In Situ Chemical And Isotopic Compositions Of The Tiechagou Carbonatites In Lesser Qinling Orogenic Belt:Implications Of The Petrogenesis And Fe Mineralization

China is blessed with vast deposits of carbonatites,among them the known Lesser Qinling in North China Craton,which is known for its huge deposit of REE,Mo,Fe and other metals.Tiechagou located in Lesser Qinling is a Fe-related carbonatite and its age is estimated at around 220 Ma.It is particularly special in its uncommon association of calcitic and dolomitic carbonatite.In such a case,it is crucial to understand the evolution of carbonatite magma.Till now,the genesis of both calcite and dolomite in Tiechagou carbonatite remains unknown.Magnetite-ilmenite is the main iron-rich mineral in the carbonatite that shows an interesting intergrowth,but the mineralization mechanism of iron in this deposit has not been clearly determined.Magnetite is another essential mineral in Tiechagou carbonatite and is used in this study to constrain the evolution of Fe-Ti oxides.Ilmenite oxy-exsolution in magnetite recording different textures gives information on the Fe-Ti oxides cooling history.A careful study of different textures in Tiechagou carbonatites was conducted using optical microscopy and scanning electron microscope.In-situ major,minor and trace elements of carbonates（calcite and dolomite）,silicates（olivine and clinopyroxene）and Fe-Ti oxides（magnetite-ilmenite）were carried out using EMPA and LA-ICP-MS,and in-situ Sr and Mg isotopes of calcite and dolomite were conducted using LA-MC-ICP-MS.The mineralogical characteristics showed that calcio-carbonatites contain mainly calcite associated with olivine and magnetite,whereas magnesio-carbonatites form a massive body and are sometimes associated with magmatic magnetite.Clinopyroxene is mainly found in the carbonatite-wall rock contact zone and associated with apatite and biotite.Minor and trace elements of calcite and dolomite showed enrichment of Sr and LREE（La,Ce,Pr,Sr,Nd and Sm）but depletion in some of HFSE like,Nb,Zr,Hf,Ta,Th and U.Geochemical data of olivine showed low content of Ni and Co and higher Mn relative to magmatic olivine,which may indicate the cause of alteration.The major element of clinopyroxene proved its diopside compositions(Wo_49.5-En_49.2-Fs_1.3)with Mg#varying from 87.1 to 97.7.In situ isotopic compositions gave ⁸⁷Sr/⁸⁶Sr ratios varying from0.70493 to 0.70638 for calcite,whereas the dolomite showed different variations（0.70240to 0.70397）indicating that they are from different sources.Theδ²⁶Mg isotope of dolomite varies from-0.78‰to-0.52‰.The lightδ²⁶Mg isotope of dolomite which is lower relative to the normal mantle implies recycled sediments,and the modeling successfully showed that 10%of recycled sedimentary carbonates were involved in Tiechagou dolomitic melt.Calcio-carbonatite resulted from the interaction between dolomitic melt and wall rock.Based on petrography,the magmatic association has been identified as thin lamellae（Type1）,thick lamellae（Type2）,fine-grained granular（Type3）and coarse-grained granular oxy-exsolutions（Type4）.Type5 is the intergrowth of magnetite and coarse-grained granular ilmenite,whereas Type6 consist of disseminated subhedral and anhedral magnetite grains mainly associated with serpentine,olivine,calcite,and biotite.Bulk（magnetite-ilmenite intergrowth）Mg O and Ti O₂ generally decrease from Type1 to Type2and Type3 to Type4.Type5 and Type6 are in general characterized with relatively low content of Ti O₂compared to the magmatic types.Bulk minor-and trace-element concentrations of all magmatic magnetite-ilmenite intergrowth exhibit similar trends in continental crust-normalized diagrams.Different magnetite and ilmenite genesis are characterized by variable minor and trace element compositions,and have been successfully discriminated using Principal Component Analysis with the most discriminant elements of V,Cr,Ni,Co,Zn,Mn and Mg.Temperature and oxygen fugacity have been determined using Win MIgob,software developed by Yavuz（2021）.Textural and chemical features of magnetite and ilmenite in the Tiechagou carbonatite combined with their re-equilibration temperature and oxygen fugacity highlight their evolution history,which is presented into two petrogenetic models.The first model records a significant decrease in temperature from 683 to 338°C and oxygen fugacity（logf O₂）from-14.58 to-34.27,which results in a progressive evolution of ilmenite oxy-exsolution from thin to thick lamellae.The second records a continuing evolution of the magnetite-ilmenite intergrowths from fine-grained to coarse-grained granular oxy-exsolutions through subsolvus oxidation,which is restrained by increase of the average oxygen fugacity（logf O₂）from-28.38 to-26.93 at an intermediate temperature of 500-400°C.Carbon in the oxidized form probably plays a significant role as the oxidizing agent especially when the oxygen fugacity is decreasing and/or low in carbonatite systems.Geothermo-oxybarometric data helped to trace the Fe-Ti oxides evolution of Tiechagou carbonatites,which was elaborated in two models.In the first model,titanomagnetite evolves to magnetite hosting thin and thick lamellae ilmenite due to a progressive decrease in temperature and oxygen fugacity.In the second model,titanomagnetite evolves to magnetite hosting fine-and coarse-grained granular ilmenite oxy-exsolutions controlled by increasing oxygen fugacity with slightly decreasing temperature.To sum up,Tiechagou magnesio-carbonatite resulted from the mantle being recycled with sedimentary carbonates,while calcio-carbonatite was the result of the interaction between dolomitic magma with the wall-rock to give an assemblage of calcite,olivine and magnetite（later generations of magnetite,e.g.,mag5 and mag6）.Earlier generations of magnetite（e.g.,mag1,mag2,mag3 and mag4）crystallized within magnesio-carbonatite.Temperature and oxygen fugacity are the key control factors in the formation of various magnetite-ilmenite oxy-exsolutions in various Fe-Ti oxides deposits.