| Kulongshan complex pluton lies in the northern margin uplift belt of North China Craton and Southwest section of the Guyuan-Honshanzi uranium belts,which between the Chifeng-Kaiyuan fault zone and Shangyi-Pinquan fault zone.Kulongshan complex pluton located at the Kulongshan area,Fengning,northern Hebei.It exposes 120 km2,which are consist of medium-coarse alkali-feldspar granites and medium-fine alkali-feldspar granites.Based on the field geological survey,the age of SHRIMP zircon U-Pb dating,petrography,geochemistry of element and Sr-Nd-Pb-O isotope geochemistry have been studied of the Kulongshan granites in northern Hebei.The formation age and geochemical characteristics of the rock mass are determined,its genetic types,material sources,tectonic setting and the relationship with uranium mineralization are analyzed.It is helpful for uranium exploration work and is of great significance for the tectonic evolution of the north margin of the North China Craton.SHRIMP zircon U-Pb dating indicate that the complex pluton are consist of outside medium-coarse alkali-feldspar granites and inside medium-fine alkali-feldspar granites,both for intrusive contact,zircon 206Pb/238U ages were 129.4ฑ1.5 Ma?MSWD=1.2?and134.0ฑ1.7 Ma?MSWD=1.8?,they formed in the late stage of Early Cretaceous and in the early stage of Early Cretaceous.Kulonghsan granites with high SiO2?76.6%77.8%,mean value is 77.2%?,K2O?4.24%4.65%,mean value is 4.41%?,Na2O?3.57%4.14%,mean value is 3.99%?,high ratio of FeOT/MgO?10.327.6,mean value is 19.8?,poor Al2O3?11.5%12.1%,mean value is 11.8%?,MgO?0.05%0.11%,mean value is 0.08%?,CaO?0.25%1.01%,mean value is 0.38%?,P2O5?0.006%0.016%,mean value is 0.008%?.The higher differentiation index?DI=95.496.6,mean value is 95.9?and lower Mg value?Mg#=7.1715.7,mean value is 10.1?indicate that the magma differentiation evolution degree is relatively high.A/CNK=0.951.06,?=1.862.05,K2O/Na2O=1.031.30,belong to the Peraluminous-weakly peraluminous high potassium calcium alkaline series rock.In the SiO2 vs?Na2O+K2O?-CaO and SiO2 vs FeOT/?FeOT+MgO?diagrams,they all fall into the A type granite area.In the Harker diagram of the major elements,there is a negative correlation between SiO2 and Al2O3,FeOT,MgO,TiO2.The total amount of REE is 222302ื10-6(mean value is 261ื10-6),light REE is167230ื10-6(mean value is 197ื10-6),heavy REE is 44.273.2ื10-6(mean value is64.3ื10-6).The ratio of light REE to heavy REE is 2.534.03?mean value is 3.10?,belong to light REE enrichment type.?La/Yb?N=1.613.04?mean value is 2.16?,the fractionation of REE is not obvious.?La/Sm?N=1.952.62?mean value is 2.29?,?Gd/Yb?N=0.620.88?mean value is 0.70?,compared with heavy REE,light REE has certain fractionation.?Ce=1.001.02?mean value is 1.01?,the anomaly is not obvious,?Eu=0.02,show strong negative Eu anomaly,It indicates that the plagioclase has undergone separation crystallization or remains in the source area during partial integration.In the standardized REE distribution curve of chondrite,Kulongshan granites like swallow type distribution,which shows the characteristics of A type granite.Enrichment Rb,Th,U,Nb,Ta,Zr,Hf,Y and other elements,loss Sr,Ba,P,Ti and other elements,Z+Nb+Ce+Y=402713ื10-6(>350ื10-6),10000Ga/Al=4.635.66?>2.6?,Characteristics of trace elements for A type granite.Rb/Sr,Ti/Y and Ti/Zr ratio of Kulongshan granites are 28.9154?mean value is100?,3.255.03?mean value is 3.96?,1.265.21?mean value is 2.61?,belonging to the range of crustal source magma.Kulongshna granites with abnormal initial ratio of Sr((87Sr/86Sr)i=0.7505620.839814),relative low Nd??Nd?t?=-14.3-13.2?and young Nd modle age(TDM2=20841996 Ma),they exhibit low radiogenic Pb istopic compositions((206Pb/204Pb)t=16.79717.010,(207Pb/204Pb)t=15.40615.434,(208Pb/204Pb)t=37.47737.540).In the Pb isotope model diagram 206Pb/204Pb-207Pb/204Pb,they fall into the lower crust region,which above the tropic of cancer.Meanwhile,they also fall in the EM I enriched mantle region.In the Pb isotope model diagram 206Pb/204Pb-207Pb/204Pb,they fall between the lower crustal evolution line and the mantle evolution line,Moreover,they are closer to the lower crustal evolution line and located at the edge of the mantle region of EM I.The above features show that the magma source of the Kulongshan granites are closely related to the lower crust,and the rich mantle component participates in the formation of the granite.?18OV-SMOW-SMOW in medium-coarse alkali-feldspar granites and medium-fine alkali-feldspar granites were 5.36.2and 0.13.1,the later is lower than the normal value of granite?610?and mantle?.7ฑ0.3?,it may be related to the addition of recirculating oceanic crust material.In the struction environment discrimination diagrams of MgO-Fe OT,SiO2-FeOT/?FeOT+MgO?,CaO-FeOT+MgO and SiO2-Al2O3,Kulongshan granites fall into the POG?post-orogenic granite?area or edge.In the R1-R2 and SiO2-lg[CaO/?Na2O+K2O?]diagrams show the characteristics of post orogenic stretching environment.In the Y-Nb,Yb-Ta,Y+Nb-Rb and Yb+Ta-Rb struction environment discrimination diagrams,all of them fall into WPG?intraplate granite?range.It is obvious that the Kulongshan granites belong to the intraplate post orogenic granite,which is formed in the extensional tectonic setting in the plate.Synthetically analyze the tectonic background of contemporary A type granites in the region,it shows that the granite is the product of extensional tectonic system related to the early Cretaceous and craton destruction in the northern margin of the North China Craton.This tectonic system may be related to the subduction of the Pacific plate.Kulongshan complex pluton and its surrounding areas of uranium polymetallic mineralization anomalies are controlled by NE faults,No.291 mineralization belt with large scale,many useful components and surface orebody pinchout in shallow,but the alteration belt extends clearly to the deep,have obvious thickening trend and better continuity,it is a favorable area for finding uranium,thorium and polymetallic minerals. |
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