Crustal Magmatic Response And Evolution In The Inner Zone Of Emeishan Igneous Province:Constraints From The Late Permian Intermediateacid Intrusions In The Panxi Region

The intermediate-acid intrusions in the Panxi region,inner zone of the Emeishan large igneous provinces（ELIP）,record the accretion and anatexis of the crustal materials.The source characteristics of these intermediate-acid intrusion are great indicators to reveal the crustal material response and evolution at different depths in the western margin of the Yangtze Block.Based on the summaries and compilations of the previous researches in Panxi region,combined with the characteristics and evolution of magma source,the intermediate-acid intrusions in the inner zone of the ELIP can be divided into three stages:～262 Ma,～259 Ma and～255 Ma.The following achievements and understanding are obtained:（1）At～262 Ma,the eruption of early-stage Emeishan basalts effectively represents the onset of volcanism associated with the ELIP.The typical intrusions in this period are Lianhecun amphibole syenite with coeval mafic enclaves.They yield zircon U-Pb ages of262±0.8 Ma and 261±0.8 Ma,respectively.The mafic enclaves mainly consist of alkaline feldspar,plagioclase,clinopyroxene and amphibole.The rocks have moderate Si O₂（59.79～60.48 wt.%）contents and high Mg#values（48.2～51.1）,and are characterized by enrichment in LILEs without obvious anomalies in Nb,Ta,Zr,Hf.Combined with negative zirconε_Hf（t）（-5.2～-1.4）and whole-rockε_Nd（t）（-5～-7.25）values,the mafic enclaves were derived from partial melting of enriched lithosphere mantle.The mafic enclaves display high K₂O（6.38～6.94 wt.%）content and K₂O/Na₂O（1.83～2.00）ratios with low Rb/Sr and high Rb/Ba ratios,suggesting that the main LILEs-bearing mineral is amphibole in the mantle source.The liquidus temperature of the mafic enclaves is 1370℃calculated by P-Melts software under the condition of oxygen fugacity showing?FMQ-1.0.The Lianhecun amphibole syenites are mainly composed of alkaline feldspar,plagioclase,amphibole and biotite.The amphibole grains have relatively high Fe O and Al₂O₃contents,and the composition of amphibole is ferro-edenite,which is obviously different with the Mg-rich amphibole from mantle-derived magma.The amphibole syenites display moderate Si O₂（60.24～64.75 wt.%）contents,and are enriched in Th,U,Ra,and Ba,and depleted in middle rare earth element（MREE）,Na and Ta contents,indicating the lower crustal affinity.Considering their low Mg#values,and Cr and Ni contents,the heterogeneous zircon Hf composition(ε_Hf（t）=-3.7～+2.0)may be caused by disequilibrium partial melting process.The crystallization temperature and pressure of the amphibole syenitic melt in equilibrium with amphiboles were 721～759℃,2.91 to 4.43 kbar,respectively.We suggest that the upwelling of mantle plume,which provided the necessary heat and triggered the partial melting of the enriched lithospheric mantle,produced primary mafic magma.Then,the mafic magma underplated and caused partial melting of lower crustal material in the inner zone of the ELIP.The mafic magma injected into a felsic magma chamber,forming the mafic enclaves in the host amphibole syenites.The Lianhecun pluton records multi materials participated in the intermediate-acid magmatic evolution in the early stage of the magmatic activity of the ELIP,reflecting the heterogeneity of the deep materials in the western margin of the Yangtze at that time.（2）～259 Ma is the most intense period of magmatic activity in ELIP.Immense magma chambers and plumbing systems were formed due to increasing magma supply rate during decompression melting of the mantle plume.Basaltic magma underplated at the bottom of the crust in the inner zone of the ELIP,resulting in the thickening of the crust,while it leads to the remelting of juvenile crustal anatexis and forming of Baima fayalite syenite and Huangcao pyroxene syenite.The Baima fayalite syenites mainly consist of alkali feldspar,clinopyroxene and fayalite,and yielded zircon U-Pb ages of 259±1.0 Ma.The clinopyroxene and fayalite grains form Baima fayalite syenites display higher Fe O and lower Mg O content than those of the picrite and basalt in the ELIP.Baima fayalite syenites display moderate Si O₂（60.14～63.58 wt.%）,and high Fe₂O₃T（4.28～8.52 wt.%）and alkaline（Na₂O+K₂O=10.91～12.75 wt.%）contents with positive zirconε_Hf（t）values（+1.5～+12.9）.They display enriched light rare earth element（LREE）,slightly negative to obvious positive Eu anomalies（Eu/Eu*=0.86～1.83）,and are depleted in Th,U,Zr and Hf components.These characteristics are consistent with gabbro accumulation units at the bottom of the Baima layered intrusion.Combined with homogeneous isotope between them,we proposed that Baima fayalite syenites are originated from the remelting of Fe-rich refractory juvenile crustal materials in a high temperature（>1100°C）and H₂O-poor condition.Huangcao pyroxene syenite yielded zircon U-Pb ages of 259±1.0 Ma.The main rock assembles consist of alkali feldspar,clinopyroxene and amphibole.The rocks have positive zirconε_Hf（t）value（+3.3～+14.0）with single Hf model ages of 327～775 Ma,which is close to the zircon U-Pb,indicating that their source is juvenile crust.The pyroxene syenite have moderate Si O₂（61.89～63.13 wt.%）and low Mg O（0.80～1.06 wt.%）contents,as well as high Sr content and Sr/Y ratio,implying the residual amphibole and garnet in the source.According to the partial melting model,we suggest that the Huangcao pyroxene syenite were derived from the high degree（30～40%）partial melting of juvenile thickened lower crust in the inner zone of the ELIP.Then,the emplacement of these high-temperature syenitic magma further caused partial melting of the shallow crustal materials,and forming a small-scale Baima A-type granite,which yield zircon U-Pb ages of 258.7±1.0 Ma.The granites have Si O₂（72.10～72.98 wt.%）and alkaline（Na₂O+K₂O=8.3～9.1 wt.%）contents with negative zirconε_Hf（t）（-8.1～-0.6）values.We propose that the granite was derived from the partial melting of ancient crustal material under low pressure condition.Thus,we suggest that the heat has begun to transfer to the shallow crustal materials at～259 Ma,resulting in crustal melting events at different levels.（3）After the peak of magmatic activity of the ELIP,the magma activity continued at least to～254 Ma.Granite is the main magma type in this stage.The typical representative granites are Sangezhuang granite and Xinfawan granite with S-type granite affinity.They yielded zircon U-Pb ages of 256±1.0 Ma and 254±1.0 Ma,respectively.These granites have high silicon（Si O₂=72.10～72.98 wt.%）and high aluminum（Al₂O₃=13.34～14.18wt.%）with high A/CNK ratios of 1.03～1.13,and negativeε_Hf（t）values,which resemble the features of mature continental crustal components.Sangezhuang granite and Xinfawan granite have low liquidus temperature（991℃and 973℃,respectively）and crystallized at<800℃.Different from the characteristics of～259 Ma granites with A-type affinity,the late period granites show mainly I-type and S-type affinity,implying the heat of the Emeishan mantle plume has declined and the intrusive magmatic activity in the ELIP was near the end.（4）Based on the systematic research of the intermediate-acid intrusions in the Panxi region,we have present that the depth of magma source（enriched lithosphere mantle,mafic lower crust,juvenile crust and mature continental crust）is generally shallowing from the early period of Emeishan mantle plume activity to the late.According to thermal conductivity software simulation,it is suggested that the underplating of mantle-derived magma has a great influence on the thermal state of the mafic lower crust in the inner zone of the ELIP.However,the contribution of mantle-derived magma to heat conduction is quite weak for shallow crustal materials in 5 Ma,and the melting of shallow crustal materials is mostly related to the intraplating intrusion of magma.Intense underplating and intraplating process of mantle-derived magma has significantly changed the crustal composition in the inner zone of the ELIP,resulting in the higher high field strength elements contents（i.e.,Nb,Ta）and oxygen fugacity values than normal continental crustal materials.