Petrogenesis And Nb-Ta Mineralization Of The Neoproterozoic Fangcheng Alkaline Magmatic Complex,East Qinling Orogen

As a strategic rare metal element,niobium plays an increasingly crucial role in the modern cutting-edge industries.A detailed study with petrology,whole-rock geochemistry,zircon U-Pb geochronology,Nd-Pb-Hf isotope and mineral chemistry was carried out for the Fangcheng Nb-enriched alkaline complex in the East Qinling Orogen,aiming to probe into its petrogenesis,tectonic implication and Nb-Ta mineralization.The Fangcheng alkaline complex consists of concomitant silica-undersaturated nepheline syenite,silica-saturated biotite syenite and aegirine-augite syenite as well as silica-oversaturated quartz syenite and syenogranite.The first three are marked by the presence or enrichment of nepheline,biotite and aegirine-augite,respectively.The key to the latter two is the occurrence and enrichment of quartz.The Fangcheng alkaline rocks are intermediate-acid?SiO₂=57-72 wt%?and are characterized by enrichment in large ion lithophile element?LILE?,light rare earth element?LREE?and high field-strength element?e.g.,Nb,Ta,Zr,Hf?as well as significant depletion of Ba,Sr,P,Ti and Eu.From the nepheline syenite to the syenogranite,the rocks changed from sodium to potassium,the silicon content increased significantly,the Al₂O₃ and CaO content decreased,and show an escalating trend in magma evolution.Whole-rock Nd and Pb isotope(?Nd?t?=-0.69^-4.87;²⁰⁶Pb/²⁰⁴Pb=16.186-17.760,²⁰⁷Pb/²⁰⁴Pb=15.164-15.524,²⁰⁸Pb/²⁰⁴Pb=23.395-37.612)of the Fangcheng alkaline complex are similar to the adjacent mafic rocks,indicating that the parental magma is originate from partial melting of an enriched lithospheric mantle.Their isotope are homogeneous but silica-oversaturated rocks are relatively more enriched(?Nd?t?=-4.50^-4.87;?Hf?t?=-4.19^-6.85;²⁰⁷Pb/²⁰⁴Pb=15.164¹5.409,²⁰⁸Pb/²⁰⁴Pb=23.395³1.809),suggesting that these rocks are product of cogenetic mantle-derived magma differentiation,but contaminated with some crustal materials during magma upwelling.Therefore,the primary alkaline magma is derived from low-degree of partial melting of enriched lithospheric mantle.The Na-rich minerals?nepheline?firstly crystallized to form the nepheline syenite with magma evolution,leading to gradual K enrichment in the residual melt.Subsequently,the residual melt evolved into the silica-oversaturated alkali granitoids combined with crustal contamination,thus finally forming a Fangcheng alkaline rock combination.SIMS and LA-ICP-MS zircon U-Pb dating yield the identical ages in variable rocks,namely quartz syenite?850-867 Ma?,syenogranite?857 Ma?,aegirine-augite syenite?862 Ma?and nepheline syenite?869 Ma?.The alkaline rocks are generally produced in extensional regime.Combined with geochemical and regional data,this paper suggest that the Fangcheng complex was formed in a post-collisional setting,implying that the orogeny of the East Qinling Orogen ended at ca.869 Ma and subsequently entered an extension regime.Accordingly,the exposure of Fangcheng complex could represent a preclude of the Rodinia supercontinent breakup.The Fangcheng alkaline rocks have high Nb contents,but nepheline syenite is the most.Whole-rock Nb content shows an obvious positive correlation with the F content,indicating that halogen F plays an important role in Nb enrichment.The Nb-rich minerals are mainly titanite and pyrochlore,and the former is widely found in each rock types,but the latter only occurs in nepheline syenite and biotite syenite.A small number of ilmenite and euxenite can be seen in the syenogranite.Based on petrography and mineral chemical features,titanite can be divided into magmatic and hydrothermal types,the latter of which are relatively more enriched in F and Nb?Nb₂O₅ up to 3.6 wt%?.Pyrochlore in nepheline syenite is mostly hydrothermal with obvious metasomatic textures,which the replaced region is higher in Nb?Nb₂O₅=55.5-61.5 wt%?.Two-stage zircon ages?869 Ma and 440 Ma?in nepheline syenite indicate that these alkaline rocks experienced at least twice Nb enrichment.In the early evolutionary stage?Neoproterozoic?,crystal fractionation of Nb-poor minerals led to an escalating F and Nb in residual melt?F effectively improves the solubility of Nb in magma?,and finally crystallized high Nb magmatic titanite.At the Early Paleozoic?second stage?,the Qinling Orogen suffered from large-scale magmatic and hydrothermal activities.In this orogenic event,these rocks,especially nepheline syenite,undergone intense hydrothermal modification.Thus,the previously formed niobium-bearing minerals are replaced by the Si and F-rich fluids along their fissures,causing them to collapse and leading to the residual fluid components to change.High concentrations of F induce niobium remobilization and migrate to form hydrothermal titanite and pyrochlore.