages over a wide range.Mass-balance calculations,based on mass proportions and the REE contents of monazite-（Ce）and apatite,demonstrate that the quantity of metasomatized early Silurian monazite-（Ce）is far higher than the proportion of monazite-（Ce）resulting from the metasomatic alteration of the apatite.Therefore,Triassic metamorphic events largely reset the U-Th-Pb isotopic system of the primary monazite-（Ce）and apatite,but only had limited or local effects on REE remobilization in the carbonatite-alkaline complexes in the South Qinling Belt.To conclude,alkaline and carbonatite rocks in the South Qinling Belt were generated by partial melting of amphibole-bearing mantle reservoir,which was metasomatized by Neoproterozoic subducted oceanic crust.In the early Paleozoic,the opening of Mianlue Ocean resulted in partial melting of the metasomatized mantle and the formation of CO₂-rich alkaline basaltic magma,which later evolved into various rocks.Then,the magmatic evolution processes,such as fractional crystallization and liquid immiscibility,will lead to gradual enrichment and final mineralization of Nb and REE.Niobium mineralization occurs mainly in late-stage silicate rocks such as syenite and trachyte and early calcite carbonatite,suggesting a magmatic origin.REE mineralization mainly occurs in the late stage of carbonatite evolution,namely Fe-Mg-Mn-Sr-REE rich salt-melt stage.Then,the Silurian carbonatite-alkaline complexes were overprinted by the regional Triassic metamorphic events,which largely reset the U-Th-Pb isotopic system of the primary monazite-（Ce）but only had limited effects on REE remobilization or mineralization.